Frontrun

A library for deterministic concurrency testing.

pip install frontrun

Overview

Frontrun is named after the insider trading crime where someone uses insider information to make a timed trade for maximum profit. The principle is the same here, except you use insider information about event ordering for maximum concurrency bugs.

The core problem: race conditions are hard to test because they depend on timing. A test that passes 95% of the time is worse than a test that always fails, because it breeds false confidence. Frontrun replaces timing-dependent thread interleaving with deterministic scheduling, so race conditions either always happen or never happen.

Four approaches, in order of decreasing interpretability:

DPOR — Systematically explores every meaningfully different interleaving. When it finds a race, it tells you exactly which shared-memory accesses conflicted and in what order. Powered by a Rust engine using vector clocks to prune redundant orderings.
Bytecode exploration — Generates random opcode-level schedules and checks an invariant under each one. Often finds races very efficiently (sometimes on the first attempt), and can catch races that are invisible to DPOR (e.g. shared state inside C extensions). The trade-off: error traces show what happened but not why — you get the interleaving that broke the invariant, not a causal explanation.
Marker schedule exploration — Exhaustive exploration of all interleavings at the # frontrun: marker level. Much smaller search space than bytecode exploration, with completeness guarantees.
Trace markers — Comment-based synchronization points (# frontrun: marker_name) that let you force a specific execution order. Useful when you already know the race window and want to reproduce it deterministically in a test.

All four have async variants. A C-level LD_PRELOAD library intercepts libc I/O for database drivers and other opaque extensions.

DPOR deadlock detection (dining philosophers)

DPOR explores thread interleavings and detects deadlocks via wait-for-graph cycle analysis. Here it finds the circular wait in the classic 3-philosopher dining problem:

Deadlock diagram showing DPOR exploration of the dining philosophers problem. Three threads each acquire one fork (lock) then block waiting for the next, forming a cycle.

The timeline shows each thread's lock acquisitions (green), context switches (pink arrows), and the point where the deadlock is detected. Run make screenshot to regenerate this image from examples/dpor_dining_philosophers.py.

Quick Start: Bank Account Race Condition

A pytest test that uses trace markers to trigger a lost-update race:

from frontrun.common import Schedule, Step
from frontrun.trace_markers import TraceExecutor

class BankAccount:
    def __init__(self, balance=0):
        self.balance = balance

    def transfer(self, amount):
        current = self.balance  # frontrun: read_balance
        new_balance = current + amount
        self.balance = new_balance  # frontrun: write_balance

def test_transfer_lost_update():
    account = BankAccount(balance=100)

    # Both threads read before either writes
    schedule = Schedule([
        Step("thread1", "read_balance"),    # T1 reads 100
        Step("thread2", "read_balance"),    # T2 reads 100 (both see same value!)
        Step("thread1", "write_balance"),   # T1 writes 150
        Step("thread2", "write_balance"),   # T2 writes 150 (overwrites T1's update!)
    ])

    executor = TraceExecutor(schedule)
    executor.run({
        "thread1": lambda: account.transfer(50),
        "thread2": lambda: account.transfer(50),
    }, timeout=5.0)

    # One update was lost: balance is 150, not 200
    assert account.balance == 150

Case Studies

46 concurrency bugs found across 12 libraries by running bytecode exploration directly against unmodified library code: TPool, threadpoolctl, cachetools, PyDispatcher, pydis, pybreaker, urllib3, SQLAlchemy, amqtt, pykka, and tenacity. See detailed case studies.

Usage Approaches

1. Trace Markers

Trace markers are special comments (# frontrun: <marker-name>) that mark synchronization points in multithreaded or async code. A sys.settrace callback pauses each thread at its markers and waits for a schedule to grant the next execution turn. This gives deterministic control over execution order without modifying code semantics — markers are just comments.

