Fix "Too many open files" errors when running on large repo on OSX

libcst/codemod/_cli.py

@@ -19,7 +19,7 @@
 from multiprocessing import Process, Queue, cpu_count
 from pathlib import Path
 from queue import Empty
-from typing import AnyStr, List, Optional, Sequence, Union, cast
+from typing import AnyStr, Dict, List, Optional, Sequence, Set, Union, cast
 
 from libcst import PartialParserConfig, parse_module
 from libcst.codemod._codemod import Codemod
@@ -682,30 +682,31 @@ def parallel_exec_transform_with_prettyprint(  # noqa: C901
     pending_processes: List[Process] = []
 
     # Start processes
+    filename_to_process: Dict[str, Process] = {}
     for f in files:
-        pending_processes.append(
-            Process(
-                target=_parallel_exec_process_stub,
-                args=(
-                    queue,
-                    transform,
-                    f,
-                    repo_root,
-                    unified_diff,
-                    include_generated,
-                    generated_code_marker,
-                    format_code,
-                    formatter_args,
-                    blacklist_patterns,
-                    python_version,
-                ),
-            )
+        process = Process(
+            target=_parallel_exec_process_stub,
+            args=(
+                queue,
+                transform,
+                f,
+                repo_root,
+                unified_diff,
+                include_generated,
+                generated_code_marker,
+                format_code,
+                formatter_args,
+                blacklist_patterns,
+                python_version,
+            ),
         )
+        pending_processes.append(process)
+        filename_to_process[f] = process
 
     # Start the processes, allowing no more than num_processes to be running
     # at once.
     results_left = len(pending_processes)
-    joinable_processes: List[Process] = []
+    joinable_processes: Set[Process] = set()
     processes_started = 0
 
     interrupted = False
@@ -714,7 +715,7 @@ def parallel_exec_transform_with_prettyprint(  # noqa: C901
             try:
                 # Move this process to the joinables
                 process = pending_processes.pop(0)
-                joinable_processes.append(process)
+                joinable_processes.add(process)
 
                 # Start it, bookkeep that we did
                 process.start()
@@ -756,6 +757,15 @@ def parallel_exec_transform_with_prettyprint(  # noqa: C901
 
         warnings += len(result.transform_result.warning_messages)
 
+        # Join the process to free any related resources.
+        # Remove all references to the process to allow the GC to
+        # clean up any file handles.
+        process = filename_to_process.pop(result.filename, None)
+        if process:
+            process.join()
+            if process in joinable_processes:
+                joinable_processes.remove(process)
+
     # Now, join on all of them so we don't leave zombies or hang
     for p in joinable_processes:
         p.join()