Motivation / Problem

Fixes #9730.

Description

As by commit message:

A race condition happens when an IPv6 connection takes more than
250ms to report an error, but does return before the IPv4 connection
is established.
In result, an invalid socket might be used for that connection.

Bit difficult story:

• You have an IPv4 and IPv6 connection, and are connecting to an IPv4+IPv6 server
• You try IPv6 first, but it takes more than 250ms to get a reply from the network stack
• Happy Eyeballs kicks in, and IPv4 connection is attempted too
• The IPv6 responds before the IPv4 does, and says: no connection

The code never assumed this situation to happen. As such, it didn't check if any of the remaining connections was actually connected, but instead assumed at least one of the remaining was. This in result means that connected_socket became INVALID_SOCKET and things went from bad to worse from there.

So now instead, check if any non-error'd socket is in writable state. If so, only than safely assume one of the sockets is connected.

The if (this->sockets.empty()) { meant this worked for most people, as it is rare two connections are actually pending, and the first reporting an error before the second manages to connect. Just when timing was bad, it broke.
(Normally IPv6 just times out, which takes a few seconds. The IPv4 attempt will succeed well before that, and that case would work perfectly fine).

The if (this->sockets.empty()) { can safely be removed, as the start of the function does exactly this already. So next CheckActivity the exact same code is executed and starts another connection attempt if needed. Less code duplication is beter :)

Limitations

None

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker