sync: Fix semaphore release bug that causes thread 'leaks'

When repo syncs a manifest that utilizes multiple branches
in the same project, then the sync will use an extra
thread for each "duplicate".  For example, if
the manifest includes the project "foo" and "bar"
twice, then "repo sync -jN" will fetch with N+2 threads.

This is caused by _FetchHelper() releasing the thread semaphore
object each time it's called, even though _FetchProjectList()
may call this function multiple times within the scope of a
single thread.

Fix by moving the thread semaphore release to
_FetchProjectList(), which is only called once per thread
instance.

Change-Id: I1da78b145e09524d40457db5ca5c37d315432bd8

subcmds/sync.py

@@ -255,7 +255,7 @@ later is required to fix a server side protocol bug.
                  dest='repo_upgraded', action='store_true',
                  help=SUPPRESS_HELP)
 
-  def _FetchProjectList(self, opt, projects, *args, **kwargs):
+  def _FetchProjectList(self, opt, projects, sem, *args, **kwargs):
     """Main function of the fetch threads when jobs are > 1.
 
     Delegates most of the work to _FetchHelper.
@@ -263,15 +263,20 @@ later is required to fix a server side protocol bug.
     Args:
       opt: Program options returned from optparse.  See _Options().
       projects: Projects to fetch.
+      sem: We'll release() this semaphore when we exit so that another thread
+          can be started up.
       *args, **kwargs: Remaining arguments to pass to _FetchHelper. See the
           _FetchHelper docstring for details.
     """
-    for project in projects:
-      success = self._FetchHelper(opt, project, *args, **kwargs)
-      if not success and not opt.force_broken:
-        break
+    try:
+        for project in projects:
+          success = self._FetchHelper(opt, project, *args, **kwargs)
+          if not success and not opt.force_broken:
+            break
+    finally:
+        sem.release()
 
-  def _FetchHelper(self, opt, project, lock, fetched, pm, sem, err_event):
+  def _FetchHelper(self, opt, project, lock, fetched, pm, err_event):
     """Fetch git objects for a single project.
 
     Args:
@@ -283,8 +288,6 @@ later is required to fix a server side protocol bug.
           (with our lock held).
       pm: Instance of a Project object.  We will call pm.update() (with our
           lock held).
-      sem: We'll release() this semaphore when we exit so that another thread
-          can be started up.
       err_event: We'll set this event in the case of an error (after printing
           out info about the error).
 
@@ -340,7 +343,6 @@ later is required to fix a server side protocol bug.
     finally:
       if did_lock:
         lock.release()
-      sem.release()
 
     return success
 
@@ -365,10 +367,10 @@ later is required to fix a server side protocol bug.
       sem.acquire()
       kwargs = dict(opt=opt,
                     projects=project_list,
+                    sem=sem,
                     lock=lock,
                     fetched=fetched,
                     pm=pm,
-                    sem=sem,
                     err_event=err_event)
       if self.jobs > 1:
         t = _threading.Thread(target = self._FetchProjectList,