lock: fixed race condition in trylock/testlock (issue3506) stable

lock: fixed race condition in trylock/testlock (issue3506)

Suppose the following scenario:

1. Process A takes the lock (e.g. on commit).
2. Process B wants to grab the lock. Since lock file exists
   the exception is raised. In the catch block the testlock
   function is called.
3. Process A releases the lock.
4. Process B tries to read the lock file as a part of testlock
   function. This results in OSError (ENOENT) and since we're
   not inside the exception handler function this is propagated
   and aborts the whole operation.

To fix this we now check in testlock function whether lock file
actually exists and if not (i.e. if readlock fails) we just return.

--- a/mercurial/lock.py	Sat Sep 29 13:33:55 2012 +0200
+++ b/mercurial/lock.py	Thu Sep 27 14:38:03 2012 -0700
@@ -97,7 +97,12 @@
         The lock file is only deleted when None is returned.
 
         """
-        locker = util.readlock(self.f)
+        try:
+            locker = util.readlock(self.f)
+        except OSError, why:
+            if why.errno == errno.ENOENT:
+                return None
+            raise
         try:
             host, pid = locker.split(":", 1)
         except ValueError: