Restructure Checker Manager for serial retries

flake8/checker.py

@@ -1,4 +1,5 @@
 """Checker Manager and Checker classes."""
+import errno
 import logging
 import os
 import sys
@@ -15,6 +16,21 @@ from flake8 import utils
 
 LOG = logging.getLogger(__name__)
 
+SERIAL_RETRY_ERRNOS = set([
+    # ENOSPC: Added by sigmavirus24
+    # > On some operating systems (OSX), multiprocessing may cause an
+    # > ENOSPC error while trying to trying to create a Semaphore.
+    # > In those cases, we should replace the customized Queue Report
+    # > class with pep8's StandardReport class to ensure users don't run
+    # > into this problem.
+    # > (See also: https://gitlab.com/pycqa/flake8/issues/74)
+    errno.ENOSPC,
+    # NOTE(sigmavirus24): When adding to this list, include the reasoning
+    # on the lines before the error code and always append your error
+    # code. Further, please always add a trailing `,` to reduce the visual
+    # noise in diffs.
+])
+
 
 class Manager(object):
     """Manage the parallelism and checker instances for each plugin and file.
@@ -62,8 +78,6 @@ class Manager(object):
 
         if self.jobs > 1:
             self.using_multiprocessing = True
-            self.process_queue = multiprocessing.Queue()
-            self.results_queue = multiprocessing.Queue()
 
     @staticmethod
     def _cleanup_queue(q):
@@ -232,28 +246,52 @@ class Manager(object):
         if not self.using_multiprocessing:
             self._report_after_serial()
 
+    def run_parallel(self):
+        """Run the checkers in parallel."""
+        # NOTE(sigmavirus24): Initialize Queues here to handle serial retries
+        # in one place.
+        self.process_queue = multiprocessing.Queue()
+        self.results_queue = multiprocessing.Queue()
+
+        LOG.info('Starting %d process workers', self.jobs - 1)
+        for i in range(self.jobs - 1):
+            proc = multiprocessing.Process(
+                target=self._run_checks_from_queue
+            )
+            proc.daemon = True
+            proc.start()
+            self.processes.append(proc)
+        proc = multiprocessing.Process(target=self._report_after_parallel)
+        proc.start()
+        LOG.info('Started process to report errors')
+        self.processes.append(proc)
+
+    def run_serial(self):
+        """Run the checkers in serial."""
+        for checker in self.checkers:
+            checker.run_checks(self.results_queue)
+
     def run(self):
         """Run all the checkers.
 
-        This handles starting the process workers or just simply running all
-        of the checks in serial.
+        This will intelligently decide whether to run the checks in parallel
+        or whether to run them in serial.
+
+        If running the checks in parallel causes a problem (e.g.,
+        https://gitlab.com/pycqa/flake8/issues/74) this also implements
+        fallback to serial processing.
         """
-        if self.using_multiprocessing:
-            LOG.info('Starting %d process workers', self.jobs)
-            for i in range(self.jobs):
-                proc = multiprocessing.Process(
-                    target=self._run_checks_from_queue
-                )
-                proc.daemon = True
-                proc.start()
-                self.processes.append(proc)
-            proc = multiprocessing.Process(target=self._report_after_parallel)
-            proc.start()
-            LOG.info('Started process to report errors')
-            self.processes.append(proc)
-        else:
-            for checker in self.checkers:
-                checker.run_checks(self.results_queue)
+        try:
+            if self.using_multiprocessing:
+                self.run_parallel()
+            else:
+                self.run_serial()
+        except OSError as oserr:
+            if oserr.errno not in SERIAL_RETRY_ERRNOS:
+                LOG.exception(oserr)
+                raise
+            LOG.warning('Running in serial after OS exception, %r', oserr)
+            self.run_serial()
 
     def start(self):
         """Start checking files."""