storage: in deadlock avoidance, fix performance issue that could freeze the cluster

In the worst case, with many clients trying to lock the same oids,
the cluster could enter in an infinite cascade of deadlocks.

Here is an overview with 3 storage nodes and 3 transactions:

 S1     S2     S3     order of locking tids          # abbreviations:
 l1     l1     l2     123                            #  l: lock
 q23    q23    d1q3   231                            #  d: deadlock triggered
 r1:l3  r1:l2  (r1)   # for S3, we still have l2     #  q: queued
 d2q1   q13    q13    312                            #  r: rebase

Above, we show what happens when a random transaction gets a lock just after
that another is rebased. Here, the result is that the last 2 lines are a
permutation of the first 2, and this can repeat indefinitely with bad luck.

This commit reduces the probability of deadlock by processing delayed
stores/checks in the order of their locking tid. In the above example,
S1 would give the lock to 2 when 1 is rebased, and 2 would vote successfully.

neo/lib/handler.py

@@ -70,9 +70,9 @@ class EventHandler(object):
                 raise UnexpectedPacketError('no handler found')
             args = packet.decode() or ()
             method(conn, *args, **kw)
-        except DelayEvent:
+        except DelayEvent, e:
             assert not kw, kw
-            self.getEventQueue().queueEvent(method, conn, args)
+            self.getEventQueue().queueEvent(method, conn, args, *e.args)
         except UnexpectedPacketError, e:
             if not conn.isClosed():
                 self.__unexpectedPacket(conn, packet, *e.args)
@@ -311,35 +311,54 @@ class _DelayedConnectionEvent(EventHandler):
 class EventQueue(object):
 
     def __init__(self):
-        self._event_queue = deque()
+        self._event_queue = []
         self._executing_event = -1
 
-    def queueEvent(self, func, conn=None, args=()):
-        self._event_queue.append(func if conn is None else
-            _DelayedConnectionEvent(func, conn, args))
+    def queueEvent(self, func, conn=None, args=(), key=None):
+        assert self._executing_event < 0, self._executing_event
+        self._event_queue.append((key, func if conn is None else
+            _DelayedConnectionEvent(func, conn, args)))
+        if key is not None:
+            self._event_queue.sort()
+
+    def sortAndExecuteQueuedEvents(self):
+        if self._executing_event < 0:
+            self._event_queue.sort()
+            self.executeQueuedEvents()
+        else:
+            # We can't sort events when they're being processed.
+            self._executing_event = 1
 
     def executeQueuedEvents(self):
         # Not reentrant. When processing a queued event, calling this method
         # only tells the caller to retry all events from the beginning, because
         # events for the same connection must be processed in chronological
         # order.
+        queue = self._event_queue
+        if queue: # return quickly if the queue is empty
             self._executing_event += 1
             if self._executing_event:
                 return
-        queue = self._event_queue
-        n = len(queue)
-        while n:
+            done = []
+            while 1:
+                try:
+                    for i, event in enumerate(queue):
                         try:
-                queue[0]()
+                            event[1]()
+                            done.append(i)
                         except DelayEvent:
-                queue.rotate(-1)
-            else:
-                del queue[0]
-            n -= 1
+                            pass
                         if self._executing_event:
+                            break
+                    else:
+                        break
+                finally:
+                    while done:
+                        del queue[done.pop()]
                 self._executing_event = 0
-                queue.rotate(-n)
-                n = len(queue)
+                # What sortQueuedEvents could not do immediately is done here:
+                if event[0] is not None:
+                    queue.sort()
             self._executing_event = -1
 
     def logQueuedEvents(self):

neo/storage/checker.py

@@ -109,7 +109,7 @@ class Checker(object):
         self.source = source
         def start():
             if app.tm.isLockedTid(max_tid):
-                app.tm.queueEvent(start)
+                app.tm.read_queue.queueEvent(start)
                 return
             args = partition, CHECK_COUNT, min_tid, max_tid
             p = Packets.AskCheckTIDRange(*args)

neo/storage/handlers/client.py

@@ -40,7 +40,7 @@ class ClientOperationHandler(BaseHandler):
 
     def getEventQueue(self):
         # for read rpc
-        return self.app.tm
+        return self.app.tm.read_queue
 
     def askObject(self, conn, oid, serial, tid):
         app = self.app
@@ -106,10 +106,11 @@ class ClientOperationHandler(BaseHandler):
         try:
             self._askStoreObject(conn, oid, serial, compression,
                 checksum, data, data_serial, ttid, None)
-        except DelayEvent:
+        except DelayEvent, e:
             # locked by a previous transaction, retry later
             self.app.tm.queueEvent(self._askStoreObject, conn, (oid, serial,
-                compression, checksum, data, data_serial, ttid, time.time()))
+                compression, checksum, data, data_serial, ttid, time.time()),
+            *e.args)
 
     def askRebaseTransaction(self, conn, *args):
         conn.answer(Packets.AnswerRebaseTransaction(
@@ -118,10 +119,10 @@ class ClientOperationHandler(BaseHandler):
     def askRebaseObject(self, conn, ttid, oid):
         try:
             self._askRebaseObject(conn, ttid, oid, None)
-        except DelayEvent:
+        except DelayEvent, e:
             # locked by a previous transaction, retry later
             self.app.tm.queueEvent(self._askRebaseObject,
-                conn, (ttid, oid, time.time()))
+                conn, (ttid, oid, time.time()), *e.args)
 
