How to debug a TLSv1.3 protocol problem?

[Claus Assmann]

On Wed, May 20, 2020, Matt Caswell wrote:

[Thanks for still helping me with this!]

> Can you test the underlying socket to see if it has been closed down in
> an orderly way (i.e. recv() should return 0 on the socket if so)? That

Yes, that what happens.

So I added some debugging to the client and it shows:

SSL info:
read server hello
write change cipher spec
write client hello

and I/O layer (only last/relevant part):
want_write=523, want_read=5
... write ... and then
want_read=5 -< read=OK, num_read=5
want_write=0, want_read=1
want_read=1 -> read=OK, num_read=1
want_write=0, want_read=5
want_read=5 -> read=TIMEOUT, num_read=0

Explanation:
want_write = BIO_ctrl_pending(network_bio)
want_read = BIO_ctrl_get_read_request(network_bio)

I didn't instrument the write part, only the read part.
want_read>0 -> invoke read, check result:  read=STATUS, num_read=N
So the last read does not get the data the library wants,
and hence the client fails and closes the connection.
In that moment the server fails in SSL_accept and issues
the misleading error message.

Now the question seems to be:
did the server actually not send the data (not sure how to check
that, maybe looking at tcpdump?)
or has the client a bug not reading the data?