lock: block signal interrupt while making a lock file On Windows where symlink isn't supported, util.makelock() could leave an empty file if interrupted immediately after os.open(). This empty lock never dies as it has no process id recorded. ld = os.open(pathname, os.O_CREAT | os.O_WRONLY | os.O_EXCL) # an interrupt may occur here os.write(ld, info) os.close(ld) This was a long-standing bug of TortoiseHg which runs a command-server and kills it by CTRL_C_EVENT, reported by random Windows users. https://bitbucket.org/tortoisehg/thg/issues/4873/#comment-43591129 At first, I tried to fix makelock() to clean up a stale lock file, which turned out to be hard because any instructions may be interrupted by a signal. ld = None try: # CALL_FUNCTION # os.open(...) # an interrupt may occur here # STORE_FAST # ld = ... ld = os.open(pathname, os.O_CREAT | os.O_WRONLY | os.O_EXCL) os.write(ld, info) ... return True except: if ld: ... os.unlink(pathname) return False So I decided to block signals by temporarily replacing the signal handlers so makelcok() and held = 1 will never be interrupted. Many thanks to Fernando Najera for investigating the issue.