Fix race condition in luigi.lock.acquire_for (#2357)

luigi/lock.py

@@ -21,6 +21,7 @@
 """
 from __future__ import print_function
 
+import errno
 import hashlib
 import os
 import sys
@@ -104,8 +105,14 @@ def acquire_for(pid_dir, num_available=1, kill_signal=None):
 
     # Check if there is a pid file corresponding to this name
     if not os.path.exists(pid_dir):
-        os.mkdir(pid_dir)
-        os.chmod(pid_dir, 0o777)
+        # Try/except for handling race condition
+        try:
+            os.mkdir(pid_dir)
+            os.chmod(pid_dir, 0o777)
+        except OSError as exc:
+            if exc.errno != errno.EEXIST:
+                raise
+            pass
 
     # Let variable "pids" be all pids who exist in the .pid-file who are still
     # about running the same command.