A marker gates the code that follows it: the thread pauses at the marker and only executes the gated code after the scheduler says so. Name markers after the operation they gate (e.g. read_value, write_balance) rather than with temporal prefixes like before_ or after_.

from frontrun.common import Schedule, Step
from frontrun.trace_markers import TraceExecutor

class Counter:
    def __init__(self):
        self.value = 0

    def increment(self):
        temp = self.value  # frontrun: read_value
        temp += 1
        self.value = temp  # frontrun: write_value

def test_counter_lost_update():
    counter = Counter()

    schedule = Schedule([
        Step("thread1", "read_value"),
        Step("thread2", "read_value"),
        Step("thread1", "write_value"),
        Step("thread2", "write_value"),
    ])

    executor = TraceExecutor(schedule)
    executor.run({
        "thread1": counter.increment,
        "thread2": counter.increment,
    }, timeout=5.0)

    assert counter.value == 1  # One increment lost

2. DPOR (Systematic Exploration)

DPOR (Dynamic Partial Order Reduction) systematically explores every meaningfully different thread interleaving. It automatically detects shared-memory accesses at the bytecode level — attribute reads/writes, subscript accesses, lock operations — and uses vector clocks to determine which orderings are equivalent. Two interleavings that differ only in the order of independent operations (two reads of different objects, say) produce the same outcome, so DPOR runs only one representative from each equivalence class.

When a race is found, the error trace shows the exact sequence of conflicting accesses and which threads were involved:

import frontrun

class Counter:
    def __init__(self):
        self.value = 0

    def increment(self):
        temp = self.value
        self.value = temp + 1

def test_counter_is_atomic():
    result = frontrun.explore(
        setup=Counter,
        workers=Counter.increment,
        count=2,
        invariant=lambda c: c.value == 2,
    )
    result.assert_holds()

This test fails because Counter.increment is not atomic. The result.explanation shows the conflict:

Race condition found after 2 interleavings.

  Write-write conflict: threads 0 and 1 both wrote to value.

  Thread 0 | counter.py:7             temp = self.value
           | [read Counter.value]
  Thread 0 | counter.py:8             self.value = temp + 1
           | [write Counter.value]
  Thread 1 | counter.py:7             temp = self.value
           | [read Counter.value]
  Thread 1 | counter.py:8             self.value = temp + 1
           | [write Counter.value]

  Reproduced 10/10 times (100%)

DPOR explored exactly 2 interleavings out of the 6 possible (the other 4 are equivalent to one of the first two). For a detailed walkthrough of how this works, see the DPOR algorithm documentation.

Search strategies: The default DFS strategy is optimal for exhaustive exploration (stop_on_first=False) — it produces the minimum number of executions. When the trace space is very large and you have a limited execution budget (stop_on_first=True or a low max_executions), use a non-DFS strategy like search="bit-reversal" to spread exploration across diverse conflict points early, finding bugs faster on average. See search strategy documentation for details.

Scope and limitations: DPOR tracks Python bytecode-level conflicts (attribute and subscript reads/writes, lock operations) plus I/O. Redis key-level conflicts are detected by intercepting redis-py's execute_command(); activate with detect_io=True (works in both sync and async from 0.5). SQL conflicts are detected by intercepting DBAPI cursor.execute(). These key/table-level detectors are important: raw socket detection uses host:port as the resource ID, so every send and recv to the same server appears to conflict — without key-level or SQL-level refinement this causes a combinatorial explosion of spurious interleavings. C-extension shared state (NumPy arrays, etc.) is not tracked at all. The frontrun CLI adds C-level socket interception via LD_PRELOAD for opaque drivers, also at the coarse host:port level.

3. Bytecode Exploration (Random Strategy)

Bytecode exploration generates random opcode-level schedules and checks an invariant under each one, in the style of Hypothesis. Each thread fires a sys.settrace callback at every bytecode instruction, pausing to wait for its scheduler turn. No markers or annotations needed.

The random strategy often finds races very quickly — sometimes on the first attempt. It can also find races that are invisible to DPOR, because it doesn't need to understand why a schedule is bad; it just checks whether the invariant holds after the threads finish. If a C extension mutates shared state in a way that breaks your invariant, random exploration will stumble into it. DPOR won't, because it can't see the C-level mutation.

The trade-off: error traces are less interpretable. You get the specific opcode schedule that broke the invariant and a best-effort interleaved source trace, but not the causal conflict analysis that DPOR provides.

import frontrun

class Counter:
    def __init__(self, value=0):
        self.value = value

    def increment(self):
        temp = self.value
        self.value = temp + 1

def test_counter_is_atomic():
    result = frontrun.explore(
        setup=lambda: Counter(value=0),
        workers=Counter.increment,
        count=2,
        invariant=lambda c: c.value == 2,
        strategy="random",
        max_attempts=200,
        max_ops=200,
        seed=42,
    )
    result.assert_holds()

This fails with output like:

Race condition found after 1 interleavings.

