fix: timeout for uds socket (backport 4.5) (#16778)

## Description

Backport to 4.5 of #16776



Co-authored-by: juanjo.alvarezmartinez <juanjo.alvarezmartinez@datadoghq.com>

Author: juanjux

ddtrace/internal/uds.py

@@ -2,9 +2,13 @@
 import socket
 from typing import Any  # noqa:F401
 
+from .constants import DEFAULT_TIMEOUT
 from .http import HTTPConnectionMixin
 
 
+_GLOBAL_DEFAULT_TIMEOUT = getattr(socket, "_GLOBAL_DEFAULT_TIMEOUT", None)
+
+
 class UDSHTTPConnection(HTTPConnectionMixin, httplib.HTTPConnection):
     """An HTTP connection established over a Unix Domain Socket."""
 
@@ -23,5 +27,11 @@ def __init__(
     def connect(self):
         # type: () -> None
         sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+        timeout = self.timeout
+        if timeout is _GLOBAL_DEFAULT_TIMEOUT:
+            timeout = socket.getdefaulttimeout()
+        if timeout is None:
+            timeout = DEFAULT_TIMEOUT
+        sock.settimeout(timeout)
         sock.connect(self.path)
         self.sock = sock

releasenotes/notes/uds-socket-timeout-c1c2055f1b661827.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Add a timeout to Unix socket connections to prevent thread I/O hangs during pre-fork shutdown.

tests/tracer/test_writer.py

@@ -969,6 +969,73 @@ def test_flush_connection_timeout(endpoint_test_timeout_server, writer_class):
             writer.flush_queue(raise_exc=True)
 
 
+def test_periodic_thread_uds_callback_unblocks_with_timeout():
+    import os
+    import tempfile
+
+    from ddtrace.internal._threads import PeriodicThread
+    from ddtrace.internal.uds import UDSHTTPConnection
+
+    sock_dir = tempfile.mkdtemp(prefix="ddtrace-uds-fork-repro-")
+    sock_path = os.path.join(sock_dir, "blackhole.sock")
+    server_ready = threading.Event()
+    server_stop = threading.Event()
+    callback_started = threading.Event()
+    callback_done = threading.Event()
+
+    def _blackhole_server():
+        srv = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+        conn = None
+        try:
+            srv.bind(sock_path)
+            srv.listen(1)
+            server_ready.set()
+            conn, _ = srv.accept()
+            # Keep the connection open and never send a response so the client
+            # can only make progress via its socket timeout.
+            while not server_stop.wait(0.05):
+                pass
+        finally:
+            if conn is not None:
+                conn.close()
+            srv.close()
+
+    def _callback():
+        callback_started.set()
+        conn = UDSHTTPConnection(sock_path, "localhost", 80)
+        try:
+            conn.request("GET", "/")
+            conn.getresponse().read()
+        except Exception:
+            # Timeout (or an equivalent network error path) is expected.
+            pass
+        finally:
+            try:
+                conn.close()
+            except Exception:
+                pass
+            callback_done.set()
+
+    server_thread = threading.Thread(target=_blackhole_server, daemon=True)
+    server_thread.start()
+    assert server_ready.wait(timeout=2)
+
+    worker = PeriodicThread(interval=60.0, target=_callback, name="repro:UDSForkTimeout", no_wait_at_start=True)
+    worker.start()
+    assert callback_started.wait(timeout=2)
+
+    try:
+        assert callback_done.wait(timeout=5)
+    finally:
+        server_stop.set()
+        worker.stop()
+        worker.join(timeout=1.0)
+        server_thread.join(timeout=1.0)
+        if os.path.exists(sock_path):
+            os.unlink(sock_path)
+        os.rmdir(sock_dir)
+
+
 @pytest.mark.parametrize("writer_class", (AgentWriter, CIVisibilityWriter, NativeWriter))
 def test_flush_connection_reset(endpoint_test_reset_server, writer_class):
     with (