Added SafeRename to better handle problems with git processes locking directories.

This solves a problem with "os.rename" sometimes failing with an
exception after cloning a dependency to a temporary directory. It's
possible the dying git processes still hold a handle to the directory.
Since gclient delete the temporary directory regardless of the success
of the process, it results in a lot of GB downloaded for nothing.

SafeRename solves this by retrying a few times if the renaming fails,
sleeping one second every time to get other processes the time to
release their lock on the directory. It gives up retrying after 15 times.

BUG=
R=maruel@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23875041

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@224372 0039d316-1c4b-4281-b951-d872f2087c98

gclient_scm.py

@@ -873,8 +873,8 @@ class GitWrapper(SCMWrapper):
           print(str(e))
           print('Retrying...')
       gclient_utils.safe_makedirs(self.checkout_path)
-      os.rename(os.path.join(tmp_dir, '.git'),
-                os.path.join(self.checkout_path, '.git'))
+      gclient_utils.safe_rename(os.path.join(tmp_dir, '.git'),
+                                os.path.join(self.checkout_path, '.git'))
     finally:
       if os.listdir(tmp_dir):
         print('\n_____ removing non-empty tmp dir %s' % tmp_dir)

gclient_utils.py

@@ -96,6 +96,29 @@ def FileWrite(filename, content, mode='w'):
     f.write(content)
 
 
+def safe_rename(old, new):
+  """Renames a file reliably.
+
+  Sometimes os.rename does not work because a dying git process keeps a handle 
+  on it for a few seconds. An exception is then thrown, which make the program 
+  give up what it was doing and remove what was deleted.
+  The only solution is to catch the exception and try again until it works. 
+  """
+  # roughly 10s
+  retries = 100
+  for i in range(retries):
+    try:
+      os.rename(old, new)
+      break
+    except OSError:
+      if i == (retries - 1):
+        # Give up.
+        raise
+      # retry
+      logging.debug("Renaming failed from %s to %s. Retrying ..." % (old, new))
+      time.sleep(0.1)
+
+
 def rmtree(path):
   """shutil.rmtree() on steroids.
 

tests/gclient_utils_test.py

@@ -38,8 +38,8 @@ class GclientUtilsUnittest(GclientUtilBase):
         'GCLIENT_CHILDREN_LOCK', 'GClientChildren',
         'SplitUrlRevision', 'SyntaxErrorToError', 'UpgradeToHttps', 'Wrapper',
         'WorkItem', 'codecs', 'lockedmethod', 'logging', 'os', 'pipes', 'Queue',
-        're', 'rmtree', 'safe_makedirs', 'stat', 'subprocess', 'subprocess2',
-        'sys', 'tempfile', 'threading', 'time', 'urlparse',
+        're', 'rmtree', 'safe_makedirs', 'safe_rename', 'stat', 'subprocess', 
+        'subprocess2', 'sys', 'tempfile', 'threading', 'time', 'urlparse',
     ]
     # If this test fails, you should add the relevant test.
     self.compareMembers(gclient_utils, members)