  Lost update: threads 0 and 1 both read value before either wrote it back.

  Thread 1 | counter.py:7             temp = self.value
           | [read value]
  Thread 0 | counter.py:7             temp = self.value
           | [read value]
  Thread 1 | counter.py:8             self.value = temp + 1
           | [write value]
  Thread 0 | counter.py:8             self.value = temp + 1
           | [write value]

  Reproduced 10/10 times (100%)

The reproduce_on_failure parameter (default 10) controls how many times the counterexample schedule is replayed to measure reproducibility. Set to 0 to skip.

Note: Opcode-level schedules are not stable across Python versions. CPython does not guarantee bytecode compatibility between releases, so a counterexample from Python 3.12 may not reproduce on 3.13. Treat counterexample schedules as ephemeral debugging artifacts.

Automatic I/O Detection

Both the bytecode explorer and DPOR automatically detect socket and file I/O operations (enabled by default via detect_io=True). When two threads access the same network endpoint or file path, the operation is reported as a conflict so the scheduler explores their reorderings.

Python-level detection (monkey-patching):

Sockets: connect, send, sendall, sendto, recv, recv_into, recvfrom
Files: open() (read vs write determined by mode)

Resource identity is derived from the socket's peer address (host:port) or the file's resolved path — two threads hitting the same endpoint or file conflict; different endpoints are independent.

Redis Key-Level Conflict Detection

DPOR goes beyond coarse socket-level detection for Redis: it intercepts execute_command() on redis-py clients, classifies each command as a read or write on specific keys, and reports per-key resource IDs to the engine. Two threads operating on different Redis keys are independent; only operations on the same key (with at least one write) trigger interleaving exploration.

Sync DPOR — Redis patching is active automatically when detect_io=True (the default):

import frontrun
import redis

def test_redis_counter_race(redis_port):
    class State:
        def __init__(self):
            r = redis.Redis(port=redis_port, decode_responses=True)
            r.set("counter", "0")
            r.close()

    def increment(state):
        r = redis.Redis(port=redis_port, decode_responses=True)
        val = int(r.get("counter"))
        r.set("counter", str(val + 1))
        r.close()

    result = frontrun.explore(
        setup=State,
        workers=[increment, increment],
        invariant=lambda s: int(redis.Redis(port=redis_port).get("counter")) == 2,
        detect_io=True,   # default — activates Redis key-level patching
    )
    assert not result.property_holds  # DPOR finds the lost-update race

Async DPOR — detect_io=True covers Redis in async too:

import frontrun
import redis.asyncio as aioredis

async def test_async_redis_race(redis_port):
    async def increment(state):
        r = aioredis.Redis(port=redis_port, decode_responses=True)
        val = int(await r.get("counter"))
        await r.set("counter", str(val + 1))
        await r.aclose()

    result = await frontrun.explore(
        setup=lambda: None,
        workers=increment,
        count=2,
        invariant=lambda s: True,  # check Redis directly in a real test
        detect_io=True,
    )

The same key-level precision applies to hashes (HGET/HSET), lists, sets, sorted sets, and all other Redis data structures — 160+ commands are classified. See the Redis technical details for a full walkthrough.

C-Level I/O Interception

When run under the frontrun CLI, a native LD_PRELOAD library (libfrontrun_io.so) intercepts libc I/O functions directly. This covers opaque C extensions — database drivers (libpq, mysqlclient), Redis clients, HTTP libraries, and anything else that calls libc's send(), recv(), read(), write(), etc.

