pytest-failure-instrumentation
Most test failures explain themselves. A pytest_runtest_makereport gives you
an assertion, a traceback and a node id, and there is nothing left to
investigate.
The failures that happen outside the call phase explain nothing. A worker is killed, a run wedges, workers disagree about which tests exist, pytest raises inside its own machinery — and what reaches your reporting is a placeholder string, a line in a log, or nothing at all. This plugin records what those failures cannot say for themselves, works out whose code is responsible, and hands you one structured incident per problem.
[worker_death] NATIVE_CRASH severity=critical owner=product
blamed on engine.py:6 in native_call
in flight test_crashes.py::test_crashes phase=call started=1 finished=0
· died while running test_crashes.py::test_crashes (call)
· exit status -11 - SIGSEGV: segmentation fault in native code (pid 805, via waitid)
· the worker wrote a stack before dying
· segmentation fault in native code
The problem
When a pytest-xdist worker dies, this is the whole report:
[gw7] node down: Not properly terminated
An OOM kill, a segfault in a C extension and a stray os._exit(1) are
indistinguishable at that point. Not because nobody bothered to print the
difference — because by the time anything can ask, the difference is gone.
There are three independent reasons, and all three have to be worked around
separately.
1. The cause never leaves the process. SIGSEGV and SIGKILL end a process
without running any Python. No finally, no atexit, no __del__, no
pytest_sessionfinish. Whatever the worker knew about what it was doing, it
knew only in memory, and that memory is gone. Anything you want to know
afterwards has to have been written down before — while the run was healthy
and the cost of writing it lands on every passing test.
2. The exit status is read and thrown away. The kernel keeps one number
that separates all these cases, and the parent process is the only thing
allowed to read it. execnet does read it — Group.terminate calls
gw._io.wait() in execnet/multi.py — and discards the return value. Nothing
in xdist asks for it either.
3. There is no field to put it in. In xdist/workermanage.py,
process_from_remote handles the channel closing: it asks execnet for a remote
error, and when there is none — because the remote never got to send anything —
substitutes a literal string:
err = "Not properly terminated" # lost connection?
That string is then passed to pytest_testnodedown(node, error) as the error.
The hook is not withholding the cause. By the time it fires, the placeholder is
genuinely all that exists.
Why you never see a MemoryError
The most common way a worker dies is also the one Python is least able to
report. On Linux, malloc returning successfully is not a promise that the
memory exists — overcommit hands out address space and resolves it on first
touch. There is no allocation failure for CPython to raise MemoryError from.
The process is killed later, from outside, with SIGKILL, which cannot be
caught, blocked or handled.
And the exit status is -9 for all of it: the kernel OOM killer, a cgroup
limit, a cancelled CI job, a kill -9 from a stray script. There is no
distinct code for "out of memory". The only in-process evidence that separates
them is the cgroup v2 memory.events oom_kill counter, which is why
OOM_KILLED is claimed only when that counter moved during this run, and
SIGKILLED — "something killed it, and here is what that could have been" —
when it did not.
The failures that reach no hook at all
A worker's death at least fires a hook. Four others do not.
A run that stalls. pytest_testnodedown needs a dead process; a wedged
one is alive. And the controller hears from a worker only when a phase
completes, so from outside, a twenty-minute test and a deadlock are the same
event: nothing. The run does not fail — it never ends, and CI kills the job an
hour later with no artifact naming a test.
Without xdist there is no outside at all, which sounds like the harder case and
is the easier one: a main thread blocked on a lock or a socket does not stop
the other threads in its process, so the run can be asked what it is doing by a
thread of its own. A plain pytest that deadlocks now names the test, prints
the stack of the thread that is stuck, and says so while it is still stuck.
Workers that collected different tests. xdist notices, writes a unified diff per differing worker into its own log, and aborts. Nothing structured reaches a hook. With sixty workers and one odd node that is fifty-nine complete diffs, every one of them naming the majority as the deviation.
A run that never came back. Every failure above is reported by a process
that survived to report it. A run that was itself killed has no survivor: a
plain pytest that segfaults, a controller reclaimed along with its workers, a
CI job cancelled mid-suite. Nothing fires, nothing is written, and the only
trace is a job that stopped. What that run recorded is on disk and complete —
so it is reported by the next run over the same directory, which was already
walking it to clear it out.
An internal error. pytest sets ExitCode.INTERNAL_ERROR, which is not in
summary_exit_codes in _pytest/terminal.py — so pytest_terminal_summary
never fires for it. Under xdist it is worse: a worker's internal error is
relayed to the controller as a flat string and re-raised there, so the
INTERNALERROR> block you read names xdist's frame, not the failure.
Who this is for
You ship a library into other people's test suites. Their run dies and the
bug report names your package. Nothing in the output can confirm or refute it,
and "cannot reproduce" is not an answer anyone accepts. owner is the field
that settles it — product, third-party, customer-code or runtime — and
it comes from a stack, not from a guess.
You own CI for a large suite. Runs fail with no test named. Was that the
OOM killer, or the runner getting reclaimed mid-job? -9 is identical either
way, and the answer decides whether you buy more memory or file a ticket with
your CI vendor.
Your suite hangs sometimes. Nothing fails, the job times out, and there is
no evidence at all because the process that would produce it is the one that is
stuck. worker_stall names the test, says whether the thread is blocked or the
whole process is frozen, and prints the stack of the thread actually stuck.
You run enough workers that they disagree. A conftest keyed on an environment variable, a machine-dependent skip, a plugin that collects conditionally. The run aborts and the reason is buried in N−1 diffs.
You are collecting failures across machines you do not control.
fingerprint groups recurrences so one defect on twelve workers is one row
with a count, and capabilities records what each machine could measure — so a
missing memory figure on a customer's Windows box reads as "unmeasurable here"
rather than "fine".
A worked example: xdist #1362
A worker dies in the window after it has sent its collection but before
scheduling begins, while a second worker is still collecting. The stale entry
in registered_collections is never cleaned up, and the run dies with a
KeyError naming an object rather than a problem
(pytest-dev/pytest-xdist#1362).
This is what the plugin reports for it — two incidents, because two things went wrong:
[worker_death] SIGKILLED severity=needs-triage owner=unknown
no test in flight started=0 finished=0
· died before running any test (startup or collection)
· exit status -9 - SIGKILL: uncatchable kill (OOM killer or external kill) (pid 21780, via waitid)
· resident memory 31 MB at last checkpoint
· SIGKILL with no cgroup OOM event: a host-level OOM killer, a container or CI cancellation, or an external kill
[internal_error] INTERNAL_ERROR severity=high owner=runtime run-ending
blamed on loadscope.py:275 in _assign_work_unit
KeyError: <WorkerController gw1>
· raised on the controller itself and captured first-hand
· raised above informational: a framework-owned defect that ended the run - no test is at fault, so nothing else will surface it
owner=runtime is the load-bearing part. No test is at fault and no worker is
at fault, so nothing else in the run will ever surface this — which is exactly
why it is the one case where a framework defect is raised above
informational.
Install
pip install pytest-failure-instrumentation
It registers itself as a pytest11 entry point. Implement one hook to receive
what it finds:
# conftest.py, or your own plugin
def pytest_failure_incident(incident):
database.save(incident.model_dump())
alerts.send(str(incident))
Tell it which packages are yours, so a failing frame in your code can be told from one in a dependency or in the customer's own tests:
[pytest]
failure_packages = yourcore, yourcore_ext
failure_product_version = 4.2.0
Without the hook it still writes its evidence to .pytest-failures/.
Disable it entirely with -p no:failure_instrumentation.
Installing it from your own framework
If you ship a test framework rather than consume one, ini is the wrong place for these settings. Your packages, your artifact directory and your build id are things your framework already knows, and an ini block every consuming team has to copy — and keep in step with you — is a migration that never finishes.
Call install from your plugin instead:
# yourframework/pytest_plugin.py
from pytest_failure_instrumentation import install
def pytest_configure(config):
install(config,
packages=deployment.owned_packages,
directory=deployment.artifact_dir,
product_version=deployment.version,
run_id=ci.build_id)
Keyword arguments layer on top of whatever ini said, so a team that has set
failure_stall_seconds keeps it. Passing a whole Settings instead replaces
ini entirely, for when your framework owns the configuration outright:
from pytest_failure_instrumentation import Settings, install
install(config, Settings(packages=("yourcore",), stall_seconds=600))
Settings has a default for every field, coerces a list of packages and a
string path to what it needs, and enforces its own invariants — so a
hand-built one cannot skip a floor that a resolved one obeys.
Three things this handles for you:
Load order. A conftest's pytest_configure runs before this plugin's, and
a plugin loaded as an entry point runs after it. Registration here is
trylast and only claims what nobody has installed, so your settings win
either way.
Workers. A worker is a separate process. Settings you computed in Python do
not exist there and your framework's code may not even be loaded — so whatever
is in force is pushed down through workerinput, and a worker prefers it over
anything it could read itself. run_id travels with it, which is what lets a
build id group a whole run's incidents.
Turning auto-registration off. -p no:failure_instrumentation skips the
entry point entirely; install puts back the hookspecs so
pytest_failure_incident, pytest_failure_worker_sample and
pytest_failure_server_ready all still reach their implementers. Note that it also skips pytest_addoption, so
failure_* ini keys become unknown config options — which is the point if your
framework owns the settings, and a reason to leave the entry point enabled and
just call install if it does not.
install is idempotent and returns the settings in force. A second call keeps
the first one's and warns rather than silently losing to it;
installed_settings(config) reads them back. Like everything else here it
warns instead of raising — the one exception is a misspelled setting name,
which is your bug and is worth an error rather than a run that quietly
attributes nothing.
