bpo-38377: Add support.skip_if_broken_multiprocessing_synchronize() (GH-20944) (GH-20962) (GH-20966)

On Linux, skip tests using multiprocessing if the current user cannot
create a file in /dev/shm/ directory. Add the
skip_if_broken_multiprocessing_synchronize() function to the
test.support module.

(cherry picked from commit ddbeb2f3e02a510c5784ffd74c5e09e8c70b5881)
(cherry picked from commit b1e736113484c99acb57e4acb417b91a9e58e7ff)

1 files changed, 23 insertions, 0 deletions

diff --git a/Lib/test/support/__init__.py b/Lib/test/support/__init__.py
index 0906e7a..b75dbd2 100644
--- a/Lib/test/support/__init__.py
+++ b/Lib/test/support/__init__.py
@@ -3377,3 +3377,26 @@ def save_restore_warnings_filters():
         yield
     finally:
         warnings.filters[:] = old_filters
+
+
+def skip_if_broken_multiprocessing_synchronize():
+    """
+    Skip tests if the multiprocessing.synchronize module is missing, if there
+    is no available semaphore implementation, or if creating a lock raises an
+    OSError.
+    """
+
+    # Skip tests if the _multiprocessing extension is missing.
+    import_module('_multiprocessing')
+
+    # Skip tests if there is no available semaphore implementation:
+    # multiprocessing.synchronize requires _multiprocessing.SemLock.
+    synchronize = import_module('multiprocessing.synchronize')
+
+    try:
+        # bpo-38377: On Linux, creating a semaphore is the current user
+        # does not have the permission to create a file in /dev/shm.
+        # Create a semaphore to check permissions.
+        synchronize.Lock(ctx=None)
+    except OSError as exc:
+        raise unittest.SkipTest(f"broken multiprocessing SemLock: {exc!r}")