Write config blocks synchronously in Orderer

Assume the config block contains a transaction which removes the
node from consenters, the consensus chain will switch to follower
chain which reads last config block from ledger directly. Because of
writing config block asynchronously, follower chain maybe read a
stale config block in which the node is still a consenter. So the
node will switch back to a consenter again.

Writting config blocks synchronously would fix this bug.

Signed-off-by: Alex-duzhichao <duzhichaomail@gmail.com>

orderer/common/multichannel/blockwriter.go

@@ -156,7 +156,7 @@ func (bw *BlockWriter) WriteConfigBlock(block *cb.Block, encodedMetadataValue []
 		logger.Panicf("Told to write a config block with unknown header type: %v", chdr.Type)
 	}
 
-	bw.WriteBlock(block, encodedMetadataValue)
+	bw.WriteBlockSync(block, encodedMetadataValue)
 }
 
 // WriteBlock should be invoked for blocks which contain normal transactions.
@@ -175,6 +175,23 @@ func (bw *BlockWriter) WriteBlock(block *cb.Block, encodedMetadataValue []byte)
 	}()
 }
 
+// WriteBlockSync is same as WriteBlock, but commits block synchronously.
+// Note: WriteConfigBlock should use WriteBlockSync instead of WriteBlock.
+//       If the block contains a transaction that remove the node from consenters,
+//       the node will switch to follower and pull blocks from other nodes.
+//       Suppose writing block asynchronously, the block maybe not persist to disk
+//       when the follower chain starts working. The follower chain will read a block
+//       before the config block, in which the node is still a consenter, so the follower
+//       chain will switch to the consensus chain. That's a dead loop!
+//       So WriteConfigBlock should use WriteBlockSync instead of WriteBlock.
+func (bw *BlockWriter) WriteBlockSync(block *cb.Block, encodedMetadataValue []byte) {
+	bw.committingBlock.Lock()
+	bw.lastBlock = block
+
+	defer bw.committingBlock.Unlock()
+	bw.commitBlock(encodedMetadataValue)
+}
+
 // commitBlock should only ever be invoked with the bw.committingBlock held
 // this ensures that the encoded config sequence numbers stay in sync
 func (bw *BlockWriter) commitBlock(encodedMetadataValue []byte) {