What you get
incident is a pydantic model, one class per kind, discriminated on
incident.kind. A segfault's resident memory and a run summary's exit code
have nothing to say to each other, so they are not fields of the same object:
kind |
Model | Raised on |
|---|---|---|
worker_death |
WorkerDeathIncident |
any run |
worker_stall |
WorkerStallIncident |
any run |
collection_mismatch |
CollectionMismatchIncident |
needs xdist |
internal_error |
InternalErrorIncident |
any run |
run_summary |
RunSummaryIncident |
any run |
Only the two that are about workers need workers. Everything else is raised whether or not you run under xdist, because the process that records is whichever one runs the tests — under xdist a worker, and without it the session itself.
They share verdict, confidence, severity, owner, fingerprint,
run_id, worker and evidence. str(incident) is the alert text — every
block quoted in this README is what it prints. A stored row comes back as the
model it was written from, and the union is a schema you can migrate a table
against:
from pytest_failure_instrumentation.incidents import registry
incident = registry.parse(json.loads(row)) # -> WorkerDeathIncident, ...
registry.json_schema()
The stack is in the payload but out of str(incident), because forty
frames turn a readable incident into a wall and whether they belong in an alert
is your call. incident.raw_stack() returns them as lines whatever the kind is;
top_frame and blamed_frame are the two already parsed, each with file,
line, function, module and owner.
What you get is the deepest frames of one thread from the most recent dump —
the other threads in a worker are this plugin's own heartbeat and execnet's
receiver, and reporting those blames the instrumentation. It is capped (40
frames for a death, 14 for a stall, 4000 characters for an internal error) and
a cut stack ends with ... and N more frames rather than pretending to be
whole. The complete dump stays in <worker>.crash on the runner.
def pytest_failure_incident(incident):
body = str(incident)
frames = incident.raw_stack()
if frames:
body += "\n\n" + "\n".join(frames)
alerts.send(body)
owner is the field that settles arguments — product, third-party,
customer-code, runtime, or unknown. It comes from walking outward past
runtime frames to the first one that belongs to somebody: the deepest frame is
usually ctypes.string_at, which tells nobody anything. A stack with no owned
frame at all is not unknown — it is a positive finding that the framework
itself failed.
severity follows from ownership rather than from how loud the failure
was, so a customer's segfaulting test does not page you. The exception is a
framework defect that ends the run, above.
fingerprint is stable across runs and excludes worker id, pid and
timings, so one defect on twelve workers is one incident with a count.
capabilities says what the machine could measure, so an absent figure is
never read as a healthy one.
suspect_owner is kept apart from owner on purpose. When no stack names
anybody, the test that was in flight is a lead worth recording — but a guess
must never sit in the column a reader takes for a finding.
Handing one to an agent
An incident is written to be read without context, which is most of what an LLM
triaging a CI failure lacks. .claude/skills/reading-failure-incidents/SKILL.md
is that context in one file: the anatomy of the alert text, what each shared
field licenses a reader to conclude, the verdicts per kind, and the handful of
places where the text is a summary and the payload is the number — so an agent
reports what the incident found rather than what a -9 looks like.
Verdicts
A worker died
| Verdict | Told apart by |
|---|---|
OOM_KILLED |
-9 and the cgroup OOM counter moved |
SIGKILLED |
-9, counter flat — host OOM, CI cancellation, external kill |
NATIVE_CRASH |
SIGSEGV/SIGABRT/SIGBUS/SIGILL/SIGFPE, or a Windows NTSTATUS |
SIGNAL_<n> |
SIGTERM/SIGINT/SIGHUP — a request to stop, not a defect |
SELF_EXIT |
any exit code with no signal, 0 included — a worker that left the run was not asked to |
PROBABLY_SIGNALLED |
exit code 128–191, a wrapper ate the signal |
UNKNOWN |
no status obtainable (remote gateway) |
A worker stalled
Silence proves nothing on its own: the controller hears from a worker only when a phase completes, so a twenty-minute test and a deadlock look identical from outside. What separates them is the worker's own heartbeat, which carries CPU time.
| Verdict | Heartbeat | CPU | Means |
|---|---|---|---|
STALLED_BLOCKED |
alive | none | the test thread is waiting on something that is not coming |
STALLED_FROZEN |
stopped | — | native code is holding the GIL, or the process is stopped |
STALLED_SILENT |
never ran | — | the watchdog is off, so there is no passive evidence either way |
| (not reported) | alive | burning | slow, not stuck |
The verdict is reached from beats already on disk. A stack is asked for afterwards, once the decision is made, because asking a wedged process a question can change its answer — see below.
In a run with no workers the assessment is the same assessment, from the same files, made by a thread inside the process it is about. The stack is read the way every live process here is read — py-spy, from outside — even though this one's frames are also directly to hand: a second reader for the single case that could avoid the first is a second set of failure modes and a second source to explain. It reads memory rather than asking the process to run anything, so no signal is sent that could return a blocked syscall early and dissolve the stall, and Windows — where no process can be asked for a stack — gets a current one like everywhere else. Without py-spy installed the verdict is unchanged, since it comes from beats rather than frames, and the stack is whatever the watchdog last wrote.
The exception is STALLED_FROZEN, which is precisely the case where no Python
runs: the watcher thread cannot run either, so a frozen lone run reports
nothing until somebody reads what its fallback timer left behind.
Workers collected different tests
| Verdict | Means |
|---|---|
COLLECTION_MEMBERSHIP_DIFFERS |
a test exists on one machine and not another |
COLLECTION_ORDER_DIFFERS |
same tests, different sequence — fatal too, since xdist addresses tests by position |
COLLECTION_PARAMETERS_UNSTABLE |
same tests, different parameter values — a parametrize that is not deterministic |
Sixty workers never produce sixty collections. They produce two or three variants, so this reports one row per variant, measured against the largest:
[collection_mismatch] COLLECTION_MEMBERSHIP_DIFFERS severity=needs-triage owner=unknown run-ending
no stack; suspect customer-code (owner of a module the workers disagreed about (test_collect.py))
2 workers produced 2 different collections
baseline: 1 worker collected 3 tests, and everything below is measured against that list
1 worker is missing 1 test, in test_collect.py (gw1)
- test_collect.py::test_two
whole collections written to .pytest-failures; the difference above travels in the incident
· xdist addresses tests by position rather than by id, so any difference between the lists is fatal - a reordering as much as a missing test
· the initial collections disagreed, so the run was aborted
At sixty workers it stays the same shape, because the row count follows the number of variants rather than the number of workers:
58 workers produced 3 different collections
baseline: 55 workers collected 300 tests, and everything below is measured against that list
2 workers are missing 1 test, in test_payments.py (gw41, gw58)
- test_payments.py::test_case_017
1 worker has 6 extra tests, in test_legacy.py (gw17)
+ test_legacy.py::test_extra_0
+ test_legacy.py::test_extra_1
+ test_legacy.py::test_extra_2
and 3 more
Read it as: how many distinct opinions existed, which workers held each, and how the minority differs from the majority. Magnitude leads each line and identity follows, samples use diff notation, and a truncated sample always says how much it withheld — a sample that looks like the whole story is worse than no sample at all.
The whole difference travels in the payload, not just the three ids the
text prints. missing and extra carry every differing node id, up to 500 per
side, with missing_count and extra_count as the true totals so you can see
whether that cap was reached. The distinction matters: a collection is
unbounded — sixty workers times fifty thousand node ids is hundreds of
megabytes — but a difference is almost always one test or one module's worth.
Only the digest is held per worker, and the whole collections are written to
collection-<digest>.txt for whoever still has the machine. That file is on a
runner which may be gone by the time anyone reads the alert, which is exactly
why the difference itself does not live there.
An order difference instead reports where the two lists first disagree, which is the one fact a unified diff of a reordered list destroys.
A parametrize whose values are drawn at collection time — random, a
timestamp, an unordered set — gives every worker a different id for the same
test, and reported as membership that reads as thousands of tests appearing and
disappearing. It is caught by asking a second question: are these the same
tests once the parameters are stripped from the ids? When they are, the report
names the parametrized tests responsible, drops the per-variant rows — which
would otherwise be one near-identical block per worker — and prints what each
of a few workers actually collected:
[collection_mismatch] COLLECTION_PARAMETERS_UNSTABLE severity=needs-triage run-ending
6 workers produced 6 different collections
the tests are the same on every worker - only the parameter values differ, so these are not tests appearing and disappearing
test_billing.py::test_invoice
gw0 collected acct-1791, acct-3471, acct-6305, acct-7468
gw1 collected acct-2186, acct-2542, acct-6991, acct-9779
gw2 collected acct-1614, acct-1950, acct-4517, acct-9313
compare those values: a parametrize evaluated at collection time - a random number, a timestamp, an unordered set, a call to something live - gives every worker a different id for the same test, and xdist requires the ids to match
The values are the diagnosis. Naming the test says where to look; three rows of disjoint account ids say a fetch is running at collection time, and three rows of floating-point noise say a random number is. Neither is apparent from one worker's list, which is the only thing xdist ever shows you.
That case is also why full id lists are held for only the first few variants.
