performance regression when sniffio is missing

[CedricCabessa]

httpcore tries to import sniffio

Until recently, the import was successful as anyio depends on sniffio

However, since agronholm/anyio#1021 it is not the case.

It doesn't change the behavior as we fallback to asyncio

However the cost of failing import is real (python doesn't cache failing import)

I suggest to move this import at module level like it is done for trio or anyio

Here is a microbenchmark to observe the issue (I can open a PR with my patched code)

import asyncio
import time

import httpcore

REQUESTS = 500
RESPONSE = b"HTTP/1.1 200 OK\r\nContent-Length: 2\r\n\r\nok"

async def handle(reader, writer):
    try:
        while await reader.readuntil(b"\r\n\r\n"):
            writer.write(RESPONSE)
            await writer.drain()
    except (asyncio.IncompleteReadError, ConnectionResetError):
        pass
    writer.close()

async def main():
    try:
        import sniffio  # noqa: F401

        status = "INSTALLED"
    except ImportError:
        status = "ABSENT"

    server = await asyncio.start_server(handle, "127.0.0.1", 0)
    url = f"http://127.0.0.1:{server.sockets[0].getsockname()[1]}/"

    async with httpcore.AsyncConnectionPool() as pool:
        await pool.request("GET", url)  # warm up the pool

        cpu = time.process_time()
        for _ in range(REQUESTS):
            await pool.request("GET", url)
        cpu = time.process_time() - cpu

    print(f"sniffio {status}: {cpu / REQUESTS * 1e6:.0f} us CPU per request")
    server.close()

asyncio.run(main())

On my machine

httpcore	sniffio	CPU / req
master	installed	~330 µs
master	missing	~500 µs
patched	missing	~330 µs
patched	installed	~330 µs

As a workaround, we add sniffio in our dependency even if we don't need it just to fix this issue.

Anyone installing httpcore[asyncio] without trio will suffer from this performance hit as soon as they move to anyio >= 4.11

[CedricCabessa]

as the fix is easy, I'm adding a PR #1103

Ready to rework it as maintainers see fit

[Avalon2222]

Confirming this, and it is much worse on Windows. Your benchmark unchanged, httpcore 1.0.9, anyio 4.16.0 (which no longer pulls sniffio in), Python 3.14 on Windows 11:

sniffio	CPU / request
installed	~219 µs
missing	~1810–1840 µs

So ~8× rather than ~1.5×. The reason is that a failing import sniffio is not cached in sys.modules: every call to current_async_library() walks all of sys.path and stats the filesystem again, and that is far more expensive on NTFS than on Linux. Since current_async_library() is called from AsyncEvent.setup(), AsyncShieldCancellation.__init__() and friends, it runs several times per request.

Until #1103 lands, anyone who notices this can just add sniffio back to their own dependencies (pip install sniffio); it is a pure-Python, dependency-free package and restores the fast path. Pinning an older anyio would also bring it back transitively, but adding sniffio directly is the cleaner fix.