fix: pin uvicorn loop to asyncio (loop=auto silently re-enables uvloop)

s3proxy/app.py

@@ -157,7 +157,13 @@ def create_lifespan(settings: Settings, credentials_store: dict[str, str]) -> As
     @asynccontextmanager
     async def lifespan(app: FastAPI) -> AsyncIterator[None]:
         _silence_health_probe_access_logs()
-        logger.info("Starting", endpoint=settings.s3_endpoint, port=settings.port)
+        loop = asyncio.get_running_loop()
+        logger.info(
+            "Starting",
+            endpoint=settings.s3_endpoint,
+            port=settings.port,
+            event_loop=f"{type(loop).__module__}.{type(loop).__qualname__}",
+        )
 
         # Initialize Redis FIRST, then create manager with correct store
         await init_redis(settings.redis_url or None, settings.redis_password or None)

s3proxy/main.py

@@ -23,15 +23,20 @@ def main():
     # uvloop 0.22.1 on Python 3.14 dies with a libuv abort (uv__io_poll assertion,
     # preceded by "OSError: [Errno 9] Bad file descriptor" from TCPTransport) when
     # the backend drops connections under load — killed 3 pods on 2026-07-16 with
-    # every in-flight upload. Off unless explicitly re-enabled.
-    if os.environ.get("S3PROXY_UVLOOP", "0") == "1":
+    # every in-flight upload. Off unless explicitly re-enabled. Skipping
+    # uvloop.install() here is NOT enough: uvicorn's default loop="auto" installs
+    # uvloop on its own whenever the package is importable (confirmed by a
+    # 2026-07-18 crash trace with uv__io_poll frames on a build without this
+    # install call), so the loop choice must be forced in the uvicorn config too.
+    use_uvloop = os.environ.get("S3PROXY_UVLOOP", "0") == "1"
+    if use_uvloop:
         try:
             import uvloop
 
             uvloop.install()
             logger.info("Using uvloop for improved performance")
         except ImportError:
-            pass
+            use_uvloop = False
 
     parser = argparse.ArgumentParser(description="S3Proxy - Transparent S3 encryption")
     parser.add_argument("--ip", default="0.0.0.0", help="Bind address")
@@ -61,6 +66,7 @@ def main():
         "host": settings.ip,
         "port": settings.port,
         "log_level": settings.log_level.lower(),
+        "loop": "auto" if use_uvloop else "asyncio",
     }
 
     if settings.memory_limit_mb > 0:

tests/unit/test_main_loop_config.py

@@ -0,0 +1,42 @@
+"""uvicorn must not auto-install uvloop: the loop has to be pinned to asyncio.
+
+2026-07-18: a build that skipped uvloop.install() still died with libuv's
+uv__io_poll abort — uvicorn's default loop="auto" installs uvloop by itself
+whenever the package is importable. The uvicorn config must pass
+loop="asyncio" unless S3PROXY_UVLOOP=1 opts back in.
+"""
+
+import json
+import sys
+
+import uvicorn
+
+from s3proxy import main as main_module
+
+_CREDS = json.dumps([{"access_key": "AK", "secret_key": "SK", "kek": "kek-secret"}])
+
+
+def _captured_uvicorn_config(monkeypatch, extra_env=None):
+    captured = {}
+    monkeypatch.setattr(uvicorn, "run", lambda **config: captured.update(config))
+    monkeypatch.setenv("S3PROXY_CREDENTIALS", _CREDS)
+    for key in ("IP", "PORT", "NO_TLS", "CERT_PATH", "REGION", "LOG_LEVEL"):
+        monkeypatch.delenv(f"S3PROXY_{key}", raising=False)
+    for k, v in (extra_env or {}).items():
+        monkeypatch.setenv(k, v)
+    monkeypatch.setattr(sys, "argv", ["s3proxy", "--no-tls"])
+    main_module.main()
+    return captured
+
+
+def test_loop_defaults_to_asyncio(monkeypatch):
+    config = _captured_uvicorn_config(monkeypatch)
+    assert config["loop"] == "asyncio"
+
+
+def test_loop_auto_only_when_uvloop_opted_in(monkeypatch):
+    import uvloop
+
+    monkeypatch.setattr(uvloop, "install", lambda: None)
+    config = _captured_uvicorn_config(monkeypatch, {"S3PROXY_UVLOOP": "1"})
+    assert config["loop"] == "auto"