"A handful of variants" is the assumption the whole design rests on, and
unstable ids turn it into one variant per worker. Past that limit a variant is
reported as not compared rather than diffed against a list nobody kept —
comparing two absent lists reports "the same tests in a different order", which
is a finding invented out of missing data.
A mismatch is run-ending usually, not always: xdist aborts when the initial
collections disagree, but silently drops a worker that registers a differing
collection after scheduling has begun. The run then continues one worker short,
and run_ending reflects which of the two happened.
How it knows
Whichever process runs the tests is the one that records. Everything below
is written from inside that process, because a process that is about to be
killed gets no warning and nothing it knew only in memory survives. Under
xdist that process is a worker and the controller reads what it left. Without
xdist there is one process and it does both jobs — so a plain pytest writes
the same state slot, the same heartbeat and the same stacks a worker does,
under the name main, and the live view, the sampler and the stall watcher
read them the same way.
The one thing it does not take, unless asked, is the fatal dump.
faulthandler keeps exactly one destination for a fatal signal and pytest's
own plugin has already pointed it at stderr; a worker claims it and loses
nothing, because that stderr is shared with fifteen others and a dump written
into it belongs to nobody. A run with no workers would be taking the crash out
of a terminal somebody is watching, so it leaves it there — see
failure_crash_stack, which is that trade written down.
A fixed-size state file. Which test and phase is open right now is written
to a fixed-size slot with os.pwrite — one syscall, no append, no growth, and a
file that is the same size after a million tests as after one. That is what
separates "died in teardown" from "died mid-call": pytest's own logfinish
fires only after the whole protocol, so it cannot tell them apart. The slot is
5 KiB, holding a node id of around 4950 characters whole — past any real one by
an order of magnitude, since a path, a class, a test name and a couple of
content hashes together use a twentieth of it. The size is close to free: one
write of one buffer costs the same syscall from 256 bytes to 8 KiB, and 5 KiB
per worker is 320 KiB across a 64-way run. An id longer than that gives up its
middle, never the record: truncating the encoded record leaves it
unparseable, which costs the reader
the phase and the counters as well and reports a worker that died mid-call as
one that died before running anything. The middle goes rather than the tail
because both ends carry something the other does not — the head is the module
attribution reads, and the tail is where a parametrize puts the value saying
which case this was.
The slot carries two node ids, and the difference is why it is read at all.
test_in_flight is the test currently running and is cleared when teardown
returns; last_test is the most recent test whether or not it finished. One
field cannot be both, and being both is how a worker that died in the gap
between two tests came to be reported as having died in the one that had
already passed — attributed to whoever owns it, with an owner and a severity on
it. Where nothing is in flight the incident says so, and the lead it offers
names itself as the last test rather than the running one.
It carries the run id too. The evidence directory outlives a run and clearing it is best-effort — on Windows a file another process still has open cannot be unlinked at all — so a record stamped with a different run is refused rather than read as this one's. That check is load-bearing beyond the report: the pid in a stale record is a pid this run's stack probe would otherwise signal.
The exit status, taken from the OS. Where a Popen object survives, its
return code. Otherwise waitid(P_PID, pid, WEXITED | WNOWAIT | WNOHANG) —
WNOWAIT reads the status without consuming it, so execnet's own reaping
still works afterwards and nothing is broken by looking. Only a parent may do
this, which is why it happens on the controller, and why a remote gateway
honestly reports UNKNOWN rather than guessing. macOS does not expose
os.waitid at all and falls back to the Popen object. capabilities.exit_status
records which mechanism this machine has; exit_status_source on the incident
records which one actually answered — so a figure is never read as having come
from a mechanism that was never used. On Windows the code is normalised to its
unsigned form first: an NTSTATUS is above 2³¹, so 0xC000013A arrives signed
or unsigned depending on who answered — and a negative status means "killed by
signal N" to everything downstream.
faulthandler, pointed at a per-worker file. pytest's own faulthandler
plugin enables at configure time with trylast, aimed at shared stderr where
every worker's output interleaves. This claims the handler back afterwards, in
pytest_sessionstart. Its C handler is async-signal-safe and writes while the
GIL is held — which is the case that matters, since native code holding the
GIL is exactly what a frozen worker looks like.
Separate dump files. A fatal dump goes to <worker>.crash; the slow-test
watchdog's goes to <worker>.slow, and the frozen-interpreter fallback's to
<worker>.frozen. They are the same shape and only
the banner separates them — Fatal Python error against Timeout (…) — and a
watchdog dump is written by tests that go on to pass. Sharing one file made
"a stack exists" ambiguous, and on the Windows path where the dump is the only
thing distinguishing abort() from os._exit(3), ambiguous means a slow test
that passed can be reported as the crash that killed the worker, blamed on
whatever that stack happened to be doing.
Choosing the right dump, and the right thread inside it. Two things have to be picked here, and getting either wrong blames code that was not running.
A file holds as many dumps as were written to it, and the crash file accumulates: an on-demand stack taken while a worker was merely stalled precedes the fatal dump that ends it. The dump that describes the present is the last one. The watchdog's file holds only ever one, because each dump is written beside it and renamed into place — a reader that caught it mid-write would get the threads faulthandler had reached and not the one running the test.
Within a dump, all_threads=True means every thread is present, and the first
printed in a pytest worker is this plugin's own heartbeat thread; the second is
execnet's receiver. Reporting the first section would blame the instrumentation
for the failure it came to explain. The section reported is the one the fault
or signal reached (Current thread), else the one carrying pytest's runtest
protocol, else anything that is not ours. Current thread is skipped when it
is ours: the watchdog's dump is taken by the heartbeat thread, so
faulthandler labels that one current, and believing the label reported the
heartbeat as the frozen test.
Saying how old a stack is. A stack is evidence about a moment, and the
frames look the same whether they were taken just now or left behind four
minutes ago. A stall that could be probed reports a current stack; one that
falls back to the watchdog's file says stack written 47s ago by the slow-test watchdog, not taken just now, and carries stack_age_seconds. A death reports
crash_stack_age_seconds alongside, so a dump that predates the death reads as
the context it is rather than as the crash.
A watchdog on a cadence, written by the heartbeat thread. A test still
running after failure_slow_test_seconds has its stack written for it, and
keeps having it rewritten every interval, so whatever is on disk is at most one
interval old. That bound is the point: on Windows nothing can ask a live
process for a stack, so this is the only one a stalled worker will ever have.
The clock starts at setup and stops at the end of teardown, so a fixture
blocking on a container and a finalizer blocking on a connection are covered as
well as the test body — those are the commonest real hangs there are, and the
state slot has always told them apart. Once for the whole test rather than per
phase, or a test that spent most of the interval in setup and the rest in the
call would never reach it. The default is 20 seconds, and the file is dropped
when the test ends, so only the running test's stack is ever on disk (about
5 KB) and a healthy suite leaves nothing.
The obvious design instead has the controller signal a stalled worker and let
faulthandler answer. It has two flaws: Windows has no SIGUSR1 and os.kill
there cannot deliver one, and on POSIX the signal perturbs the subject —
PEP 475 makes Python retry on EINTR, but a C extension blocked in a raw
syscall need not, so it returns early and the stall being measured disappears.
The signal path remains as an extra, for asking an already-diagnosed worker for
a fresher stack.
The next design, and what this was until it was measured, is
faulthandler.dump_traceback_later(repeat=True). It needs no signal and works
on Windows, and it dumps even while native code holds the GIL — but it dumps
from a C thread that does not hold the GIL, walking every other thread's
frames while those threads push and pop them. A dump landing while the
interpreter is executing rather than blocked reads a frame being torn down, and
the worker segfaults. Over a suite whose tests were four times the cadence
long, that killed the worker in 10 runs out of 10, against 0 with the repeat
turned off; it left the dump ending mid-frame with a nonsense line number, and
the crash file empty because the fault was inside the dumper. Instrumentation
that crashes what it is watching is worse than no instrumentation, so the
cadence is driven from the heartbeat thread instead, in Python, holding the
GIL — nothing else can be mutating what is being walked.
A fallback for the one stack a Python thread cannot take. When native code
holds the GIL, no Python thread runs and the watchdog above writes nothing.
That is the case the C timer exists for, and also the case that makes it
dangerous — so it is armed such that it can only fire when it is safe. Every
heartbeat pushes its deadline out by three intervals, so while Python runs at
all the deadline is always in the future and the timer never fires; when three
beats in a row are missed it fires once, and by then nothing is executing for
it to trip over. Missing beats for that long has one realistic cause: a thread
holding the GIL and running C. A main thread running Python releases the GIL
every few milliseconds, and a machine loaded badly enough to starve a daemon
thread for three intervals would starve the timer's own thread with it. The
dump goes to <worker>.frozen, because it means something the watchdog's does
not — not "this test is slow" but "this process stopped responding" — and the
incident says which file its stack came out of in stack_source.
It stands down where pytest is using that timer. There is exactly one
faulthandler.dump_traceback_later timer per process, and arming it cancels
whatever was armed before. pytest's own faulthandler plugin arms it at the
start of every test when faulthandler_timeout is set, and this fallback
re-arms every second — so the fallback always won, and a configured timeout
silently never fired, faulthandler_exit_on_timeout and all. Where that ini is
set the fallback is not armed at all, and the worker records
frozen_fallback_stood_down in its event log saying why. It costs a stalled
worker its frozen-fallback stack, which is a worse report; the alternative is a
run that hangs past a timeout somebody configured, which is a worse run.
