[3.13] gh-87512: Fix subprocess using timeout= on Windows blocking with a large input= (GH-142058)

Lib/subprocess.py

@@ -1616,6 +1616,10 @@ def _readerthread(self, fh, buffer):
             fh.close()
 
 
+        def _writerthread(self, input):
+            self._stdin_write(input)
+
+
         def _communicate(self, input, endtime, orig_timeout):
             # Start reader threads feeding into a list hanging off of this
             # object, unless they've already been started.
@@ -1634,8 +1638,23 @@ def _communicate(self, input, endtime, orig_timeout):
                 self.stderr_thread.daemon = True
                 self.stderr_thread.start()
 
-            if self.stdin:
-                self._stdin_write(input)
+            # Start writer thread to send input to stdin, unless already
+            # started.  The thread writes input and closes stdin when done,
+            # or continues in the background on timeout.
+            if self.stdin and not hasattr(self, "_stdin_thread"):
+                self._stdin_thread = \
+                        threading.Thread(target=self._writerthread,
+                                         args=(input,))
+                self._stdin_thread.daemon = True
+                self._stdin_thread.start()
+
+            # Wait for the writer thread, or time out.  If we time out, the
+            # thread remains writing and the fd left open in case the user
+            # calls communicate again.
+            if hasattr(self, "_stdin_thread"):
+                self._stdin_thread.join(self._remaining_time(endtime))
+                if self._stdin_thread.is_alive():
+                    raise TimeoutExpired(self.args, orig_timeout)
 
             # Wait for the reader threads, or time out.  If we time out, the
             # threads remain reading and the fds left open in case the user

Lib/test/test_subprocess.py

@@ -1033,6 +1033,62 @@ def test_communicate_timeout_large_output(self):
         (stdout, _) = p.communicate()
         self.assertEqual(len(stdout), 4 * 64 * 1024)
 
+    def test_communicate_timeout_large_input(self):
+        # Test that timeout is enforced when writing large input to a
+        # slow-to-read subprocess, and that partial input is preserved
+        # for continuation after timeout (gh-141473).
+        #
+        # This is a regression test for Windows matching POSIX behavior.
+        # On POSIX, select() is used to multiplex I/O with timeout checking.
+        # On Windows, stdin writing must also honor the timeout rather than
+        # blocking indefinitely when the pipe buffer fills.
+
+        # Input larger than typical pipe buffer (4-64KB on Windows)
+        input_data = b"x" * (128 * 1024)
+
+        p = subprocess.Popen(
+            [sys.executable, "-c",
+             "import sys, time; "
+             "time.sleep(30); "  # Don't read stdin for a long time
+             "sys.stdout.buffer.write(sys.stdin.buffer.read())"],
+            stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE)
+
+        try:
+            timeout = 0.2
+            start = time.monotonic()
+            try:
+                p.communicate(input_data, timeout=timeout)
+                # If we get here without TimeoutExpired, the timeout was ignored
+                elapsed = time.monotonic() - start
+                self.fail(
+                    f"TimeoutExpired not raised. communicate() completed in "
+                    f"{elapsed:.2f}s, but subprocess sleeps for 30s. "
+                    "Stdin writing blocked without enforcing timeout.")
+            except subprocess.TimeoutExpired:
+                elapsed = time.monotonic() - start
+
+            # Timeout should occur close to the specified timeout value,
+            # not after waiting for the subprocess to finish sleeping.
+            # Allow generous margin for slow CI, but must be well under
+            # the subprocess sleep time.
+            self.assertLess(elapsed, 5.0,
+                f"TimeoutExpired raised after {elapsed:.2f}s; expected ~{timeout}s. "
+                "Stdin writing blocked without checking timeout.")
+
+            # After timeout, continue communication. The remaining input
+            # should be sent and we should receive all data back.
+            stdout, stderr = p.communicate()
+
+            # Verify all input was eventually received by the subprocess
+            self.assertEqual(len(stdout), len(input_data),
+                f"Expected {len(input_data)} bytes output but got {len(stdout)}")
+            self.assertEqual(stdout, input_data)
+        finally:
+            p.kill()
+            p.wait()
+
     # Test for the fd leak reported in http://bugs.python.org/issue2791.
     def test_communicate_pipe_fd_leak(self):
         for stdin_pipe in (False, True):

Misc/NEWS.d/next/Library/2025-11-29-03-02-45.gh-issue-87512.bn4xbm.rst

@@ -0,0 +1,5 @@
+Fix :func:`subprocess.Popen.communicate` timeout handling on Windows
+when writing large input. Previously, the timeout was ignored during
+stdin writing, causing the method to block indefinitely if the child
+process did not consume input quickly. The stdin write is now performed
+in a background thread, allowing the timeout to be properly enforced.