Fix improper _recv() callback return when ssl layer returned SSL_CLOSE_NOTIFY

[Adam5Wu]

With AsynClient, when remote initiated disconnect, tcp_ssl_read() will return SSL_CLOSE_NOTIFY.
However, at this moment, the pbuf has already been freed, and thus the _recv() callback should return ERR_OK so that LwIP can properly discard the buffer reference.

Otherwise, LwIP2 will think the data is not consumed, and attempt to maintain it in refused_data, and ultimately cause a double free and crash later.

On LwIP1, this is still an error, but it is likely to not cause any problem because LwIP1 does not attempt to maintain the unconsumed refused_data, instead it just print an assert message and overwrite the pointer with a new piece of data.

For the changes, there are three lines, only one line in src/ESPAsyncTCP.cpp directly related to this bug;
The other two lines are nice-to-have:
The second line in src/tcp_axtls.c is there just to keep a peace of mind;
The thrid line in src/tcp_axtls.h solves C99 compiler complaint of unknown bool type.

[romansavrulin]

Shouldn't we return error instead of saying ERR_OK?

[Adam5Wu]

Not really:

1. if tcp_ssl_read() returns SSL_CLOSE_NOTIFY
   • The pbuf has been freed in tcp_ssl_read(), so LwIP should not try to further maintain it, and
   • Freeing pbuf in tcp_ssl_read() is a good choice because by definition of this signal, we are closing the connection anyway.
2. if tcp_ssl_read() returns other error code
   • The block above the return is closing the connection anyway, so we don't care the left over data in pbuf
   • There are two code paths in tcp_ssl_read(), one frees the pbuf, one does not (handshake error)
     • For the code path that does not free pbuf, a reference is left in fd_data->tcp_pbuf, which will be freed when tcp_ssl_free() is called later.
   • Therefore, it is still necessary and correct to let LwIP not further maintain the pbuf.

So, in short, when we run into read_bytes < 0 situation, we will be closing the connection one way or another, and it is best to let LwIP give up ownership of the current pbuf.

And the easiest way to let LwIP do what we want is to return ERR_OK.

[Adam5Wu]

From a different way to look at it, SSL belongs to "upper" layer and TCP belongs to "lower" layer.

SSL_CLOSE_NOTIFY is an upper layer signal, it may mean some abnormal situation, only to the SSL layer.

To the lower TCP layer, everything is normal -- the TCP layer successfully delivered the close notification signal to the SSL layer, hence ERR_OK.

[ilyary]

Please, raise version in library.json: "version": "1.1.3" to be updated by PlatformIO package manager