From f10f1c665fefc6685a4bacf4a22ec52a2c03fb99 Mon Sep 17 00:00:00 2001
From: serversidehannes <hangus1995@gmail.com>
Date: Sat, 18 Jul 2026 18:28:03 +0200
Subject: [PATCH] =?UTF-8?q?fix:=20pin=20uvicorn=20loop=20to=20asyncio=20?=
 =?UTF-8?q?=E2=80=94=20loop=3Dauto=20silently=20re-enables=20uvloop?=
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---
 s3proxy/app.py                      |  8 +++++-
 s3proxy/main.py                     | 12 ++++++---
 tests/unit/test_main_loop_config.py | 42 +++++++++++++++++++++++++++++
 3 files changed, 58 insertions(+), 4 deletions(-)
 create mode 100644 tests/unit/test_main_loop_config.py

diff --git a/s3proxy/app.py b/s3proxy/app.py
index 1547fa8..41701a2 100644
--- a/s3proxy/app.py
+++ b/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)
diff --git a/s3proxy/main.py b/s3proxy/main.py
index 5f3337c..34cfa5f 100644
--- a/s3proxy/main.py
+++ b/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:
diff --git a/tests/unit/test_main_loop_config.py b/tests/unit/test_main_loop_config.py
new file mode 100644
index 0000000..8375343
--- /dev/null
+++ b/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"