Nothing is signalled that cannot be confirmed. SIGUSR1's default
disposition is to terminate, and the pid the on-demand probe would signal was
read back out of a file the worker wrote. A worker that has since exited leaves
its number to be handed on by the kernel, so signalling it does not produce a
bad report — it kills an unrelated process. The pid is signalled only once
something says it is still ours: the controller can see that process running
under this worker's gateway, or the machine can be asked and answers that it is
a child of this one. A machine that cannot be asked at all is not taken as a
yes, and the incident says which of those it was instead of showing a stack it
never had.
A heartbeat carrying CPU time. One line every five seconds per worker,
bounded by wall-clock rather than by how many tests run. time.process_time()
in each beat is what turns silence into a verdict: alive with no CPU is
blocked, stopped is frozen, alive and burning is a slow test that must be
reported as nothing at all.
Evidence written before it is needed. Every mechanism above puts its output on disk during the healthy part of the run, because a process that is about to be killed gets no warning. The controller reads files, never the corpse.
And a run that never came back is read by the next one. The corpse being a whole run rather than one worker changes only who does the reading. A starting run already walks the evidence directory to clear out the runs that are over; the same walk now asks a second question of the same marker, and two answers separate a run to report from a run to delete:
- Is it over? The owner's pid is in the marker, so a run still going is recognisable however long it has been going — which matters because several run at once, and deleting a live run's evidence is how a cleanup once broke the very reports it existed to produce.
- Did it report for itself? A run that reaches session finish stamps the marker on the way out. It raised its own incidents; re-raising them a day later against whichever run happened to notice is worse than never raising them.
A run that is over and never stamped that mark is exactly a run that could not report for itself, and each process in it that started a session and never finished one is a death:
[worker_death] UNKNOWN severity=needs-triage owner=unknown
no stack; suspect customer-code (owner of the test in flight (test_pool.py))
recovered from run-8f21c0b4e5d7, which ended without reaching session finish
in flight test_pool.py::test_writes phase=call started=12 finished=11
· died while running test_pool.py::test_writes (call)
· exit status unavailable (pid 21780): nothing was left to read it. Only a parent may, the parent was the run that died, and by the time this evidence was found the process was gone - so an OOM kill, a segfault and an os._exit cannot be told apart here
· resident memory 412 MB at last checkpoint
· no stack was kept here: this run had no workers, so its fatal dump went to the terminal pytest's faulthandler plugin writes to rather than into a file - set failure_crash_stack to keep a copy instead
recovered_from_run leads the block, because a reader who takes this for the
current run's crash goes looking for a failure that is not there. run_id is
the dead run's — that is the key anything joins on, and the run that merely
found it had no part in what happened. raised_at is now.
The exit status is the one thing genuinely lost. Only a parent may read it, the
parent was the run that died, and the process is long gone — so -9, -11 and
os._exit(1) cannot be told apart afterwards, and the incident says that
rather than guessing. Everything else survives: which test was in flight and in
which phase, how many had run, the resident memory at the last beat, and — if
the run kept one — the fatal stack, which is enough to reach NATIVE_CRASH
with a blamed frame and no status at all. That is the same position Windows is
in for a watched worker, and it is why the dump is evidence in its own right.
Live stacks over HTTP
Everything above is for reading afterwards. This is the other direction: a UI watching a run, asking what a test is doing while it is still doing it.
$ curl localhost:8080/stack?pid=48213
{"pid": 48213, "source": "py-spy", "captured_at": 1756142887.31,
"options": {"native": false, "locals": false, "nonblocking": false},
"threads": [{"thread_id": 8632442880, "thread_name": "MainThread",
"os_thread_id": 48213, "owns_gil": true, "active": true,
"frames": [{"function": "_wait_for_lease", "file": "/app/pool.py", "line": 91,
"module": null, "native": false, "locals": null},
{"function": "checkout", "file": "/app/pool.py", "line": 44,
"module": null, "native": false, "locals": null},
{"function": "test_concurrent_writes", "file": "/tests/test_pool.py",
"line": 210, "module": null, "native": false, "locals": null}]}]}
Naming the process. ?pid= is what /workers reports and what a UI already
holds. ?worker=gw3 is what a person holds — somebody looking at a stalled
worker is asking about that worker, not about the test it happens to be on, and
resolving the name at the moment of the read closes the window where xdist
replaces it between two requests. The name is compared against a directory
listing and never joined onto one, and a worker whose process has exited
resolves to nothing rather than to its last pid — pids are reused, and reading
one afterwards means reading whatever the machine has since given that number
to. Name it one way or the other; both at once is refused, because they can
disagree and there is no right one to prefer.
Asking for more than the frames. Three options, each one py-spy flag, all
off unless switched on. A bare ?locals is on: only an explicit ?locals=0 is
a no.
| option | what it adds | what it costs |
|---|---|---|
?native |
frames from C, C++ and Cython extensions | needs the process paused, and a py-spy that can unwind |
?locals |
each frame's variables, rendered as text | the largest payload here, and the data a test is holding |
?nonblocking |
reads without pausing the target at all | accuracy, and owns_gil/active, which become null |
$ curl 'localhost:8080/stack?worker=gw3&locals'
{"pid": 48219, "worker": "gw3", "source": "py-spy", "captured_at": 1756142889.02,
"options": {"native": false, "locals": true, "nonblocking": false},
"threads": [{"thread_name": "MainThread", "frames":
[{"function": "_wait_for_lease", "file": "/app/pool.py", "line": 91,
"module": null, "native": false,
"locals": [{"name": "timeout", "repr": "30.0", "argument": true},
{"name": "waited", "repr": "27.4", "argument": false}]}]}]}
options on the way back is what was applied, not what was asked for, and
any difference is a sentence in notes. --native and --nonblocking are
refused as a pair by py-spy, so asking for both drops native — --nonblocking
is a promise about the target, and honouring native instead would pause a
process somebody asked not to have paused. A py-spy that cannot unwind is
likewise a reason to return the Python frames plus a note rather than an error.
A caller that displayed its own request back to a user would be captioning
frames with a setting that did not produce them, so read the toggles back from
here.
$ curl 'localhost:8080/stack?pid=48219&native&nonblocking'
{"options": {"native": false, "locals": false, "nonblocking": true},
"notes": ["native frames need the process paused, and --nonblocking was asked
for as well; py-spy refuses that pair, ..."], ...}
locals is null when they were not asked for and [] when the frame holds
none — a native frame has no Python variables, and answering null there would
read as "you did not ask". The variables are rendered by py-spy inside its own
process while the target is stopped, so what crosses the wire is text and no
__repr__ of yours is executed to produce it. They are nonetheless the one
thing this server discloses that is the data a test is working on rather than
the shape of the code — a fixture's credentials, a customer record, a decrypted
payload — so a deployment that cannot have that leave the process turns them off
and still gets the frames:
[pytest]
failure_stack_server_locals = false
Nothing is redacted selectively. This package cannot tell a password from a lease id, and a filter that pretended to would be worse than the honest switch.
Off by default — a plugin installed for crash reporting should not start opening listening sockets on everybody who upgrades it:
[pytest]
failure_stack_server = true
$ pytest --callstack-port 8080 # also switches it on
$ pytest --callstack-host 0.0.0.0 # so does this - and needs a token
$ PYTEST_CALLSTACK_TOKEN=$(openssl rand -hex 16) pytest --callstack-host 0.0.0.0
A token does not switch it on: "authenticate the server I am already
running" and "start a server" are different requests, and an exported
PYTEST_CALLSTACK_TOKEN in a shell profile must not open a socket on every
pytest run in that shell.
Two modes, and the port number picks between them
Drawn — the default, when no port is named. The session binds whatever the kernel hands it and writes the address into the evidence directory. Nothing is shared, so nothing is contended and nothing can be lost to another session.
Named — --callstack-port 8080, or the ini equivalent. The session claims
that exact port and shares it with every other session on the machine, since a
fixed port cannot be bound twice. First to start serves; the rest wait, and take
over within five seconds of the holder exiting.
Name a port when something outside has to be told the address once and for all — a firewall rule, a UI with it compiled in, a published container port. Otherwise let one be drawn: a UI that can read the evidence directory needs no agreement about numbers, and it has to read that directory anyway to know which pid is running which test.
