[fix] Create a shim class providing compat for Pool implementations

On Windows and MacOS the multiprocess library's Pool class is used,
whereas on Linux and other platforms the concurrent.futures library's
ProcessPoolExecutor is used.  These two classes have similar but not
identical APIs.  This commit introduces a shim class which adapts the
API of multiprocess.Pool to be more similar to ProcessPoolExecutor.

This commit will close Ericsson#4338.

Author: frankplow

codechecker_common/compatibility/multiprocessing.py

@@ -13,8 +13,17 @@
 # pylint: disable=no-name-in-module
 # pylint: disable=unused-import
 if sys.platform in ["darwin", "win32"]:
-    from multiprocess import Pool  # type: ignore
+    from multiprocess.pool import Pool as MultiprocessPool
     from multiprocess import cpu_count
+
+    # Shim class intended to provide API compatibility between the
+    # pools from multiprocess and concurrent.futures.
+    class Pool(MultiprocessPool): # type: ignore
+        def __init__(self, *args, max_workers=None, **kwargs):
+            super().__init__(*args, processes=max_workers, **kwargs)
+
+        def map(self, fn, *iterables, chunksize=1):
+            return super().starmap(fn, zip(*iterables), chunksize)
 else:
     from concurrent.futures import ProcessPoolExecutor as Pool  # type: ignore
     from multiprocessing import cpu_count