     def _askRebaseObject(self, conn, ttid, oid, request_time):
         conflict = self.app.tm.rebaseObject(ttid, oid)
@@ -188,10 +189,10 @@ class ClientOperationHandler(BaseHandler):
         self.app.tm.register(conn, ttid)
         try:
             self._askCheckCurrentSerial(conn, ttid, oid, serial, None)
-        except DelayEvent:
+        except DelayEvent, e:
             # locked by a previous transaction, retry later
             self.app.tm.queueEvent(self._askCheckCurrentSerial,
-                conn, (ttid, oid, serial, time.time()))
+                conn, (ttid, oid, serial, time.time()), *e.args)
 
     def _askCheckCurrentSerial(self, conn, ttid, oid, serial, request_time):
         try:

neo/storage/transactions.py

@@ -84,14 +84,11 @@ class Transaction(object):
 class TransactionManager(EventQueue):
     """
         Manage pending transaction and locks
-
-    XXX: EventQueue is not very suited for deadlocks. It would be more
-         efficient to sort delayed packets by locking tid in order to minimize
-         cascaded deadlocks.
     """
 
     def __init__(self, app):
         EventQueue.__init__(self)
+        self.read_queue = EventQueue()
         self._app = app
         self._transaction_dict = {}
         self._store_lock_dict = {}
@@ -208,8 +205,9 @@ class TransactionManager(EventQueue):
                 self._notifyReplicated()
         # Some locks were released, some pending locks may now succeed.
         # We may even have delayed stores for this transaction, like the one
-        # that triggered the deadlock.
-        self.executeQueuedEvents()
+        # that triggered the deadlock. They must also be sorted again because
+        # our locking tid has changed.
+        self.sortAndExecuteQueuedEvents()
 
     def rebase(self, conn, ttid, locking_tid):
         self.register(conn, ttid)
@@ -337,7 +335,7 @@ class TransactionManager(EventQueue):
                 # but this is not a problem. EventQueue processes them in order
                 # and only the last one will not result in conflicts (that are
                 # already resolved).
-                raise DelayEvent
+                raise DelayEvent(transaction)
             if oid in transaction.lockless:
                 # This is a consequence of not having taken a lock during
                 # replication. After a ConflictError, we may be asked to "lock"
@@ -358,7 +356,7 @@ class TransactionManager(EventQueue):
                 self._app.master_conn.send(Packets.NotifyDeadlock(
                     ttid, transaction.locking_tid))
                 self._rebase(transaction, ttid)
-                raise DelayEvent
+                raise DelayEvent(transaction)
             # If previous store was an undo, next store must be based on
             # undo target.
             try:
@@ -387,7 +385,7 @@ class TransactionManager(EventQueue):
                 return
         elif transaction.locking_tid == MAX_TID:
             # Deadlock avoidance. Still no new locking_tid from the client.
-            raise DelayEvent
+            raise DelayEvent(transaction)
         else:
             previous_serial = self._app.dm.getLastObjectTID(oid)
         # Locking before reporting a conflict would speed up the case of
@@ -521,6 +519,7 @@ class TransactionManager(EventQueue):
         if self._replicated:
             self._notifyReplicated()
         # some locks were released, some pending locks may now succeed
+        self.read_queue.executeQueuedEvents()
         self.executeQueuedEvents()
 
     def abortFor(self, uuid):
@@ -551,6 +550,7 @@ class TransactionManager(EventQueue):
         for oid, ttid in self._store_lock_dict.iteritems():
             logging.info('    %s by %s', dump(oid), dump(ttid))
         self.logQueuedEvents()
+        self.read_queue.logQueuedEvents()
 
     def updateObjectDataForPack(self, oid, orig_serial, new_serial, data_id):
         lock_tid = self.getLockingTID(oid)

neo/tests/threaded/test.py

@@ -2060,6 +2060,40 @@ class Test(NEOThreadedTest):
         self.assertEqual(end, {0: ['AnswerRebaseTransaction',
                                    'StoreTransaction', 'VoteTransaction']})
 
+    @with_cluster()
+    def testDelayedStoreOrdering(self, cluster):
+        """
+        By processing delayed stores (EventQueue) in the order of their locking
+        tid, we minimize the number deadlocks. Here, we trigger a first
+        deadlock, so that the delayed check for t1 is reordered after that of
+        t3.
+        """
+        t1, c1 = cluster.getTransaction()
+        r = c1.root()
+        for x in 'abcd':
+            r[x] = PCounter()
+            t1.commit()
+        r['a'].value += 1
+        self.readCurrent(r['d'])
+        t2, c2 = cluster.getTransaction()
+        r = c2.root()
+        r['b'].value += 1
+        self.readCurrent(r['d'])
+        t3, c3 = cluster.getTransaction()
+        r = c3.root()
+        r['c'].value += 1
+        self.readCurrent(r['d'])
+        threads = map(self.newPausedThread, (t2.commit, t3.commit))
+        with self.thread_switcher(threads, (1, 2, 0, 1, 2, 1, 0, 2, 0, 1, 2),
+            ('tpc_begin', 'tpc_begin', 'tpc_begin', 1, 2, 3, 4, 4, 4,
+             'RebaseTransaction', 'StoreTransaction')) as end:
+            t1.commit()
+            for t in threads:
+                t.join()
+        self.assertEqual(end, {
+            0: ['AnswerRebaseTransaction', 'StoreTransaction'],
+            2: ['StoreTransaction']})
+
     @with_cluster(replicas=1)
     def testConflictAfterDeadlockWithSlowReplica1(self, cluster,
                                                   slow_rebase=False):