uv-ffi

Persistent in-process execution engine for uv's package resolver and installer.

While uv is designed as a world-class CLI tool, uv-ffi re-architects its core as a resident engine. By keeping a Tokio runtime, HTTP connection pools, site-packages metadata, and interpreter state warm in memory across calls, it achieves execution speeds limited only by filesystem I/O.

Used internally by omnipkg, but directly callable from any long-lived Python process.

Installation

Standard platforms

pip install uv-ffi

Covers Linux x86_64/aarch64, macOS universal2 (arm64+x86_64), Windows amd64/arm64 — all via ABI3 wheels (Python ≥ 3.8). macOS users wanting native-only wheels (smaller download, no Rosetta overhead) can use the extra-index-url above.

Exotic platforms (musl Alpine, armv7, riscv64, s390x, ppc64le, free-threaded CPython 3.13t, PyPy, GraalPy, Windows x86)

pip install uv-ffi --extra-index-url https://exotic-wheels.github.io/

Browse all wheels — PyPI wheels + 600+ additional wheels hosted on GitHub Releases

600+ wheels hosted on GitHub Releases, indexed at the URL above. These were removed from PyPI to stay under the 10 GB storage limit.

Build from source (requires Rust toolchain)

pip install uv-ffi --no-binary uv-ffi

Build from source for cp37

pip install uv-ffi --config-settings \"cargo-extra-args=--no-default-features\"

If the Rust build fails, pre-built wheels for your platform are almost certainly available via the extra-index-url above.

Usage

from uv_ffi import run, invalidate_site_packages_cache, patch_site_packages_cache, get_site_packages_cache, clear_registry_cache

PY = '/path/to/your/python'
BASE = f'pip install --python {PY}'  # LinkMode::Symlink);  // ← by default

# First call initializes the engine (~65-75ms, one-time cost)
rc, installed, removed, warnings = run(f'{BASE} rich==14.3.2')

# Subsequent calls use the warm engine (~5-6ms)
rc, installed, removed, warnings = run(f'{BASE} rich==14.3.3')
# -> installed=[('rich', '14.3.3')] removed=[('rich', '14.3.2')]

# Inspect engine's current in-memory view of the environment
state = get_site_packages_cache()
# ->[('rich', '14.3.3'), ('requests', '2.31.0'), ...]

The key is keeping the import alive in a long-lived process. Each new Python subprocess pays ~70ms (interpreter startup + engine init). In a warm daemon worker, the same operation costs ~6ms.

Performance

Measured wall-clock time on Linux (NVMe SSD, Python 3.11, pre-warmed uv cache).

No-op (package already satisfied)

Real swap (uninstall + reinstall different version)

Cache operations

The ~5–6ms floor on a real swap is the hardware limit — VFS symlink create/unlink on NVMe. uv-ffi eliminates all software overhead above that floor.

Important: calling uv-ffi via a new subprocess each time (~73ms avg) is slower than calling uv directly (~19ms). The gains only materialize when the engine stays warm across multiple calls in the same process.

API

run(cmd: str) -> (int, list[tuple[str,str]], list[tuple[str,str]])

Execute a uv command in-process. Returns (exit_code, installed, removed) where installed/removed are lists of (name, version) tuples.

rc, installed, removed, err = run('pip install --python /usr/bin/python3 rich==14.3.3')

Supports all flags omnipkg uses on the fast path: --python, --link-mode, --target, --index-url, --extra-index-url, --reinstall, -q. Any unrecognized flag falls back to the full clap parse path automatically.

get_site_packages_cache() -> list[tuple[str, str]]

Returns the engine's current in-memory view of the environment as [(name, version), ...]. Returns an empty list if the cache has not been populated yet (before first install call). Zero disk I/O.

state = get_site_packages_cache()
#[('rich', '14.3.3'), ('requests', '2.31.0'), ...]

invalidate_site_packages_cache()

Forces a full disk rescan on the next install call. Use when an external tool has modified the environment and you don't have the changelog. Cost: ~2.5ms on next call.

patch_site_packages_cache(installed, removed)

Surgically update the in-memory cache with a known delta. ~100× faster than a full rescan. Returns True if the cache was live and patched, False if no cache was active.

patch_site_packages_cache(
    installed=[['rich', '14.3.3']],
    removed=[['rich', '14.3.2']],
)

clear_registry_cache()

Drops the persistent RegistryClient memory cache. Use this if you suspect the internal PyPI Simple API cache is stale. Note: The engine already auto-heals on install failures, so manual invocation is rarely needed.

C ABI: omnipkg_uv_run_c

int omnipkg_uv_run_c(const char *cmd, char *out_json, int max_out);

Runs a uv command and writes a JSON changelog to out_json:

{"installed":[["rich","14.3.3"]],"removed":[["rich","14.3.2"]]}

Returns the exit code. Safe to call from Go, C++, Rust, or any language with C FFI.

Isolated "Bubble" Installs (--target)

uv-ffi provides cache-safe isolated directory installs via --target. Standard uv evaluates --target against the host interpreter's installed packages, which can produce incorrect results and poisons in-memory state. uv-ffi routes --target installs through a pre-warmed BUBBLE_ENVIRONMENT:

• The resolver sees a clean slate — no existing packages, no cross-contamination
• SITE_PACKAGES_CACHE (which reflects main env state) is never read or written during a bubble install
• The BUBBLE_INSTALL atomic flag ensures the main env cache is fully protected for the duration
• After the install, the flag is reset and the next main-env call proceeds normally

# Install into isolated dir — main env cache untouched
rc, installed, _ = run(f'pip install --python {PY} --target /tmp/app_env flask==3.0.0')

This is how omnipkg generates multiversion isolated environments entirely in-memory.

