From 315d4a9cd7be562d6fcbc6ebd619197193ae79b0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?C=C3=A9dric=20Cabessa?= <cedric.cabessa@ledger.fr>
Date: Tue, 11 Aug 2026 09:47:48 +0200
Subject: [PATCH] fix: performance regression when sniffio not installed
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`sniffio` is optional and lazily imported in `current_async_library`,
when absent we use `asyncio`

However the cost of import failure is paid every time the function is
called.

This patch moves the import at module level, like it is already done for
`anyio` or `trio`

Why it happens now?:

Httpcore never explicitly declared `sniffio` but it was installed via
`anyio` until they stopped depending on it:
https://github.com/agronholm/anyio/pull/1021

Here is a sample to reproduce the issue

```
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   |
---
 httpcore/_synchronization.py | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/httpcore/_synchronization.py b/httpcore/_synchronization.py
index 2ecc9e9c..32d9ada7 100644
--- a/httpcore/_synchronization.py
+++ b/httpcore/_synchronization.py
@@ -18,14 +18,17 @@
 except ImportError:  # pragma: nocover
     anyio = None  # type: ignore
 
+try:
+    import sniffio
+except ImportError:  # pragma: nocover
+    sniffio = None  # type: ignore
+
 
 def current_async_library() -> str:
     # Determine if we're running under trio or asyncio.
     # See https://sniffio.readthedocs.io/en/latest/
-    try:
-        import sniffio
-    except ImportError:  # pragma: nocover
-        environment = "asyncio"
+    if sniffio is None:
+        environment = "asyncio"  # pragma: nocover
     else:
         environment = sniffio.current_async_library()