Intercepted functions: connect, send, sendto, sendmsg, write, writev, recv, recvfrom, recvmsg, read, readv, close

The library maintains a process-global file-descriptor → resource map:

connect(fd, sockaddr{127.0.0.1:5432}, ...)  →  record fd=7 → "socket:127.0.0.1:5432"
send(fd=7, ...)                              →  report write to "socket:127.0.0.1:5432"
recv(fd=7, ...)                              →  report read from "socket:127.0.0.1:5432"
close(fd=7)                                  →  remove fd=7 from map

Events are transmitted to the Python side via one of two channels:

Pipe (preferred): IOEventDispatcher creates an os.pipe() and sets FRONTRUN_IO_FD to the write-end fd. The Rust library writes directly to the pipe (no open/close overhead per event), and a Python reader thread dispatches events to registered listener callbacks in arrival order. The pipe's FIFO ordering provides a natural total order without timestamps.
Log file (legacy): FRONTRUN_IO_LOG points to a temp file. Events are appended per-call (open + write + close each time) and read back in batch after execution.

from frontrun._preload_io import IOEventDispatcher

with IOEventDispatcher() as dispatcher:
    dispatcher.add_listener(lambda ev: print(f"{ev.kind} {ev.resource_id}"))
    # ... run code under LD_PRELOAD / DYLD_INSERT_LIBRARIES ...
# all events are also available as dispatcher.events

Trace Filtering (`trace_packages`)

By default, frontrun only traces user code — files outside the stdlib, site-packages, and frontrun's own internals. When the code under test lives inside an installed package (Django apps, plugin architectures, etc.), pass trace_packages to widen the filter:

import frontrun

result = frontrun.explore(
    setup=make_state,
    workers=[thread_a, thread_b],
    invariant=check_invariant,
    trace_packages=["mylib.*", "django_filters.*"],
)

Patterns use fnmatch syntax and are matched against dotted module names (e.g. django_filters.views). All exploration entry points (explore, explore_random, and their async variants) accept this parameter. See trace filtering docs for details.

Async Support

Trace markers, random interleaving exploration, and DPOR all have async support.

Async Trace Markers

from frontrun import TraceExecutor
from frontrun.common import Schedule, Step

class AsyncCounter:
    def __init__(self):
        self.value = 0

    async def get_value(self):
        return self.value

    async def set_value(self, new_value):
        self.value = new_value

    async def increment(self):
        # frontrun: read_value
        temp = await self.get_value()
        # frontrun: write_value
        await self.set_value(temp + 1)

def test_async_counter_lost_update():
    counter = AsyncCounter()

    schedule = Schedule([
        Step("task1", "read_value"),
        Step("task2", "read_value"),
        Step("task1", "write_value"),
        Step("task2", "write_value"),
    ])

    executor = TraceExecutor(schedule)
    executor.run({
        "task1": counter.increment,
        "task2": counter.increment,
    })

    assert counter.value == 1  # One increment lost

Async Exploration

Async exploration works at natural await boundaries instead of opcodes, making schedules stable across Python versions. frontrun.explore() detects async workers automatically:

import asyncio
import frontrun

class Counter:
    def __init__(self):
        self.value = 0

    async def increment(self):
        temp = self.value
        await asyncio.sleep(0)  # any natural await is a scheduling point
        self.value = temp + 1

# DPOR (default) — systematic
async def test_async_counter_dpor():
    result = await frontrun.explore(
        setup=Counter,
        workers=Counter.increment,
        count=2,
        invariant=lambda c: c.value == 2,
    )
    result.assert_holds()

# Random strategy — fast, probabilistic
async def test_async_counter_random():
    result = await frontrun.explore(
        setup=Counter,
        workers=Counter.increment,
        count=2,
        invariant=lambda c: c.value == 2,
        strategy="random",
        max_attempts=200,
    )
    result.assert_holds()

CLI

The frontrun CLI wraps any command with the I/O interception environment:

# Run pytest with frontrun I/O interception
frontrun pytest -vv tests/

# Run any Python program
frontrun python examples/orm_race.py

# Run a web server
frontrun uvicorn myapp:app

The CLI:

Sets FRONTRUN_ACTIVE=1 so frontrun knows it's running under the CLI
Sets LD_PRELOAD (Linux) or DYLD_INSERT_LIBRARIES (macOS) to load libfrontrun_io.so/.dylib
Runs the command as a subprocess

Pytest Plugin

Frontrun ships a pytest plugin (registered via the pytest11 entry point) that patches threading.Lock, threading.RLock, queue.Queue, and related primitives with cooperative versions before test collection.

