[3.13] gh-141473: Fix subprocess.Popen.communicate to send input to stdin upon a subsequent post-timeout call (GH-141477) (#142060)

* gh-141473: Fix subprocess.Popen.communicate to send input to stdin upon a subsequent post-timeout call (GH-141477)

* gh-141473: Fix subprocess.Popen.communicate to send input to stdin
* Docs: Clarify that `input` is one time only on `communicate()`
* NEWS entry
* Add a regression test.

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(cherry picked from commit 526d7a8bb4)

Co-authored-by: Artur Jamro <artur.jamro@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>

* no assertStartsWith

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Co-authored-by: Artur Jamro <artur.jamro@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>

Lib/test/test_subprocess.py

@@ -1684,6 +1684,40 @@ def test_wait_negative_timeout(self):
 
             self.assertEqual(proc.wait(), 0)
 
+    def test_post_timeout_communicate_sends_input(self):
+        """GH-141473 regression test; the stdin pipe must close"""
+        with subprocess.Popen(
+                [sys.executable, "-uc", """\
+import sys
+while c := sys.stdin.read(512):
+    sys.stdout.write(c)
+print()
+"""],
+                stdin=subprocess.PIPE,
+                stdout=subprocess.PIPE,
+                stderr=subprocess.PIPE,
+                text=True,
+        ) as proc:
+            try:
+                data = f"spam{'#'*4096}beans"
+                proc.communicate(
+                    input=data,
+                    timeout=0,
+                )
+            except subprocess.TimeoutExpired as exc:
+                pass
+            # Prior to the bugfix, this would hang as the stdin
+            # pipe to the child had not been closed.
+            try:
+                stdout, stderr = proc.communicate(timeout=15)
+            except subprocess.TimeoutExpired as exc:
+                self.fail("communicate() hung waiting on child process that should have seen its stdin pipe close and exit")
+            self.assertEqual(
+                    proc.returncode, 0,
+                    msg=f"STDERR:\n{stderr}\nSTDOUT:\n{stdout}")
+            self.assertTrue(stdout.startswith("spam"), msg=stdout)
+            self.assertIn("beans", stdout)
+
 
 class RunFuncTestCase(BaseTestCase):
     def run_python(self, code, **kwargs):