gh-141817: Add IPV6_HDRINCL constant to the socket module

[Rosuav]

Add the socket.IPV6_HDRINCL constant on platforms that support it.

There are a lot more constants that could also be added. How far should this be taken?

• Issue: Add socket constant IPV6_HDRINCL and maybe others #141817

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📚 Documentation preview 📚: https://cpython-previews--141818.org.readthedocs.build/

[vstinner]

LGTM

[Rosuav]

There are a bunch more constants not currently in the socket module. I made a list here: https://discuss.python.org/t/adding-new-socket-options-specifically-socket-ipv6-hdrincl/104964/2 Do we want to add more?

[picnixz]

Looking at the constants that were found, I think all IP_* constants and IPV6_* constants could be added. However, I wonder how useful this can be. Are these constants usable as is by our interface or is there some pre-requirements that need to be met? (maybe the constant exists but requires some additional headers, etc).

[Rosuav]

However, I wonder how useful this can be.

The script retains #if and comments in case they're indicative, and in some cases, I believe those do indeed show that the constants aren't useful. (One of them, __SOCK_SIZE__, is clearly a false positive.) A lot of them are socket options or the values for them, so you could use them with setsockopt(). Some are factual notes (IN_LOOPBACKNET and the IN_CLASS* constants), could take 'em or leave 'em, not a lot to judge either way.

I'd be inclined to include all of the socket option flags, since they can be useful without any additional support. Most of them get/set integers (in a lot of cases, they're just booleans), and even the ones that aren't, there's ways to use those in Python without help.

I've no idea what IPV6_FL_* and IPV6_TLV_* are all about, so we could omit those until such time as someone asks about them.

[vstinner]

PR merged, thank you.

[vstinner]

There are a bunch more constants not currently in the socket module. I made a list here: https://discuss.python.org/t/adding-new-socket-options-specifically-socket-ipv6-hdrincl/104964/2 Do we want to add more?

I replied there: https://discuss.python.org/t/adding-new-socket-options-specifically-socket-ipv6-hdrincl/104964/5