mbedtls and detecting non-blocking reads during handshaking

[nkolban]

If we attempt to use a non-blocking socket with mbedtls we receive a failure. I dug deeply into this and believe I have found the resolution.

If we look at this function:

https://github.com/espressif/esp-idf/blob/master/components/mbedtls/port/net.c#L204

The intent is to determine if an error reported by a previous socket call was caused because the socket is non blocking. The return code is 0 if the last error was NOT caused by being a non-blocking socket while the return code is 1 if the last error was because we WERE a non-blocking socket. The logic is:

1. If the socket is a blocking socket , return 0
2. If the last error was because we were non-blocking, return 1.
3. Else ... return 0

This sound sensible enough, however there is perhaps a flaw in the implementation. The we check "if the last error was because we were non-blocking" is to ASK the socket about its last error. We don't use errno because we are multi-threaded and premptable. Again, on the surface, that took makes sense. However in test number 1, we perform an fcntl against the socket to determine if it is blocking or non-blocking ... and that resets the error condition that was present on the socket when the function was called. What this means is that the original error is lost.

[projectgus]

Thanks @nkolban, this looks good (and correct) to me.

Are you able to please rewrite the commit message so it summarises the fix? something like:

mbedtls port: Fix non-blocking socket erro detection

For knowing when to return MBEDTLS_ERR_SSL_WANT_READ/MBEDTLS_ERR_SSL_WANT_WRITE.
Checking non-blocking socket status clears errno, so save errno first.

Closes #424

Would be ideal. If you're not comfortable rewriting git history in this way then that's OK, I can rewrite the message when I push it into our internal merge queue.

[nkolban]

Sorry sir, I'm not good at Git ... :-(