Patching is on by default when running under the frontrun CLI. When running plain pytest without the CLI, patching is off unless explicitly requested:

frontrun pytest                    # cooperative lock patching is active (auto)
pytest --frontrun-patch-locks      # explicitly enable without CLI
pytest --no-frontrun-patch-locks   # explicitly disable even under CLI

Tests that use frontrun.explore() or frontrun.explore_random() will be automatically skipped when run without the frontrun CLI, preventing confusing failures when the environment isn't properly set up.

Platform Compatibility

Feature	Linux	macOS	Windows
Trace markers (sync + async)	Yes	Yes	Yes
Bytecode exploration (sync + async)	Yes	Yes	Yes
DPOR (Rust engine)	Yes	Yes	Yes
`frontrun` CLI + C-level I/O interception	Yes	Yes	No

Linux is the primary development platform and has full support for all features including the LD_PRELOAD I/O interception library.

macOS supports all features. The frontrun CLI uses DYLD_INSERT_LIBRARIES to load libfrontrun_io.dylib. Note that macOS System Integrity Protection (SIP) strips DYLD_INSERT_LIBRARIES from Apple-signed system binaries (/usr/bin/python3, etc.). Use a Homebrew, pyenv, or venv Python to avoid this limitation.

Windows support is limited to trace markers, bytecode exploration, and DPOR — the pure-Python and Rust PyO3 components that don't rely on LD_PRELOAD. The frontrun CLI and C-level I/O interception library are not available on Windows because they depend on the Unix dynamic linker's symbol interposition mechanism, which has no direct Windows equivalent.

Development

Prefer `assert_holds()` over manual asserts

InterleavingResult exposes a convenience helper that raises AssertionError with the race explanation on failure and returns None silently on success:

result = frontrun.explore(setup=setup, workers=[thread1, thread2], invariant=invariant)
result.assert_holds()  # preferred over: assert result.property_holds, result.explanation

An optional msg_prefix is prepended to the explanation:

result.assert_holds(msg_prefix="transfer race: ")

Running Tests

# Build everything and run tests
make test-3.10

# Or via the frontrun CLI
make build-dpor-3.10 build-io
frontrun .venv-3.10/bin/pytest -v

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Algorithm	Hash digest
SHA256	`b73b407eebe7d951936a8e7095efbff5cc11e7828bd60c70877c8d23d2ad05fb`
MD5	`f25980f4f0176cd119269c0c3e8d19ee`
BLAKE2b-256	`b3a8a7044cbe5db233a51c54bb04adbbbcf4b2c04c69442c728514700664abd9`

Algorithm	Hash digest
SHA256	`18002f784de66f2657a2905f239a14de92474dc722cb10ea8111622a1e1e79f5`
MD5	`73f6c0cf3c87c077ba9ffd11f5a9c6de`
BLAKE2b-256	`85bada9443e6c371cb5bea3c95e7a2f7aa9157321892fdd7cec5355d749385cf`

Algorithm	Hash digest
SHA256	`f31201fe2f86448046e2023e8233080d7bf28653de43895aa5bc74160bbaf31f`
MD5	`2d571a2100342d63e28c3ee9407f3498`
BLAKE2b-256	`57e54481e7a154f362d68300ae6719d6639613de32c00c8ab3769897f8d1cf22`

Algorithm	Hash digest
SHA256	`a22ec2cf7bb87c9c965279b30efa06ab3d4f261d34c4eb9e3911715dcf586f52`
MD5	`9ec16a272be8c97a88e600b16f58dd04`
BLAKE2b-256	`365fdcc81f8546149ac0b6d952a3d318d80ed38c264d09c6ef6069d1ee1dba38`

Algorithm	Hash digest
SHA256	`56854f52f5a742ad875a3950c815b29f5320b5a24e67694217b60d18a11e2200`
MD5	`1d20c752c5960de7e43ed8a0a56668f5`
BLAKE2b-256	`99967c7beed087a275cf7c3bda67a0ea42a549d178dafb03a1a0ef4b7046450b`

Algorithm	Hash digest
SHA256	`6399091c457125ee665372c7723120f3cc1a497eeb7b830e840b1111e2f42ab7`
MD5	`ea7ea2fbb6a58dec3fd0ea9985a1c81e`
BLAKE2b-256	`0dfaa2c517dfe8a4d058324f2ab54197b21a0bf8130ab232d4fbbd9ecd8609b8`

