Fix long-standing race condition bug in locking unittest

Every once in a while we saw failures in the locking unittest, but
weren't able to determine the cause. This patch fixes it by using
threading.Event instead of plain threading.Condition to notify another
thread. In most cases, threading.Condition must be used with another
variable to keep the actual state.  threading.Event does this for us.
Otherwise it can happen that the waiter only gets the lock after
condition was notified.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

test/ganeti.locking_unittest.py

@@ -604,7 +604,7 @@ class TestSharedLock(_ThreadedTestCase):
 
   @_Repeat
   def testMixedAcquireTimeout(self):
-    sync = threading.Condition()
+    sync = threading.Event()
 
     def _AcquireShared(ev):
       if not self.sl.acquire(shared=1, timeout=None):
@@ -615,12 +615,8 @@ class TestSharedLock(_ThreadedTestCase):
       # Notify main thread
       ev.set()
 
-      # Wait for notification
-      sync.acquire()
-      try:
-        sync.wait()
-      finally:
-        sync.release()
+      # Wait for notification from main thread
+      sync.wait()
 
       # Release lock
       self.sl.release()
@@ -641,7 +637,7 @@ class TestSharedLock(_ThreadedTestCase):
     self.failIf(self.sl.acquire(shared=0, timeout=0.02))
 
     # Acquire exclusive without timeout
-    exclsync = threading.Condition()
+    exclsync = threading.Event()
     exclev = threading.Event()
 
     def _AcquireExclusive():
@@ -653,11 +649,8 @@ class TestSharedLock(_ThreadedTestCase):
       # Notify main thread
       exclev.set()
 
-      exclsync.acquire()
-      try:
-        exclsync.wait()
-      finally:
-        exclsync.release()
+      # Wait for notification from main thread
+      exclsync.wait()
 
       self.sl.release()
 
@@ -667,11 +660,7 @@ class TestSharedLock(_ThreadedTestCase):
     self.failIf(self.sl.acquire(shared=0, timeout=0.02))
 
     # Make all shared holders release their locks
-    sync.acquire()
-    try:
-      sync.notifyAll()
-    finally:
-      sync.release()
+    sync.set()
 
     # Wait for exclusive acquire to succeed
     exclev.wait()
@@ -690,11 +679,7 @@ class TestSharedLock(_ThreadedTestCase):
       self._addThread(target=_AcquireSharedSimple)
 
     # Tell exclusive lock to release
-    exclsync.acquire()
-    try:
-      exclsync.notifyAll()
-    finally:
-      exclsync.release()
+    exclsync.set()
 
     # Wait for everything to finish
     self._waitThreads()