Fixes Issue #26373: subprocess.Popen.communicate now correctly ignores

BrokenPipeError when the child process dies before .communicate()
is called in more (all?) circumstances.

Lib/subprocess.py

@@ -1011,8 +1011,7 @@ class Popen(object):
             try:
                 self.stdin.write(input)
             except BrokenPipeError:
-                # communicate() must ignore broken pipe error
-                pass
+                pass  # communicate() must ignore broken pipe errors.
             except OSError as e:
                 if e.errno == errno.EINVAL and self.poll() is not None:
                     # Issue #19612: On Windows, stdin.write() fails with EINVAL
@@ -1020,7 +1019,15 @@ class Popen(object):
                     pass
                 else:
                     raise
+        try:
             self.stdin.close()
+        except BrokenPipeError:
+            pass  # communicate() must ignore broken pipe errors.
+        except OSError as e:
+            if e.errno == errno.EINVAL and self.poll() is not None:
+                pass
+            else:
+                raise
 
     def communicate(self, input=None, timeout=None):
         """Interact with process: Send data to stdin.  Read data from
@@ -1661,9 +1668,15 @@ class Popen(object):
             if self.stdin and not self._communication_started:
                 # Flush stdio buffer.  This might block, if the user has
                 # been writing to .stdin in an uncontrolled fashion.
+                try:
                     self.stdin.flush()
+                except BrokenPipeError:
+                    pass  # communicate() must ignore BrokenPipeError.
                 if not input:
+                    try:
                         self.stdin.close()
+                    except BrokenPipeError:
+                        pass  # communicate() must ignore BrokenPipeError.
 
             stdout = None
             stderr = None

Lib/test/test_subprocess.py

 import unittest
+from unittest import mock
 from test.support import script_helper
 from test import support
 import subprocess
@@ -1240,6 +1241,52 @@ class ProcessTestCase(BaseTestCase):
         fds_after_exception = os.listdir(fd_directory)
         self.assertEqual(fds_before_popen, fds_after_exception)
 
+    def test_communicate_BrokenPipeError_stdin_close(self):
+        # By not setting stdout or stderr or a timeout we force the fast path
+        # that just calls _stdin_write() internally due to our mock.
+        proc = subprocess.Popen([sys.executable, '-c', 'pass'])
+        with proc, mock.patch.object(proc, 'stdin') as mock_proc_stdin:
+            mock_proc_stdin.close.side_effect = BrokenPipeError
+            proc.communicate()  # Should swallow BrokenPipeError from close.
+            mock_proc_stdin.close.assert_called_with()
+
+    def test_communicate_BrokenPipeError_stdin_write(self):
+        # By not setting stdout or stderr or a timeout we force the fast path
+        # that just calls _stdin_write() internally due to our mock.
+        proc = subprocess.Popen([sys.executable, '-c', 'pass'])
+        with proc, mock.patch.object(proc, 'stdin') as mock_proc_stdin:
+            mock_proc_stdin.write.side_effect = BrokenPipeError
+            proc.communicate(b'stuff')  # Should swallow the BrokenPipeError.
+            mock_proc_stdin.write.assert_called_once_with(b'stuff')
+            mock_proc_stdin.close.assert_called_once_with()
+
+    def test_communicate_BrokenPipeError_stdin_flush(self):
+        # Setting stdin and stdout forces the ._communicate() code path.
+        # python -h exits faster than python -c pass (but spams stdout).
+        proc = subprocess.Popen([sys.executable, '-h'],
+                                stdin=subprocess.PIPE,
+                                stdout=subprocess.PIPE)
+        with proc, mock.patch.object(proc, 'stdin') as mock_proc_stdin, \
+                open('/dev/null', 'wb') as dev_null:
+            mock_proc_stdin.flush.side_effect = BrokenPipeError
+            # because _communicate registers a selector using proc.stdin...
+            mock_proc_stdin.fileno.return_value = dev_null.fileno()
+            # _communicate() should swallow BrokenPipeError from flush.
+            proc.communicate(b'stuff')
+            mock_proc_stdin.flush.assert_called_once_with()
+
+    def test_communicate_BrokenPipeError_stdin_close_with_timeout(self):
+        # Setting stdin and stdout forces the ._communicate() code path.
+        # python -h exits faster than python -c pass (but spams stdout).
+        proc = subprocess.Popen([sys.executable, '-h'],
+                                stdin=subprocess.PIPE,
+                                stdout=subprocess.PIPE)
+        with proc, mock.patch.object(proc, 'stdin') as mock_proc_stdin:
+            mock_proc_stdin.close.side_effect = BrokenPipeError
+            # _communicate() should swallow BrokenPipeError from close.
+            proc.communicate(timeout=999)
+            mock_proc_stdin.close.assert_called_once_with()
+
 
 class RunFuncTestCase(BaseTestCase):
     def run_python(self, code, **kwargs):

Misc/NEWS

@@ -128,6 +128,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #26373: subprocess.Popen.communicate now correctly ignores
+  BrokenPipeError when the child process dies before .communicate()
+  is called in more/all circumstances.
+
 - Issue #21776: distutils.upload now correctly handles HTTPError.
   Initial patch by Claudiu Popa.