What is running where
GET /workers answers the whole question in one request, assembled from files
the run was writing anyway — no ptrace, no per-test cost, nothing written:
{"served_by": {"service": "…", "pid": 17155}, "observed_at": 1787688175.421,
"runs": [{"session": "run-19d52c2ff8e2", "run_id": "757f3cc51790…",
"controller": {"pid": 17155, "alive": true},
"schedule": {"dist": "load", "collected": 812, "unassigned": 240, "settled": false},
"workers": [
{"worker": "gw0", "pid": 21615, "nodeid": "test_slow.py::test_alpha", "phase": "call",
"status": "blocked", "why": "heartbeat 0.5s old but no CPU progress: the test thread is waiting on something",
"process_exists": true, "heartbeat_age_s": 0.5, "cpu_rate": 0.001, "rss_mb": 32,
"tests_finished": 51, "tests_running": 1, "tests_queued": 12, "tests_assigned": 64},
{"worker": "gw1", "pid": 21618, "nodeid": "test_slow.py::test_beta", "phase": "call",
"status": "gone", "why": "process 21618 no longer exists; last seen in call of test_slow.py::test_beta",
"process_exists": false,
"tests_finished": 12, "tests_running": 1, "tests_queued": 7, "tests_assigned": 20},
{"worker": "gw2", "pid": 21621, "nodeid": "test_slow.py::test_gamma", "phase": "call",
"status": "working", "why": "heartbeat 0.3s old, burning 1.00 cores", "cpu_rate": 1.0,
"tests_finished": 48, "tests_running": 1, "tests_queued": 11, "tests_assigned": 60}]}]}
?worker= narrows it to particular workers, which on a sixty-four-way run is
the difference between reading one state file and reading all of them. Both
spellings and both shapes work, and they mix:
$ curl 'localhost:8080/workers?worker=gw1'
$ curl 'localhost:8080/workers?worker=gw0,gw3'
$ curl 'localhost:8080/workers?worker=gw0&worker=gw2'
Runs left with no matching worker drop out, and names that matched nothing
anywhere come back under filter.unmatched — otherwise a caller cannot tell
"not running" from "misspelt". An empty ?worker= is treated as no filter,
because that is what a UI sends when its filter box is empty. The names are
compared against a directory listing and never joined onto one, so a value that
looks like a path is just a name that matches nothing.
A run with no workers is described the same way, under the name main. It is
the case where the two halves of the view coincide: the process serving is the
process running the tests, so controller.pid and the worker's pid are the same
number — and /stack for it is read exactly as any other pid is.
$ curl localhost:8080/workers
{"runs": [{"session": "run-8f21c0b4e5d7", "controller": {"pid": 4212, "alive": true},
"workers": [{"worker": "main", "pid": 4212, "nodeid": "test_pool.py::test_writes",
"phase": "call", "status": "blocked",
"why": "heartbeat 0.4s old but no CPU progress: the test thread is waiting on something"}]}]}
$ curl 'localhost:8080/stack?pid=4212'
{"pid": 4212, "source": "py-spy", ...}
The status vocabulary is analysis/stall.py's truth table, as
a live status rather than a post-hoc verdict:
| status | heartbeat | CPU | process |
|---|---|---|---|
working |
fresh | above 0.05 cores | exists |
blocked |
fresh | below that | exists |
frozen |
stale | — | exists |
gone |
— | — | absent |
unmeasured |
never any | — | — |
finished |
stopped with the session | — | idle until the run ends |
The last row is the one that is not a finding, and it exists because of what a
worker does when it runs out of work: it does not exit. xdist sends it
shutdown once the queue is empty, which ends its test loop and nothing else.
The worker runs its own session finish, reports workerfinished, and its
process then sits inside execnet — main thread parked in
integrate_as_primary_thread on an Event.wait — until the controller's
session finish tears every gateway down at once. Under --dist load that is
however long the slowest worker's remaining tests take. Its heartbeat stopped
with its session, so from the beats alone it is a live process that has not
beaten for a while, which is the frozen row and its wording about native code
holding the GIL. The worker writes worker_finish into its event log on the
way out, and that record outranks the beats — and outranks gone too, since
being closed at the end of the run is how a finished worker's process ends, not
a death.
Three files answer three different questions, and keeping them apart is what
makes this cheap and correct. .state says what a worker is doing and is
written before each phase runs, so it is ahead of anything the controller
knows — but a twenty-minute test writes nothing for twenty minutes, so a stale
record says nothing about liveness. .events carries a heartbeat every few
seconds whatever the test is doing, and the CPU time on each beat is the only
thing separating a worker that is working from one that is stuck. The pid
answers the narrowest question of the three, about a number that can be reused.
Two details that are easy to get wrong and are handled here:
- A killed worker is a zombie until its parent reaps it, and the kernel
accepts signals for it the whole time — so a
kill(pid, 0)check reports a worker killed a moment ago as alive, which is the opposite of what a crash view is for. Linux answers from procfs, which is cheaper than building a psutil object per worker per request; everywhere else psutil answers. - Liveness is a different mechanism per platform. Signal 0 is a POSIX question; on Windows it is not a question at all (see below), so the platform picks the mechanism before anything else happens.
cpu_rate: nullis not zero. "It burned nothing" and "we could not measure" are different findings, and a worker at full tilt whose beats collide produces the second.
Nothing here signals a worker or asks it anything: every verdict comes from beats already on disk, because asking a wedged process a question can dissolve the stall you were measuring.
nodeid and phase are null between tests, and a very long nodeid is
trimmed from both ends with nodeid_elided: true saying so.
How many tests each worker has
tests_started and tests_finished come from the worker's own state slot, and
on their own they are a numerator with no denominator: the one question anybody
watching a run actually has is is it nearly done, and nothing a worker writes
can answer it. No worker knows. It collects the whole suite and is then fed
indices a chunk at a time, so an empty queue and a pause look the same from
inside. The controller's scheduler holds what is outstanding per worker and
throws an index away the moment that test completes, so it cannot say how many
have been through either. The total exists only as the sum of the two, which is
why the controller works it out and writes it into the run's directory as
schedule.json — where /workers, the sample hook and anything else reading
the evidence pick it up like every other fact here.
| field | on | meaning |
|---|---|---|
tests_assigned |
worker | tests handed to this worker so far |
tests_finished |
worker | of those, how many it has run |
tests_running |
worker | the test in flight — 1, or 0 between tests |
tests_queued |
worker | the ones it has not begun |
collected |
run | tests in the run's whole collection |
unassigned |
run | tests that are nobody's yet — what every total can still grow by |
settled |
run | whether any worker's total can still change |
The three worker counts partition the total: finished + running + queued == assigned, always, with every test in exactly one of them. That shape is
deliberate. Reporting what was left instead read more naturally and was a
trap — "not finished" includes the test in flight, and so does tests_started,
so the two obvious numbers to add were the two that overlapped, and a row
saying started 2, pending 2, assigned 3 looked broken while being correct.
It is a running total, not a plan, and settled is how you tell. Under
--dist load and its relatives the scheduler keeps most of the suite in a queue
nobody is assigned yet and hands it out in chunks, so a worker's total grows for
as long as unassigned is above zero — a percentage drawn without it is a bar
whose end moves. Under --dist each it is settled from the first moment,
because every worker is given the whole collection at once. Under
--dist worksteal an empty queue is still not settled while a steal can
happen: that mode moves work between workers, so a total that has stopped
growing can still shrink. It takes both halves of xdist's own condition —
somebody idle to give the work to, and somebody holding more than the floor of
two to be worth taking it from — so one worker running the tail of a run is
not settled, while two workers holding two tests each are.
The three numbers in a row always agree, and that took arranging, because
they do not come from one place: the total is the controller's, written into
one file for the whole run, and tests_finished is the worker's own, written
into its own slot. Pairing a live read of one with a stale read of the other
produced rows saying a worker had finished nineteen of the fifteen tests it had
been given — not a lag a reader can interpret, but a row that cannot be true.
Two things stop it. The controller's record is rewritten whenever a test
starts rather than on a timer, so it is never more than one test behind; and
only the total comes from it — the split is measured from the worker's own
counts, and the total is floored at tests_started, because a worker cannot
begin a test it was never given either. A stale total can then only understate
the queue, never contradict the line above it.
What can still move is tests_assigned, by one, for an instant. A worker tells
the controller a test is finished before it tells the scheduler, so in
between it is counted as run and still outstanding both. Writing from the start
of a test rather than the end of one keeps that worker out of its own window;
what is left is the rarer case of another worker's two messages straddling this
one's, and it lasts until the next test starts somewhere.
A crashed worker keeps its row, so the rows can add up to more than the run.
xdist drops a dead worker's queue back into the global one and starts a
replacement under a new id, and the test it died in is reported failed rather
than reassigned. The dead worker's row stays as it was — tests_assigned: 5, tests_finished: 3, tests_running: 1, tests_queued: 1 is "it was given five,
ran three, died inside the fourth and never started the fifth", which is the
line a death is triaged with, and the replacement gets a row of its own rather
than overwriting it. What that costs is that the tests it was given and
somebody else then ran are counted in both rows. So collected is the run's
size and summing tests_assigned is not; status: gone is what marks a row as
a record of a process rather than a report on one.
Nothing is written for a run with no scheduler to ask — a single-process run,
or a distributed one whose workers have not collected yet. Those fields come
back null, which is not zero: zero pending is a worker about to finish, and
not knowing is not.
When there is no live view
Switching the server on and getting no server raises a
stack_server_unavailable incident through the same hook as everything else.
Without it the run continues perfectly well and your UI shows nothing forever
with no error anywhere — because from the outside "no server" and "no tests
running" look identical, which is the exact misreading this package exists to
prevent:
[stack_server_unavailable] PORT_TAKEN severity=informational owner=runtime
no live stacks this run: 127.0.0.1 could not serve on port 8080
port 8080 is held by something that is not a stack server (Address already in use);
pass --callstack-port with an unused port, or leave it off entirely and let one be drawn
· the run itself is unaffected; what is missing is the live view
Two verdicts, because they have different remedies. PORT_TAKEN is a stranger
on the port — name another one. BIND_REFUSED is an address that is not an
interface on this machine, or a sandbox that forbids listening — naming another
port does not help.
Neither is raised when another of your own sessions holds the port: that is the shared mode working as designed, and alerting on the ordinary case is how a kind gets filtered out entirely. It is reported once per address per run, not once per retry, though a named port held by a stranger is re-probed for the life of the run.
owner=runtime, severity=informational: no test is at fault and nothing is
broken. What is lost is a diagnostic, and somebody has to decide whether to
reconfigure it.
