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Sofia will unpredictably close a tls transport during call setup. This
occurs when the epoll event loop wakes up the socket reader and SSL_read
returns an error because there is no packet on the socket. Normally
sofia will read the last error using SSL_get_error and return
SSL_ERROR_WANT_READ. Sofia gracefully handles this error and the
transport stays open. Sometimes, however, the worker thread will call
SSL_shutdown for a different transport, which can write an error to the
internal openssl error queue. If that error is not read off the queue,
the next time that SSL_get_error is called, it will read that unrelated
error.
The documentation for SSL_shutdown explains that there are three
possible results -1, 0 and 1 with, oddly, 1 indicating success. The -1
result code occurs when there is no handshake callback registered on the
connection. It can return 0 when there is still work to be done. The
documentation suggest that it is insufficient to call it just once. This
is why I added the do {} while () construct.
Although just the fix to SSL_shutdown was enough to resolve my issue, I
a also audited other calls to SSL_* functions and found a few other
cases where an error may be generated, but was not handled.