tcp_connect does not respect the timeout parameter (IDFGH-2971)

[X-Ryl669]

In current repository, tcp_connect in esp_transport has this signature:

static int tcp_connect(esp_transport_handle_t t, const char *host, int port, int timeout_ms)

Yet, inside the function, there are 2 blocking functions being called and none will respect the timeout_ms given.
The first one is inet_pton/resolve_dns and the second one is connect.
Please notice that lwip does not respect the socket option SO_SNDTIMEO for connect.

As a result, if a server takes forever to connect (for example, by being down), tcp_connect will block indefinitively.

The correct way to have a timeout here is to set the socket as non blocking (via fcntl), call connect, then select for socket writeability within the given timeout.

[X-Ryl669]

Also, as a more general remark, the parameter timeout_ms in all those transport functions should be called inactivity_time_ms instead since the timeout option set on a socket is reset on socket activity. So if you're receiving a byte per second with a timeout_ms set to 2s, and you are receiving 5 bytes, it'll actually take 5 seconds to complete.

[anecdata]

We've noticed that TCP sockets seem to block if, for example, an unreachable numeric IPv4 address is supplied. Sounds like this may be the underlying issue.

[X-Ryl669]

You've resolved one issue but not the other. resolve_dns is still blocking.
To resolve a DNS name asynchronously, you should use LWIP's dns_gethostbyname and a semaphore.