Fix race between exists() and mkdir()

The code previously could fail if another process made
a directory with the same name after we called os.path.exists()
but before mkdir().

python/saliweb/frontend/submit.py

@@ -42,7 +42,11 @@ def is_job_in_db():
 
     job_dir = _get_job_directory(job_name)
     if not os.path.exists(job_dir) and not is_job_in_db():
-        os.mkdir(job_dir)
+        try:
+            os.mkdir(job_dir)
+        except FileExistsError:
+            # Directory may have been made between exists() check and mkdir()
+            return None
         if not is_job_in_db():  # avoid race condition
             return job_dir