Algorithm	Hash digest
SHA256	`4b4bbd58c5e5b53b9fabe3ac946994cb8dfb00b1826c36f5fed10e8f0a2f5a82`
MD5	`3afb96cfd124b699c177fe3444f49bce`
BLAKE2b-256	`10f189a1028820d2b2683fcffca5727e5a442695ff0bd53209e6c15b3a5bbbb2`

Algorithm	Hash digest
SHA256	`6741a67cc3fc12765872f849e289ab2757d39ede2fbc3bbc8ddec9edf3700011`
MD5	`8976679ce342b5adc62c44cae2759022`
BLAKE2b-256	`9b5ff08a8814856abf122db79b1b74a7187527c1d52661f89506ca567587dfc4`

Algorithm	Hash digest
SHA256	`485a33cfa4008204b300ecd4c7dcfb7ecedc5256e7202a4b6e84b8ce47cd1ef1`
MD5	`f890d6bc221d99bab00a3b52b59331b1`
BLAKE2b-256	`25bead8925bd6044cb6ed631aa263d9591b0e3b012dc3c0a6ea4c654e058ac06`

Algorithm	Hash digest
SHA256	`dfb53d13194497a5af98bc68b33e625aab89cc192090ad3caf147ffd3c01e973`
MD5	`36e0d21d40d73d785db48a6871ecb757`
BLAKE2b-256	`a837071adf8779c792d4b75bdfd9eb4df7d9220f20af1731616cd3ccd25beea3`

Algorithm	Hash digest
SHA256	`814f5e81006e9ed65411dd49f3611d0dd1b6720894df389545d0cf2edaf94386`
MD5	`48012a9df9319b781907e17ffff236bc`
BLAKE2b-256	`dfcb9a657dfe1ae83b0967ebdedea5048f5e9dd5f060dfe787426ced8e3ad011`

Algorithm	Hash digest
SHA256	`82ab957cfc28e651b6c0721a37f1eca8b3b6ef84d2dd90e46120bb4b5ef62a50`
MD5	`1f3ea32907118d17b2011096507d4a70`
BLAKE2b-256	`eafb54e77cc1c88b537d90d72bc9d4fdebf7700351a97c89c8d1d6aa4805ba6a`

Algorithm	Hash digest
SHA256	`61cca42f20baf24748cd34ede9c123ff57719f6edcead431a4e21dfababce564`
MD5	`593a237b930fd01c326951db15312c2b`
BLAKE2b-256	`4f4c814c10036790fec9ae52ec6090c347d0404be90ae0f174e115a15c6aba50`

Algorithm	Hash digest
SHA256	`3a7b544497856aa174e65f0350e326ab8ed8cf3d9f6e25f5ce512e3c949fecf2`
MD5	`45c442de1c6d3f7054d0e7ec06d19121`
BLAKE2b-256	`7b7392d528d4b0225519f6a711626684a022424642e4223101c2d5e1d2b00e2e`

Algorithm	Hash digest
SHA256	`d4167ad68f7e041ab1f877ef8676b171227962a38e47aceeb421f929bc6752bb`
MD5	`c5c928800599c053e6895595d8b18ac4`
BLAKE2b-256	`ee9d726fca34133e0baf27cffa4b06174ecc52c26f49258cce103471e85649e6`

frontrun 0.6.0

Frontrun

Overview

DPOR deadlock detection (dining philosophers)

Quick Start: Bank Account Race Condition

Case Studies

Usage Approaches

1. Trace Markers

2. DPOR (Systematic Exploration)

3. Bytecode Exploration (Random Strategy)

Automatic I/O Detection

Redis Key-Level Conflict Detection

C-Level I/O Interception

Trace Filtering (trace_packages)

Async Support

Async Trace Markers

Async Exploration

CLI

Pytest Plugin

Platform Compatibility

Development

Prefer assert_holds() over manual asserts

Running Tests

Project links

Key dates

1 maintainer

Credits

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Provides Extra

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Trace Filtering (`trace_packages`)

Prefer `assert_holds()` over manual asserts