Who may ask
Four things, and the token is only one of them.
The bind. Loopback by default, and anything else refuses to open without a token — see below.
The Host header. A request naming a host this server never bound is
refused with 403. That is not about the network, which the bind already
settles; it is about a browser. A page you visit can re-resolve its own
hostname to 127.0.0.1, at which point its origin is this server's and the
same-origin policy stops protecting you. Checking Host costs nothing and
closes that. A bind that is not loopback is exempt, because the address a
legitimate client outside a container uses is one this process never learns —
there the token is what stands in for the check.
Which pids /stack will answer for. This run's: the serving process, plus
the worker pids read out of the evidence directory. Anything else is 403. The
server reads any process it has permission to read, so without this a caller
who got past the bind could walk pids and collect the stack of every process
you own — and each read pauses its target.
A token, if you supplied one. On loopback you usually will not:
$ curl localhost:8080/workers
With a token, which is what any bind but loopback requires:
$ export PYTEST_CALLSTACK_TOKEN=$(openssl rand -hex 16)
$ pytest -n8 --callstack-host 0.0.0.0 &
$ curl -H "Authorization: Bearer $PYTEST_CALLSTACK_TOKEN" host:8080/workers
$ curl "host:8080/workers?token=$PYTEST_CALLSTACK_TOKEN" # for a hurry
The token is supplied, never minted, and never written to disk. That is the whole design, and it comes from the two halves of the problem being opposites.
The port has to be published. A port drawn at random is unguessable by construction — that is the point of drawing it — so the run must write it down for anything outside to find it.
The token does not. It is the one value both ends can agree on in advance,
because whoever starts the run picks it. Minting one here made it discoverable
instead, which meant writing it into the address file — and that turned every
question about where a run may write its evidence into a question about where a
secret may live. POSIX answers that with an 0o600. Windows does not answer
it at all: a mode there is not an ACL, so the file inherits the evidence
directory's and the promise quietly stops holding on a supported platform.
So the address file is ordinary data — a host, a port and a pid, the address of
a server anyone who can reach it may query anyway. Put failure_directory
wherever evidence goes, on any platform. And the secret arrives the way secrets
already reach a container, a CI job and a shell:
PYTEST_CALLSTACK_TOKEN |
a shell, a CI job, docker run -e — prefer this |
--callstack-token SECRET |
one run, at the cost below |
| (nothing) | no authentication — the default, and right on loopback |
There is no ini setting, deliberately: ini files live in the repository.
The two are not equally private. --callstack-token puts the secret in the
controller's command line, and a command line is public on a shared machine:
/proc/<pid>/cmdline is world-readable on Linux, so any other account can take
the token out of ps -eww for as long as the run lasts — on exactly the
machine a token is worth having. Shell history and an echoed CI command keep it
after the run has ended, too. PYTEST_CALLSTACK_TOKEN reaches the same setting
by the same path and has none of that: /proc/<pid>/environ is 0400, the
owner alone. The flag still works — runs use it, and it is unobjectionable on a
machine with one user on it — and a run that uses it warns once, saying this.
No token is the default and the right one on loopback, where the bind already bounds the reachable set to processes on this machine. On a box you share with people you would not hand a debugger to, supply one or leave the server off — "only local" and "only you" are different statements, and without a token only the first is being made.
Off loopback without a token is refused, before the socket is opened, and
reported as a stack_server_unavailable incident. Serving every local
process's stack to whatever can route to the host is not something anybody
configures on purpose, and a warning is the wrong instrument for it: by the
time one is read the port has been open for the length of the run.
/identity stays open even with a token set: it is what one session asks
another before standing down from a contested port, and two sessions that
minted nothing have no way to share a credential. It answers with a service
name, a version and a pid.
Finding the server
The run tells you, on a hook, the moment it is serving:
def pytest_failure_server_ready(server):
registry.upsert(
session=server.session_id, # names this run's evidence directory
url=server.url, # already bracketed if the host is IPv6
port=server.port, # what got bound, never the 0 you asked for
token=server.token, # what you supplied, or "" if you did not
pid=server.pid, # the controller, not any worker
)
That is the whole address, and for a drawn port it is the only way to learn it
before the run is over — nobody can configure a number that did not exist a
moment ago. server is a LiveStackServer; server.headers() gives you the
Authorization header the endpoints want — {} when this run supplied no
token, so the same client code works either way — and server.endpoint("/workers")
joins the URL, so neither the scheme nor the slash is yours to get right.
The hook fires on a thread of its own once the server is already accepting, so it is free to call straight back into the server it was just handed. It does not fire at all when the server was never switched on, nor when this session stood down because another of ours already holds a named port — that session announced itself, and one server should not be stored twice.
No run id in the payload. At the moment the server binds, xdist has usually
not built its node manager, so this run's real id does not exist yet; stamping
the placeholder onto a row you will join against later is a key that silently
matches nothing. session_id is stable from the first moment, and /workers
reports the run id per directory as soon as a worker beats.
If you would rather poll the filesystem than implement a hook, the address is
also on disk. A drawn port is written to callstack-<pid>.json in this run's
evidence directory (see the layout below), one file per serving session, and
removed when that session ends. Files left by a
session that was killed are swept by whoever publishes next — by checking the pid
in the filename, so a live session's address is never deleted.
for address in Path(".pytest-failures").glob("*/callstack-*.json"):
run = address.parent # one directory per run
server = json.loads(address.read_text())["url"]
for state in run.glob("*.state"):
record = json.loads(state.read_bytes().rstrip(b"\x00").strip())
stack = requests.get(f"{server}/stack?pid={record['pid']}").json()
print(run.name, record["nodeid"], record["phase"], stack["threads"][0]["frames"][0])
test_pool.py::test_concurrent_writes call {'function': '_wait_for_lease', ...}
Pushing samples instead of polling for them
/workers and /stack are a pull: something outside asks, when it wants to
know. Where a dashboard can reach the run, that is the better route — it reports
more per worker than a sample does, at whatever cadence it chooses, and costs
nothing at all while nobody is watching.
What it needs is a listening socket, and there are runs that cannot have one: a
CI job forbidden to open a port, a container with nothing routed into it, a run
too short-lived for anything to discover and poll before it is over.
failure_sample_seconds turns the same information around and pushes it out of
the process instead, with no port and nothing to discover:
def pytest_failure_worker_sample(sample):
# collected / unassigned / settled ride on the sample too: a worker's
# total is what it has been given so far, so a bar drawn without them
# has a moving end — and this is the path for runs that cannot open a
# port, where there is no /workers to ask instead.
for worker in sample.workers:
rows.insert(session=sample.session_id, at=sample.observed_at,
worker=worker.worker, nodeid=worker.nodeid,
phase=worker.phase, status=worker.status, why=worker.why,
rss_mb=worker.rss_mb, cpu_rate=worker.cpu_rate,
assigned=worker.tests_assigned, done=worker.tests_finished,
queued=worker.tests_queued)
A run with no workers pushes one row per pass, for main, from the same files.
Off by default. It is the only hook here that fires when nothing is wrong, so it
is the only one with a running cost — and that cost is a directory walk: every
field above comes from the .state and .events files the run was writing
anyway, and nothing is asked of a worker itself. No ptrace, no subprocess, no
pause. A sample of sixty-four workers is a few kilobytes of statuses.
No frames, deliberately. Reading a stack per stuck worker per pass was tried
here and taken out again: blocked is the status of any worker under 0.05
cores, so on an I/O-bound suite every healthy worker waiting on a database
qualified, and each pass paid a subprocess and a pause for each of them. Frames
are worth that when a human is asking about one worker — /stack?pid=, on
demand — rather than for every stuck worker on a timer. session_id and the
worker's pid are what join a sample to a stack fetched that way.
Containers
--callstack-host 0.0.0.0 is what a container needs: its UI is outside, and
127.0.0.1 inside a container is unreachable from there. That bind requires a
token and is refused without one — see Who may ask — which
suits a container better than the alternative did:
$ docker run -e PYTEST_CALLSTACK_TOKEN -p 8080:8080 yourimage \
pytest -n8 --callstack-host 0.0.0.0 --callstack-port 8080
The UI outside reads the same value from the same place. Nothing has to be mounted out for it to find a secret in a file, which is what a minted token would have required.
Two things about containers make this easier than it looks:
- Each container has its own network namespace, so
8080inside one pod is not8080inside another. The port contention that the named mode exists to resolve does not arise between pods at all — it is a bare-metal and laptop problem. Name a port in a container, publish it, and every pod can use the same number. - Each container has its own PID namespace, so a server in one pod could not read another pod's workers even with every permission granted. Sharing is neither possible nor needed there.
Reading a process needs ptrace, which modern Docker permits under its default seccomp profile. Where it is refused, the endpoint says so and names the fix rather than returning nothing:
{"pid": 48213, "source": "py-spy", "error": "Operation not permitted (os error 1)
- ptrace is not permitted: check /proc/sys/kernel/yama/ptrace_scope (0 or 1 allows
this; 1 requires the target to be a descendant of the reader, which xdist workers
are), and add --cap-add=SYS_PTRACE if this is a container"}
ptrace_scope is a host-wide sysctl and is not namespaced, so a container
inherits the node's value and cannot change it. At ptrace_scope=1 the
tracer must be an ancestor of what it reads — and the tracer is not the
controller but py-spy, which the controller spawns. py-spy and a worker are
both children of the controller, so they are siblings, and a sibling is not an
ancestor.
