Check for a port being in use before binding to the port

[cloutiertyler]

It's possible for one process to bind to the IPv4 loopback port (e.g. spacetime start) and another process to bind to the IPv6 loopback port (e.g. docusaurus), leading to an enormous waste of time.

[egormanga]

This is normal POSIX behavior, just needs to be known. The result depends on what localhost resolves to on your system. It does not, however, depend on whether the same port is being used by another process on the other protocol.

This is absolutely not specific to SpacetimeDB, and OTOH it actually is useful to be able to bind the socket on a certain protocol. Users who deploy self-hosted should generally already be familiar with this nitpick.

Moreover, it might be a difficult task to check for this during binding. For example, if you tell StDB to listen on localhost, and DNS resolves it to ::1, then the check would also be performed against ::1. Such check is redundant, as the kernel already does it before binding (e.g. EADDRINUSE).

As a result, you'll have to treat localhost a special case, which is severely non-obvious to the end user. No other software does that.

The solution I can propose is to bind to "" (empty address) instead of "0.0.0.0" (IPv4) by default, being effectively “all interfaces”. This is exactly what all software does.