The node reported "Handle sync block error" and no blocks were synchronized in the next 20 seconds.

[liuyifei001]

System information

Java-tron version: v4.7.4
OS & Version: Linux & macOS

Expected behavior

Synchronization of blocks should be normal, no exceptions should occur.

Actual behavior

The node reported a Handle sync block error exception, and no blocks were synchronized for 20 seconds after the exception occurred.

[zeusoo001]

@liuyifei001 The pop function threw an exception. It should be that syncBlockToFetch object has multi-threaded access.

if (blockId.equals(peer.getSyncBlockToFetch().peek())) {
        peer.getSyncBlockToFetch().pop();

[jwrct]

@zeusoo001 Can you please describe in detail the scenario of multiple-thread access? For example, which threads will access syncBlockToFetch? What scenarios can cause program exceptions?

[zeusoo001]

@jwrct Three threads accessed the syncBlockToFetch variable at the same time.

Thread 1: When the peer disconnects, the clear method of the object is called.
Thread 2: When processing the synchronization block, the pop method of the object is called.
Thread 3: When processing the chain inventory message, the pop method of the object is called.

Concurrent access by thread 1 and thread 2 may cause the exception. When thread 2 calls the peek method of the object, then thread 1 clears the data, and then an exception is thrown when thread 2 pops the data.

[jwrct]

@zeusoo001 As you said, I have a question, do Thread 1 and Thread 3 have concurrent access to the syncBlockToFetch issue? What about Thread 2 and Thread 3?

[zeusoo001]

@jwrct Thread 3 uses the try catch statement to solve the concurrency problem, and there is no problem. A lock is used between thread 2 and thread 3, so there is no concurrency problem.

[zeusoo001]

Fixed in this PR: #5832