A worker therefore nominates its parent as a permitted tracer at startup, via
prctl(PR_SET_PTRACER, <controller pid>) — the exception Yama provides for
exactly this. worker_start records whether it was granted, so a refused read
has an answer beside it rather than only a message.
Yama admits the nominated pid and every descendant of it, so this is wider than "the controller's py-spy may read this worker". The controller's descendants are the whole process tree of the run: every other worker, and any subprocess a test spawns while the declaration stands. The reader it exists for is one of them and is not the only one.
So the declaration is only made where something is going to read a worker's
stack — the live stack server, or the sampler (failure_sample_seconds) —
and is off on every run where neither is on, which is most runs. The
controller resolves that, being the only process that can see either, and hands
each worker the answer; a worker never judges it for itself. failure_tracer
says which declaration such a run makes, not that one is made.
A named port shared across sessions still reads only the workers of the
session hosting it: another session's workers nominated their controller, not
this one. failure_tracer = any is what lifts that — it drops the relationship
requirement entirely, so any reader on the machine that could already ptrace is
permitted. That is the setting a shared server needs and the one a private run
does not, which is why it is not the default.
Reading a live process needs py-spy
pip install pytest-failure-instrumentation[stacks]. There is no way to walk
another process's frames from Python, so a live stack is read from outside the
target: py-spy reads its memory rather than asking it to run anything, and
stops it before reading, so it never walks a frame that is being torn down.
That is what makes it work on a worker whose GIL is held by native code, and it
is why nothing here has to be signalled.
Every pid is read that way, including the process doing the reading. Its
own frames are also directly to hand — sys._current_frames(), no subprocess
and no permission — and answering from them was a second reader with a second
source for the one process that could avoid the first. A caller that has to
know which mechanism answered has been handed two APIs, and a UI is where that
ends up encoded. One reader, one shape, one thing to keep working.
Reading yourself is a child tracing its parent, which is the wrong direction
for Yama. The declaration that admits it names this process — so it permits
this run's own descendants and nothing else, narrower than the parent policy
above — and it is made at the moment of the read rather than at startup, so
only the runs that read a stack make it.
Without py-spy the endpoint still answers, with the reason instead of a stack. A UI that is told why it has no stack can tell a dead process from a missing permission; one that gets an empty response cannot. The same is true of a stall: the verdict comes from heartbeats rather than frames, so it is reached either way, and the stack falls back to whatever the watchdog last wrote with its age attached.
On Windows there is no ptrace and no equivalent restriction: any process can
read another running as the same user at the same integrity level, so the
descendant rule above simply does not apply. Reading an elevated process from
an unelevated one needs SeDebugPrivilege.
One directory per run
.pytest-failures/
run-70a514cc7a93/ <- this pytest process's own name for itself
owner.json <- the controller's pid, and the only thing that makes
gw0.state this directory ours to delete
gw0.events <- every line carries the *reported* run id
gw1.state
schedule.json <- how many tests each worker has been given, which is
callstack-4213.json the one fact no worker can write about itself
A run with no workers writes the same files under main, since it is the
process running the tests:
.pytest-failures/
run-8f21c0b4e5d7/
owner.json
main.state
main.events
Runs used to share a flat directory and name their files after the worker,
which works exactly until two runs happen at once — and on a laptop or a
bare-metal runner that is the ordinary case. Every worker is gw0, so the
second run's gw0.state is the first run's gw0.state: one run reads the
other's evidence, believes it, and attributes a stall to a test a different run
is running. The old start-of-run cleanup made it worse rather than better,
because it deleted the files of a run still using them.
A directory per run removes the class of bug rather than a symptom. Nothing inside is named for the run, because the directory already is, so every path a reader builds is unchanged.
Why the directory is not named after the run id you see on incidents. The
obvious name is xdist's own run id, and it cannot be used: it does not exist
until xdist has built its node manager, and there is no hook order that
reliably puts that first. trylast does not do it, because xdist's own session
start is also trylast — so which of the two runs first comes down to which
plugin registered first, and that differs between installing from the entry
point and installing from a framework's pytest_configure. The directory is
therefore named by something this process fixes for itself and nothing can
reorder. The reported run id still prefers xdist's, so incidents still line up
with xdist's logs, and every .events line inside carries it — which is how a
directory is matched back to a run.
PYTEST_RUN_ID names the directory if you set it, which is also the way to
make two runs deliberately share one. The value has to be a name rather
than a path: 1–128 characters of letters, digits, ., - and _, and
neither . nor ... Anything else is refused with a warning and the run
names itself — so a slugified branch works and a raw feature/x does not,
because a separator in there would put this run's evidence somewhere other
than failure_directory.
What gets cleaned up, and what is read first. Whole directories of runs
that are over — not old — and never before they have been asked whether they
have anything left to report.
The controller's pid is in owner.json, so a run still going is recognisable
as such however long it has been going, which matters precisely because several
run at once. A directory without that marker is not ours and is left alone
whatever it looks like, which includes the flat files an older version of this
plugin left behind: they cannot be mistaken for a current run's evidence,
because a current run does not look there for any.
Cost
A passing test must cost as close to nothing as possible, because that is the overwhelming majority of what runs.
- Per test: six fixed-size writes to a file that never grows — two per phase,
one as it opens and one as it closes, which is what separates "died in
teardown" from "died mid-call" — plus two clock reads. No append log, no
/procread, no allocation tracking. - Per test that outlives
failure_slow_test_seconds(measured setup through teardown): one ~5 KB stack dump every interval, written and renamed by the heartbeat thread, and an unlink when the test ends. Nothing accumulates across tests, and nothing is written for a test that finishes in time. - Per 5 seconds, per worker: one heartbeat carrying CPU time and resident memory. Per second, per worker: two deadline comparisons and a timer rearm, which is why the first stack of a wedged test does not wait for a beat.
- Per test, on the controller only:
schedule.jsonrewritten, which is a length read off the scheduler per worker and one small write at a fixed offset — 6µs at eight workers, 32µs at sixty-four. It was written by rename at first, at 54µs a time, and throttled to twice a second to pay for that; the throttle is what let a worker's row contradict itself, so the write got cheap instead. Nothing here scales with the number of tests: diffing a queue against its last reading would, and so would reading the loadscope family's per-test done flags — that one was measured at 1.7ms a write on a sixty-thousand-test--dist loadfilerun, near two minutes of controller time over the run, and is now a length per scope instead (133µs, 8s). - Off by default:
tracemalloc(needed to attribute an OOM kill to a source line) and the live-object census — walking the heap on a worker near its ceiling is exactly the instrumentation that makes things worse. - The live stack server, when switched on: one thread per session, which either serves or retries the claim every five seconds. Nothing is sampled and nothing is written unless something asks.
- pydantic is imported on the controller, and only when xdist is active. A worker never loads it, so nothing about the per-test path changed when the payload became typed.
- Nothing in the reporting path may raise. A failure while gathering an
incident degrades it to what survived, because an exception in a reporting
hook becomes an
INTERNALERRORthat ends the customer's run.
Settings
| Setting | Default | Purpose |
|---|---|---|
failure_packages |
— | Your top-level packages, for attribution |
failure_directory |
.pytest-failures |
Where evidence is written; each run gets a subdirectory under it |
failure_product_version |
— | Version recorded on every incident, for telling which build a failure came from |
failure_watchdog |
true |
Memory and liveness sampling |
failure_heartbeat_interval |
5.0 |
Seconds between liveness beats (floor 1.0) |
failure_tracemalloc_depth |
0 |
1 names the allocating line for OOM attribution |
failure_object_census |
false |
Count live objects at a high-water mark |
failure_high_water_mb |
auto | Memory mark for a snapshot; defaults to a share of the discovered limit |
failure_memory_limit_mb |
0 |
Soft cap (POSIX) turning an OOM kill into a MemoryError |
failure_slow_test_seconds |
20 |
How often a running test refreshes its stack (setup through teardown; needs failure_watchdog) |
failure_stall_seconds |
300 |
Silence before a stall is assessed |
failure_stack_probe |
true |
Ask a diagnosed stalled worker for a fresh stack (POSIX) |
failure_crash_stack |
false |
Keep the fatal stack of a run that has no workers, instead of leaving it on the stderr pytest points it at. A worker keeps its own either way |
failure_tracer |
parent |
Who may read a worker on Linux under Yama: parent, any, off. Declared only when the stack server is on — a run with no reader declares nothing whatever this says |
failure_sample_seconds |
0 |
Push a worker sample this often while the run is going. 0 is off |
failure_stack_server |
false |
Serve live stacks over HTTP |
failure_stack_server_port |
0 |
0 draws a free port and writes it down; any other is claimed and shared (--callstack-port) |
failure_stack_server_host |
127.0.0.1 |
What it binds; 0.0.0.0 for a container (--callstack-host) |
failure_stack_server_locals |
true |
Whether /stack?locals answers with each frame's variables |
There is deliberately no ini setting for the token. It comes from
PYTEST_CALLSTACK_TOKEN or --callstack-token and nowhere else: an ini file
lives in the repository, and a credential in the repository is the thing this
design exists to avoid. Prefer the environment variable — a token on the
command line is readable by every other user of the machine. See
Who may ask.
failure_slow_test_seconds and failure_stall_seconds are not independent.
