gh-87512: Fix subprocess communicate() timeout on Windows for large stdin

On Windows, Popen._communicate() previously wrote to stdin synchronously,
which could block indefinitely if the subprocess didn't consume input
quickly and the pipe buffer filled up. The timeout parameter was only
checked when joining the reader threads, not during the stdin write.

This change moves the stdin writing to a background thread (similar to
how stdout/stderr are read in threads), allowing the timeout to be
properly enforced. If timeout expires, TimeoutExpired is raised promptly
and the writer thread continues in the background. Subsequent calls to
communicate() will join the existing writer thread.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: gpshead

Lib/subprocess.py

@@ -1613,6 +1613,10 @@ 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.
@@ -1631,8 +1635,23 @@ 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