Beast: "Assertion 'ws_.wr_block_ == tok_' failed" in write.ipp, line 468, Boost 1.66

[fpingas]

Hi,
I am trying to establish a websocket session, that keeps a pending async_read command, while processing a queue of write requests, one async_write at a time.

I keep running into this assert even with only a single thread running the pertinent io_context. I have tried to keep all the calls to these asynchronous operations on explicit strands, but I still run into the same error.

It is, to my understanding, possible to maintain these two concurrent operations, given the correct use of strands. Still, I have been unable to make it work as I expected. I would appreciate some guidance in this issue.

This happens with Beast 144 with Boost 1.66, on Windows 10 with mingw 5.3.0 32bit.

I can reproduced the issue with the following routine:
https://gist.github.com/Pinguinha/f282e049102cec63f28053e3622166bb

Thanks in advance.

[vinniefalco]

Just a note, in your function onWrite if you just always post the string to the strand, and then append to the queue or initiate a send inside the posted completion handler, you won't need a mutex. Strands already come with a mutex, you're adding another one on top of it.

I am investigating, thanks for the report.

[vinniefalco]

Update: I was able to build the example program after fixing a trivial compilation error in the construction of the endpoint. I also get the assert. Nothing looks wrong with your program at first glance, but the assert means an invariant has been broken so I am continuing to study it - thanks!

[djarek]

@Pinguinha
Probably not the root cause, but after a 5 minute glance I noticed you might have a data race in main() in line 228:

client1->socket().async_close(boost::beast::websocket::close_code::normal, [](auto){});

Looks like the access to this socket is not protected with the strand used elsewhere in WssSession

[vinniefalco]

I get the assert before the async_close ever has a chance to run but you're right, it needs a strand (implicit or explicit).

[bostjanv]

I think I've stumbled upon the same issue. I made small modifications to websocket_client_async.cpp and websocket_server_async.cpp examples which are able to trigger the assert. Please take a look:

https://gist.github.com/bostjanv/5f7d6a6867f6e7e022176494730780db
https://gist.github.com/bostjanv/7240bbd40976e35696942133da684bec

I'm using Beast from Boost 1.66 and gcc 7.2.0.

[vinniefalco]

This took way longer than it should have to figure out but now we know the problem and it will be fixed in Boost 1.67.0 (and the master/develop branches here). Thanks for reporting this and thanks a ton for the helpful example program, without which I could not have tracked down the problem. If you want a cheap temporary fix, change the token class in <boost/beast/websocket/stream.hpp> to use unsigned long instead of char. I will publish a solid fix shortly.

[bostjanv]

This is good news! Thanks for taking the time and figuring it out.