The stack a stalled worker is reported with is whatever the watchdog last
wrote, so the cadence has to have fired before the stall is assessed — a stall
judged sooner is judged with no stack at all, and on Windows that is every
stall. Neither is clamped, but an inverted pair warns.
failure_crash_stack is a trade rather than a switch, and it is off because of
which way the trade goes by default. faulthandler keeps exactly one
destination for a fatal signal, and pytest's own plugin has already pointed it
at stderr. A worker takes it and loses nothing — that stderr is shared with
fifteen other workers and a dump written into it belongs to nobody. A run with
no workers would be taking the crash out of a terminal somebody is watching, in
exchange for one that cannot be reported until a later run reads the file.
There is no having both: faulthandler.register(SIGSEGV, chain=True) is
refused by CPython itself.
What is lost by leaving it off is the stack, not the report — see the
recovered incident above, which still names the test, the phase, the counters
and the memory, and still offers a suspect_owner. What it cannot carry is a
blamed frame. Turn it on where the incident is the artefact that gets read and
the terminal is not, which is most of CI.
failure_memory_limit_mb is worth a note: an RLIMIT_AS cap makes the
allocation fail inside the process, so you get a MemoryError with a
traceback and a node id instead of an uncatchable kill with neither. It costs
you a hard ceiling per worker, which is why it is opt-in.
failure_directory is safe to share between runs going at once, and it has to
be: worker ids start at gw0 in every run, so a flat directory would have two
sessions writing the same file names. Each run gets a subdirectory of its own
named for its session, holding an owner marker with the pid that made it.
Cleanup prunes whole directories whose owner is no longer running rather than
a list of file suffixes, so a live run's evidence is never what a starting run
deletes, and a coverage report that happens to live there is never touched at
all.
Two things back that up rather than repeating it. Every record carries the run that wrote it, and a reader refuses one naming a different run — so even a directory that somehow got crossed yields missing evidence rather than another run's attributed to yours. And a run that finds a live session already owning its directory says so.
Platform coverage
| Capability | Linux | macOS | Windows |
|---|---|---|---|
| Test in flight, phase, exit status | yes | yes | yes |
| Crash stack | yes | yes | yes |
| Stack from a slow or hung test | yes | yes | yes |
| Current memory | procfs | psutil | psapi |
| Container limit, OOM counter | yes | n/a | n/a — no OOM killer |
| On-demand stack from a stalled worker | yes | yes | no |
| Live stack of a running process (needs py-spy) | yes | root only | yes |
| Stack from a worker that stopped running Python | yes | yes | yes |
The last row is the frozen-interpreter fallback, and it is the one capability a
setting takes away on every platform rather than a platform taking away: where
faulthandler_timeout is set, pytest owns the single
dump_traceback_later timer it would arm, and it stands down rather than
cancel a timeout somebody configured. On Windows that leaves a worker frozen in
native code with no stack at all, since the on-demand probe is not available
there either — which is why the worker records frozen_fallback_stood_down in
its event log rather than leaving the absence to be guessed at.
Two Windows differences are worth knowing about, because they change what you will see rather than how it is reported.
ctypes wraps every foreign function call in structured exception handling, so
an access violation raised through ctypes comes back as an OSError and the
worker survives it. A fault inside a real C extension still ends the process —
but the reproduction that segfaults a worker on Linux may simply fail a test on
Windows.
And a Windows process that dies from a fault reports an NTSTATUS as its exit
code rather than a signal, while abort() reports plain 3 — the same code a
deliberate os._exit(3) gives. What separates a crash from a clean exit there
is whether a dump was written, not the exit status, which is why the crash
stack is evidence in its own right rather than a decoration on the verdict.
psutil is a dependency, imported like any other. It is the only
cross-platform way to ask whether a process is still there, and the POSIX way
is actively dangerous on Windows: os.kill(pid, 0) sends a console event only
for CTRL_C_EVENT and CTRL_BREAK_EVENT, and calls TerminateProcess for
every other value — including zero. A liveness check written the obvious way
would kill each worker it inspected, and the live view inspects every worker on
every request. psutil also carries the memory figures on macOS and Windows,
which procfs cannot.
Tests
pip install -e ".[test]"
pytest
The integration tests run a real pytest in a subprocess through pytester,
crash or wedge a worker for real, and read back what the plugin raised — so
they exercise the mechanism rather than a mock of it. Every one of them also
round-trips its incidents through registry.parse and asserts model_dump()
equals the stored row, which makes the payload contract a property of every
scenario rather than a test of its own.
CI runs the suite on Linux, macOS and Windows across Python 3.9–3.13 — every
platform path in the table above is executed on the platform it was written
for. The
probes are platform code — procfs, psapi, waitid, GetExitCodeProcess,
cgroup counters — and none of the Windows or macOS paths can be exercised on a
Linux runner, which is the whole reason the matrix exists. Two axes matter as
much as the operating system, so each gets its own job:
- without
pytest-xdist, wherepytest_testnodedownhas no hookspec at all and an unspecced hookimpl is a registration error — the failure mode that once made a plainpytestrun report nothing. - against the declared minimums,
pytest==7.0.1on Python 3.9. Every other job installs whatever is newest, so a hook signature or an ini type that arrived later would pass all of them and fail on a user's pinned pytest.
ruff and mypy run as their own job, and fail first because they are cheap.
The source carries # noqa and # type: ignore markers, which are only worth
writing if something reads them.
Two of the tests are about the plugin rather than about a failure: a run whose evidence directory cannot be created has to keep running, and a directory shared with somebody else's artifacts has to come out of a run with those artifacts still in it. A reporting tool that ends a run, or eats a file, has cost more than the failure it came to explain.
Releasing
Tag the commit and the rest runs itself:
git tag v0.2.0 && git push origin v0.2.0
The tag is the only input. .github/workflows/release.yml builds the sdist and
wheel, refuses to continue if the tag disagrees with the version in
pyproject.toml, installs the built wheel on Linux, macOS and Windows and
runs the whole suite against it, publishes to PyPI, and then creates the GitHub
release with the artifacts attached.
The wheel is tested rather than the checkout because this plugin is one entry
point. If packaging drops it the import still succeeds, the suite still passes,
and nothing is instrumented at all — the one failure mode a green test run
cannot rule out. So the release explicitly asserts the entry point exists and
that the package under test came from site-packages.
Credentials
There is no API token to create and no secret to add to the repository.
Publishing uses trusted publishing:
PyPI verifies this workflow's OIDC identity at upload time, so nothing
long-lived exists to leak or rotate. GITHUB_TOKEN is supplied by Actions
automatically.
What it does need is configuration, once, on each side.
On PyPI — Your account → Publishing. The project does not exist there yet, so this is an "Add a new pending publisher", not a setting on an existing project; a pending publisher is how a first upload is authorised for a name nobody has claimed. It becomes a normal publisher after that first release.
| Field | Value |
|---|---|
| PyPI project name | pytest-failure-instrumentation |
| Owner | Heknon |
| Repository name | pytest-failure-instrumentation |
| Workflow name | release.yml |
| Environment name | pypi |
On GitHub — Settings → Environments → New environment, named pypi.
Under it, tick Required reviewers and add yourself. That is the manual gate:
the run pauses before anything reaches PyPI, shows you the tag it is about to
publish, and waits. Nothing is uploaded until someone approves, and waiting does
not consume the job's timeout.
Worth setting at the same time, under Deployment branches and tags: restrict
the environment to the tag pattern v*, so the only thing that can ever reach
PyPI is a tagged commit.
TestPyPI is a separate site with a separate account, so rehearsing needs its
own pending publisher at test.pypi.org with the environment named testpypi.
Leave that environment without reviewers — the point of a rehearsal is that it
does not need one.
Licence
MIT — see LICENSE. Declared as an SPDX expression under PEP 639 rather than a classifier, since PyPI rejects a distribution carrying both.
Status
All five kinds and every verdict in the tables above are covered, on all three platforms, with and without xdist — a run with no workers records, serves, watches and is recovered through the same code paths a distributed one uses, and the tests drive it the same way: a real run, wedged or killed for real, read back through the hook.
Most are produced for real: a worker is crashed, killed, signalled, wedged or
made to disagree about its collection, and the incident is read back from the
hook. Two cannot be, by anyone: OOM_KILLED needs a kernel that has just
killed something, and UNKNOWN needs a remote gateway with no local process to
query. Those branches are exercised against a constructed incident instead — as
are the Windows NTSTATUS decodes, which additionally run against a process that
really exits with one.
The opt-in paths are covered too: the memory ceiling turning an uncatchable
kill into a MemoryError that names the test, and the high-water snapshot
naming the line holding the memory.
The probes are also called directly, because in normal use they shadow each
other — psutil answers before psapi, and execnet's Popen before waitid — so
the fallbacks a customer's machine actually runs were never being executed.
That includes the claim WNOWAIT rests on: the status is read, and the process
is still reapable afterwards with the same answer.
The first cross-platform run paid for itself twice. It found that a Windows
\Lib\ in sysconfig and a \lib\ in a traceback made every stdlib frame
look like nobody's code, so a blocked test was blamed on threading.py and
then on the customer who called it — a runtime frame reported as customer code,
which is the one direction this must never fail in. Only the 3.9 cell caught
it. And it found that ctypes cannot raise an uncaught fault on Windows at all,
which is a fact about what users will see rather than about the plugin.
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