core: added a running flag to prevent panics in the chainmanager

The running flag will determine whether the chain manager is still
running or not. This will prevent the quit channel from being closed
twice resulting in a panic. This PR should fix this issue.

Closes #1559

core/chain_manager.go

@@ -76,7 +76,8 @@ type ChainManager struct {
 	cache        *lru.Cache // cache is the LRU caching
 	futureBlocks *lru.Cache // future blocks are blocks added for later processing
 
-	quit chan struct{}
+	quit    chan struct{}
+	running int32 // running must be called automically
 	// procInterrupt must be atomically called
 	procInterrupt int32 // interrupt signaler for block processing
 	wg            sync.WaitGroup
@@ -443,6 +444,9 @@ func (bc *ChainManager) setTotalDifficulty(td *big.Int) {
 }
 
 func (bc *ChainManager) Stop() {
+	if !atomic.CompareAndSwapInt32(&bc.running, 0, 1) {
+		return
+	}
 	close(bc.quit)
 	atomic.StoreInt32(&bc.procInterrupt, 1)