Improve warning when connection to unix socket fails

[JelteF]

PgBouncer is hard to debug when it's trying to connect to Postgres
server over a UNIX socket but that connection fails. You would get an
unhelpful error message like this:

WARNING sbuf_connect failed: No such file or directory

This change starts including the path in the error message:

WARNING sbuf_connect failed to connect to UNIX socket /aaaaa/.s.PGSQL.5432: No such file or directory

It also includes the address+port for IPv4/IPv6 connection attempts.

Fixes #835

[eulerto]

Why don't you provide messages for IPv4 and IPv6? AFAICS, this log_warning call does not provide a PgSocket as first parameter so address information (postgres/euler@unix:9915) provided by log_socket_prefix is not added after the prefix (2023-02-27 16:27:24.299 -03 [33171] DEBUG). It is not possible to provide PgSocket to sbuf API (it is in a different layer), hence, you should construct a string that contains host + port or Unix socket path for sbuf. Perhaps, add this address information before the message (to be similar to what logging_prefix_cb does).

[JelteF]

I wasn't able to trigger the WARNING for IPv4/IPv6 sockets. They would error during reading instead of erroring at connect. Those errors during reading would become a LOG with the log_socket_prefix. So those are already pretty helpful.

[eulerto]

I didn't try to reproduce it but the following code can trigger it. Looking at man socket, I expect some errors (such as EACCESS, EMFILE, ENFILE, ENOMEM) in certain situations.

    sock = socket(sa->sa_family, SOCK_STREAM, 0);
    if (sock < 0) { 
        /* probably fd limit */
        goto failed;
    }

[JelteF]

Thanks for the pointer. By manually setting my file limit very low I was able to trigger the WARNING for IPv4/IPv6 sockets too. So now the patch includes a better error for those as well.

[eulerto]

Since sa2str encapsulates the print-a-network-address logic, you don't need the if/else. Use only the last else statements.

[JelteF]

Makes total sense. Pushed that now. I had a bit of a brainfart I think because I misremembered the fix for this issue: libusual/libusual@9469419

[eulerto]

LGTM