Cache Coherency

If uv-ffi is the only thing modifying the environment, no action is needed. After every install, the cache is updated directly from the resolver's in-memory changelog at zero disk I/O cost — it's always coherent with no overhead.

For environments shared with external tools, uv-ffi now auto-heals on detection:

Auto-heal (built-in, no configuration) When uv-ffi detects that its in-memory cache references a dist-info path that no longer exists on disk, it automatically forces a full rescan and retries the install transparently. The caller always gets a correct result.

Verified behavior after external version change:

[1] uv-ffi installs rich==13.9.4:          4.93ms  ← cache warm, correct
[2] external uv installs rich==13.9.3:     ~18ms   ← cache now stale
[3] uv-ffi asked for rich==14.0.0:         7.60ms  ← auto-healed, correct
    (0.5ms failed attempt + 1.6ms rescan + 5.5ms uninstall/install)

True overhead of auto-heal vs normal swap: +2.67ms (one failed attempt + one disk rescan). This is paid only when an external tool has modified the environment since the last uv-ffi call.

For zero-overhead coherency, bypass the heal entirely with a proactive delta patch:

Option A — FS watcher + delta patch (omnipkg's approach, zero heal cost) Watch site-packages for filesystem events. On each change, call patch_site_packages_cache(installed, removed). Cost: ~25µs per patch. Cache never goes stale, auto-heal never triggers.

Option B — Force rescan Call invalidate_site_packages_cache() before any call where external modification is possible. Cost: ~1.6ms on next call. Same rescan cost as auto-heal but paid upfront rather than on failure.

Architecture

Python API  →  run() / patch_site_packages_cache() / get_site_packages_cache()
C ABI       →  omnipkg_uv_run_c() → JSON changelog
Fast path   →  try_parse_ffi_install() → run_pip_install_direct() [bypasses clap entirely]
Slow path   →  clap parse → uv::run() [pip freeze, uninstall, etc.]
Globals     →  ENGINE / BUBBLE_ENVIRONMENT / SITE_PACKAGES_CACHE / REGISTRY_CLIENT / PYTHON_ENVIRONMENT

Persistent UvEngine singleton Interpreter discovery, platform tagging, cache init, and TLS pool setup happen once at import time and are held in a OnceLock. All subsequent calls skip directly to resolution. Includes a pre-warmed BUBBLE_ENVIRONMENT for --target installs.

Zero-clap fast path pip install commands are parsed directly via try_parse_ffi_install() — a hand-written token parser covering all flags omnipkg uses. Internal Rust structs are constructed directly, skipping clap entirely (~2ms saved per call). Unrecognized flags fall back to clap automatically.

Persistent RegistryClient and PythonEnvironment The HTTP client (TLS pools, connection pools) and Python environment (interpreter metadata, marker environment) are stored as global singletons after first use. Subsequent calls reuse them directly — no socket teardown, no filesystem search.

PyPI Registry Auto-Healing The FFI engine keeps PyPI API responses in RAM for maximum speed. If a newly published package version is requested and not found in the RAM cache, the engine detects the internal failure, automatically drops its registry cache, and retries the network fetch transparently. You never need to restart the process to see newly published packages.

Zero-disk post-install cache update After a successful install, SITE_PACKAGES_CACHE is updated directly from the resolver's changelog using in-memory InstalledRegistryDist construction. No dist-info directory scan, no try_from_path I/O. Post-install cache update cost: 0.0ms.

SitePackages::add_dist() A new method added to uv-installer's SitePackages that surgically inserts a distribution into the in-memory index without touching disk. Used by both the post-install zero-disk update and patch_site_packages_cache().

Idempotent initialization Logging setup (setup_logging) and miette::set_hook are now called with let _ = — safe to call repeatedly in a long-running process without double-init panics.

Persistent Tokio runtime The main() path previously created a new Tokio runtime on every call and called shutdown_background() on exit (leaving pending HTTP requests). The runtime is now stored in a OnceLock and reused across calls — no teardown overhead, no leaked requests.

Profiling

Set UV_FFI_PROFILE=1 to enable millisecond-precision phase tracing:

[UV-PROFILE] cache-reused: 0.12ms[UV-PROFILE] post-site-packages-scan: 0.08ms (cached)
[UV-PROFILE] post-settings-resolve: 0.31ms
[UV-PROFILE] post-execute-plan: 4.82ms
[UV-PROFILE] post-changelog-from-local: 4.83ms
[UV-PROFILE] post-changelog-write: 4.91ms[UV-SYNC] Zero-disk cache update: done
[UV-PROFILE] post-explicit-drop: 0.02ms
[UV-PROFILE] post-await: 5.14ms

Coexistence with vanilla uv

uv-ffi installs are fully compatible with vanilla uv operations in the same environment. uv-ffi writes complete dist-info including RECORD, INSTALLER, and REQUESTED — so uv pip uninstall, uv pip install, and other standard toolchain operations work correctly on packages uv-ffi installed.

Coexistence means no corruption, not automatic cache synchronization — see cache coherency section above.

Platform Support

Version Correspondence

uv-ffi versions track the upstream uv release they are built against.

Benchmark Methodology

• In-process tests: 10-run alternating swap (rich==14.3.2 ↔ rich==14.3.3) in a single warm Python session
• Subprocess tests: 8 separate subprocess.run calls, new Python process each time
• Interference test: uv subprocess between uv-ffi calls, 1s settle time
• uv cache pre-warmed before all runs
• Hardware: Linux, NVMe Gen4, Python 3.11.14

Attribution

This crate links against uv source code from astral-sh/uv, copyright Astral Software Inc., used under the MIT License. See NOTICE for full attribution.

Not affiliated with, endorsed by, or sponsored by Astral Software Inc.