gh-142206: multiprocessing.resource_tracker: Decode messages using older protocol

Lib/multiprocessing/resource_tracker.py

@@ -68,6 +68,13 @@ def __init__(self):
         self._exitcode = None
         self._reentrant_messages = deque()
 
+        # True to use colon-separated lines, rather than JSON lines,
+        # for internal communication. (Mainly for testing).
+        # Filenames not supported by the simple format will always be sent
+        # using JSON.
+        # The reader should understand all formats.
+        self._use_simple_format = False
+
     def _reentrant_call_error(self):
         # gh-109629: this happens if an explicit call to the ResourceTracker
         # gets interrupted by a garbage collection, invoking a finalizer (*)
@@ -200,7 +207,9 @@ def _launch(self):
             os.close(r)
 
     def _make_probe_message(self):
-        """Return a JSON-encoded probe message."""
+        """Return a probe message."""
+        if self._use_simple_format:
+            return b'PROBE:0:noop\n'
         return (
             json.dumps(
                 {"cmd": "PROBE", "rtype": "noop"},
@@ -267,6 +276,15 @@ def _write(self, msg):
         assert nbytes == len(msg), f"{nbytes=} != {len(msg)=}"
 
     def _send(self, cmd, name, rtype):
+        if self._use_simple_format and '\n' not in name:
+            msg = f"{cmd}:{name}:{rtype}\n".encode("ascii")
+            if len(msg) > 512:
+                # posix guarantees that writes to a pipe of less than PIPE_BUF
+                # bytes are atomic, and that PIPE_BUF >= 512
+                raise ValueError('msg too long')
+            self._ensure_running_and_write(msg)
+            return
+
         # POSIX guarantees that writes to a pipe of less than PIPE_BUF (512 on Linux)
         # bytes are atomic. Therefore, we want the message to be shorter than 512 bytes.
         # POSIX shm_open() and sem_open() require the name, including its leading slash,
@@ -286,6 +304,7 @@ def _send(self, cmd, name, rtype):
 
         # The entire JSON message is guaranteed < PIPE_BUF (512 bytes) by construction.
         assert len(msg) <= 512, f"internal error: message too long ({len(msg)} bytes)"
+        assert msg.startswith(b'{')
 
         self._ensure_running_and_write(msg)
 
@@ -296,6 +315,30 @@ def _send(self, cmd, name, rtype):
 getfd = _resource_tracker.getfd
 
 
+def _decode_message(line):
+    if line.startswith(b'{'):
+        try:
+            obj = json.loads(line.decode('ascii'))
+        except Exception as e:
+            raise ValueError("malformed resource_tracker message: %r" % (line,)) from e
+
+        cmd = obj["cmd"]
+        rtype = obj["rtype"]
+        b64  = obj.get("base64_name", "")
+
+        if not isinstance(cmd, str) or not isinstance(rtype, str) or not isinstance(b64, str):
+            raise ValueError("malformed resource_tracker fields: %r" % (obj,))
+
+        try:
+            name = base64.urlsafe_b64decode(b64).decode('utf-8', 'surrogateescape')
+        except ValueError as e:
+            raise ValueError("malformed resource_tracker base64_name: %r" % (b64,)) from e
+    else:
+        cmd, rest = line.strip().decode('ascii').split(':', maxsplit=1)
+        name, rtype = rest.rsplit(':', maxsplit=1)
+    return cmd, rtype, name
+
+
 def main(fd):
     '''Run resource tracker.'''
     # protect the process from ^C and "killall python" etc
@@ -318,23 +361,7 @@ def main(fd):
         with open(fd, 'rb') as f:
             for line in f:
                 try:
-                    try:
-                        obj = json.loads(line.decode('ascii'))
-                    except Exception as e:
-                        raise ValueError("malformed resource_tracker message: %r" % (line,)) from e
-
-                    cmd = obj["cmd"]
-                    rtype = obj["rtype"]
-                    b64  = obj.get("base64_name", "")
-
-                    if not isinstance(cmd, str) or not isinstance(rtype, str) or not isinstance(b64, str):
-                        raise ValueError("malformed resource_tracker fields: %r" % (obj,))
-
-                    try:
-                        name = base64.urlsafe_b64decode(b64).decode('utf-8', 'surrogateescape')
-                    except ValueError as e:
-                        raise ValueError("malformed resource_tracker base64_name: %r" % (b64,)) from e
-
+                    cmd, rtype, name = _decode_message(line)
                     cleanup_func = _CLEANUP_FUNCS.get(rtype, None)
                     if cleanup_func is None:
                         raise ValueError(

