qmemman: make memory request handling more defensive

If getting memory for new VM fails for any reason, make sure that global
lock will be released. Otherwise qmemman will stop functioning at all.

QubesOS/qubes-issues#1636

qmemman/qmemman_server.py

@@ -175,6 +175,7 @@ class QMemmanReqHandler(SocketServer.BaseRequestHandler):
         self.log = logging.getLogger('qmemman.daemon.reqhandler')
 
         got_lock = False
+        try:
             # self.request is the TCP socket connected to the client
             while True:
                 self.data = self.request.recv(1024).strip()
@@ -184,8 +185,6 @@ class QMemmanReqHandler(SocketServer.BaseRequestHandler):
                     if got_lock:
                         global force_refresh_domain_list
                         force_refresh_domain_list = True
-                    global_lock.release()
-                    self.log.debug('global_lock released')
                     return
 
                 # XXX something is wrong here: return without release?
@@ -204,8 +203,13 @@ class QMemmanReqHandler(SocketServer.BaseRequestHandler):
                     resp = "FAIL\n"
                 self.log.debug('resp={!r}'.format(resp))
                 self.request.send(resp)
-
-            # XXX no release of lock?
+        except BaseException as e:
+            self.log.exception(
+                "exception while handling request: {!r}".format(e))
+        finally:
+            if got_lock:
+                global_lock.release()
+                self.log.debug('global_lock released')
 
 
 def start_server(server):