GH-134453: Fix subprocess memoryview input handling on POSIX (GH-134949)

Fix inconsistent subprocess.Popen.communicate() behavior between Windows
and POSIX when using memoryview objects with non-byte elements as input.

On POSIX systems, the code was incorrectly comparing bytes written against
element count instead of byte count, causing data truncation for large
inputs with non-byte element types.

Changes:
- Cast memoryview inputs to byte view when input is already a memoryview
- Fix progress tracking to use len(input_view) instead of len(self._input)
- Add comprehensive test coverage for memoryview inputs

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* old-man-yells-at-ReST
* Update 2025-05-30-18-37-44.gh-issue-134453.kxkA-o.rst
* assertIsNone review feedback
* fix memoryview_nonbytes test to fail without our fix on main, and have a nicer error.

Thanks to Peter Bierma @ZeroIntensity for the code review.

Lib/subprocess.py

@@ -2102,7 +2102,10 @@ def _communicate(self, input, endtime, orig_timeout):
             self._save_input(input)
 
             if self._input:
+                if not isinstance(self._input, memoryview):
                     input_view = memoryview(self._input)
+                else:
+                    input_view = self._input.cast("b")  # byte input required
 
             with _PopenSelector() as selector:
                 if self.stdin and not self.stdin.closed and self._input:
@@ -2138,7 +2141,7 @@ def _communicate(self, input, endtime, orig_timeout):
                                 selector.unregister(key.fileobj)
                                 key.fileobj.close()
                             else:
-                                if self._input_offset >= len(self._input):
+                                if self._input_offset >= len(input_view):
                                     selector.unregister(key.fileobj)
                                     key.fileobj.close()
                         elif key.fileobj in (self.stdout, self.stderr):

Lib/test/test_subprocess.py

@@ -957,6 +957,48 @@ def test_communicate(self):
         self.assertEqual(stdout, b"banana")
         self.assertEqual(stderr, b"pineapple")
 
+    def test_communicate_memoryview_input(self):
+        # Test memoryview input with byte elements
+        test_data = b"Hello, memoryview!"
+        mv = memoryview(test_data)
+        p = subprocess.Popen([sys.executable, "-c",
+                              'import sys; sys.stdout.write(sys.stdin.read())'],
+                             stdin=subprocess.PIPE,
+                             stdout=subprocess.PIPE)
+        self.addCleanup(p.stdout.close)
+        self.addCleanup(p.stdin.close)
+        (stdout, stderr) = p.communicate(mv)
+        self.assertEqual(stdout, test_data)
+        self.assertIsNone(stderr)
+
+    def test_communicate_memoryview_input_nonbyte(self):
+        # Test memoryview input with non-byte elements (e.g., int32)
+        # This tests the fix for gh-134453 where non-byte memoryviews
+        # had incorrect length tracking on POSIX
+        import array
+        # Create an array of 32-bit integers that's large enough to trigger
+        # the chunked writing behavior (> PIPE_BUF)
+        pipe_buf = getattr(select, 'PIPE_BUF', 512)
+        # Each 'i' element is 4 bytes, so we need more than pipe_buf/4 elements
+        # Add some extra to ensure we exceed the buffer size
+        num_elements = pipe_buf + 1
+        test_array = array.array('i', [0x64306f66 for _ in range(num_elements)])
+        expected_bytes = test_array.tobytes()
+        mv = memoryview(test_array)
+
+        p = subprocess.Popen([sys.executable, "-c",
+                              'import sys; '
+                              'data = sys.stdin.buffer.read(); '
+                              'sys.stdout.buffer.write(data)'],
+                             stdin=subprocess.PIPE,
+                             stdout=subprocess.PIPE)
+        self.addCleanup(p.stdout.close)
+        self.addCleanup(p.stdin.close)
+        (stdout, stderr) = p.communicate(mv)
+        self.assertEqual(stdout, expected_bytes,
+                         msg=f"{len(stdout)=} =? {len(expected_bytes)=}")
+        self.assertIsNone(stderr)
+
     def test_communicate_timeout(self):
         p = subprocess.Popen([sys.executable, "-c",
                               'import sys,os,time;'

Misc/NEWS.d/next/Library/2025-05-30-18-37-44.gh-issue-134453.kxkA-o.rst

@@ -0,0 +1,4 @@
+Fixed :func:`subprocess.Popen.communicate` ``input=`` handling of :class:`memoryview`
+instances that were non-byte shaped on POSIX platforms. Those are now properly
+cast to a byte shaped view instead of truncating the input.  Windows platforms
+did not have this bug.