Lib/test/_test_multiprocessing.py

@@ -39,6 +39,7 @@
 from test.support import socket_helper
 from test.support import threading_helper
 from test.support import warnings_helper
+from test.support import subTests
 from test.support.script_helper import assert_python_failure, assert_python_ok
 
 # Skip tests if _multiprocessing wasn't built.
@@ -4383,6 +4384,19 @@ def test_copy(self):
         self.assertEqual(bar.z, 2 ** 33)
 
 
+def resource_tracker_format_subtests(func):
+    """Run given test using both resource tracker communication formats"""
+    def _inner(self, *args, **kwargs):
+        tracker = resource_tracker._resource_tracker
+        for use_simple_format in False, True:
+            with (
+                self.subTest(use_simple_format=use_simple_format),
+                unittest.mock.patch.object(
+                    tracker, '_use_simple_format', use_simple_format)
+            ):
+                func(self, *args, **kwargs)
+    return _inner
+
 @unittest.skipUnless(HAS_SHMEM, "requires multiprocessing.shared_memory")
 @hashlib_helper.requires_hashdigest('sha256')
 class _TestSharedMemory(BaseTestCase):
@@ -4662,6 +4676,7 @@ def test_shared_memory_SharedMemoryServer_ignores_sigint(self):
         smm.shutdown()
 
     @unittest.skipIf(os.name != "posix", "resource_tracker is posix only")
+    @resource_tracker_format_subtests
     def test_shared_memory_SharedMemoryManager_reuses_resource_tracker(self):
         # bpo-36867: test that a SharedMemoryManager uses the
         # same resource_tracker process as its parent.
@@ -4913,6 +4928,7 @@ def test_shared_memory_cleaned_after_process_termination(self):
                     "shared_memory objects to clean up at shutdown", err)
 
     @unittest.skipIf(os.name != "posix", "resource_tracker is posix only")
+    @resource_tracker_format_subtests
     def test_shared_memory_untracking(self):
         # gh-82300: When a separate Python process accesses shared memory
         # with track=False, it must not cause the memory to be deleted
@@ -4940,6 +4956,7 @@ def test_shared_memory_untracking(self):
             mem.close()
 
     @unittest.skipIf(os.name != "posix", "resource_tracker is posix only")
+    @resource_tracker_format_subtests
     def test_shared_memory_tracking(self):
         # gh-82300: When a separate Python process accesses shared memory
         # with track=True, it must cause the memory to be deleted when
@@ -7353,13 +7370,18 @@ def test_forkpty(self):
 
 @unittest.skipUnless(HAS_SHMEM, "requires multiprocessing.shared_memory")
 class TestSharedMemoryNames(unittest.TestCase):
-    def test_that_shared_memory_name_with_colons_has_no_resource_tracker_errors(self):
+    @subTests('use_simple_format', (True, False))
+    def test_that_shared_memory_name_with_colons_has_no_resource_tracker_errors(
+            self, use_simple_format):
         # Test script that creates and cleans up shared memory with colon in name
         test_script = textwrap.dedent("""
             import sys
             from multiprocessing import shared_memory
+            from multiprocessing import resource_tracker
             import time
 
+            resource_tracker._resource_tracker._use_simple_format = %s
+
             # Test various patterns of colons in names
             test_names = [
                 "a:b",
@@ -7387,7 +7409,7 @@ def test_that_shared_memory_name_with_colons_has_no_resource_tracker_errors(self
                     sys.exit(1)
 
             print("SUCCESS")
-        """)
+        """ % use_simple_format)
 
         rc, out, err = assert_python_ok("-c", test_script)
         self.assertIn(b"SUCCESS", out)

Misc/NEWS.d/next/Library/2025-12-03-09-36-29.gh-issue-142206.ilwegH.rst

@@ -0,0 +1,4 @@
+The resource tracker in the :mod:`multiprocessing` module can now understand
+messages from older versions of itself. This avoids issues with upgrading
+Python while it is running. (Note that such 'in-place' upgrades are not
+tested.)