From bdd9130c05828a158cb8088eec3561f8b1d0f956 Mon Sep 17 00:00:00 2001
From: Mission Liao <mission.liao@dexon.org>
Date: Tue, 12 Mar 2019 11:51:26 +0800
Subject: core/syncer: fix syncer deadlock (#479)

* Fix dead lock

* core/syncer: prevent selecting on a nil channel

* Remove unnecessary reader lock holding
---
 core/syncer/consensus.go | 44 ++++++++++++++++++++++++--------------------
 1 file changed, 24 insertions(+), 20 deletions(-)

(limited to 'core')

diff --git a/core/syncer/consensus.go b/core/syncer/consensus.go
index 3053396..25911ce 100644
--- a/core/syncer/consensus.go
+++ b/core/syncer/consensus.go
@@ -250,11 +250,6 @@ func (con *Consensus) SyncBlocks(
 		// syncing is done.
 		if con.checkIfSynced(blocks) {
 			con.stopBuffering()
-			con.dummyCancel, con.dummyFinished = utils.LaunchDummyReceiver(
-				context.Background(), con.network.ReceiveChan(),
-				func(msg interface{}) {
-					con.dummyMsgBuffer = append(con.dummyMsgBuffer, msg)
-				})
 			con.syncedLastBlock = blocks[len(blocks)-1]
 			synced = true
 		}
@@ -302,24 +297,38 @@ func (con *Consensus) GetSyncedConsensus() (*core.Consensus, error) {
 // This method is mainly for caller to stop the syncer before synced, the syncer
 // would call this method automatically after being synced.
 func (con *Consensus) stopBuffering() {
-	if func() bool {
+	if func() (notBuffering bool) {
 		con.lock.RLock()
 		defer con.lock.RUnlock()
-		return !con.duringBuffering
+		notBuffering = !con.duringBuffering
+		return
 	}() {
 		return
 	}
-	con.lock.Lock()
-	defer con.lock.Unlock()
-	if !con.duringBuffering {
+	if func() (alreadyCanceled bool) {
+		con.lock.Lock()
+		defer con.lock.Unlock()
+		if !con.duringBuffering {
+			alreadyCanceled = true
+			return
+		}
+		con.duringBuffering = false
+		con.logger.Trace("syncer is about to stop")
+		// Stop network and CRS routines, wait until they are all stoped.
+		con.ctxCancel()
+		return
+	}() {
 		return
 	}
-	con.duringBuffering = false
-	con.logger.Trace("syncer is about to stop")
-	// Stop network and CRS routines, wait until they are all stoped.
-	con.ctxCancel()
 	con.logger.Trace("stop syncer modules")
 	con.moduleWaitGroup.Wait()
+	// Since there is no one waiting for the receive channel of fullnode, we
+	// need to launch a dummy receiver right away.
+	con.dummyCancel, con.dummyFinished = utils.LaunchDummyReceiver(
+		context.Background(), con.network.ReceiveChan(),
+		func(msg interface{}) {
+			con.dummyMsgBuffer = append(con.dummyMsgBuffer, msg)
+		})
 	// Stop agreements.
 	con.logger.Trace("stop syncer agreement modules")
 	con.stopAgreement()
@@ -506,11 +515,6 @@ func (con *Consensus) startCRSMonitor() {
 			con.latestCRSRound = round
 		}()
 		for func() bool {
-			con.lock.RLock()
-			defer con.lock.RUnlock()
-			if !con.duringBuffering {
-				return false
-			}
 			select {
 			case <-con.ctx.Done():
 				return false
@@ -552,9 +556,9 @@ func (con *Consensus) startCRSMonitor() {
 func (con *Consensus) stopAgreement() {
 	if con.agreementModule.inputChan != nil {
 		close(con.agreementModule.inputChan)
-		con.agreementModule.inputChan = nil
 	}
 	con.agreementWaitGroup.Wait()
+	con.agreementModule.inputChan = nil
 	close(con.receiveChan)
 	close(con.pullChan)
 }
-- 
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