SigSet: Fix incorrect implementation of Eq, PartialEq and Hash

[germag]

signal::SigSet derives Eq, PartialEq and Hash, but on linux, sigfillset(3) only initializes the first 64 bits, leaving the rest uninitialized. So two signal::SigSets can be different even though both have the same set of signals.

closes #1749

[germag]

v2:

• Fix formatting issue

[rtzoeller]

@germag have you done any performance analysis of the auto-derived version (on platforms other than Linux; I acknowledge the Linux impl is broken) vs the manually implemented one?

If this new implementation is measurably slower, we might want to use the auto-derived version on platforms where it is correct.

We could also implement this on Linux by reading only the initialized bits and hashing them all at once, although that is certainly a leaky abstraction.

[germag]

@germag have you done any performance analysis of the auto-derived version (on platforms other than Linux; I acknowledge the Linux impl is broken) vs the manually implemented one?

If this new implementation is measurably slower, we might want to use the auto-derived version on platforms where it is correct.

No, I haven't, sigismember() it's a simple function, but there will certainly be a difference in performance. However, IIRC according to posix we must treat sigset_t as an opaque object. Anyway, I can implement it only for linux, but I don't know if is also required in other targets.

We could also implement this on Linux by reading only the initialized bits and hashing them all at once, although that is certainly a leaky abstraction.

I think it would be better to do that in libc. Also, the __val[] field inside sigset_t is not public.

[asomers]

I think we should merge this. It's hard for Nix to know which platforms fully initialize sigset_t and which don't. So unless they start providing sigeqsetor something like that, we should probably just do things the hard way.

[SteveLauC]

Friendly ping @germag, would you like to finish this PR? The CI failure seems to be irrelevant, rebasing your branch should resolve it :)

[germag]

Rebase

Friendly ping @germag, would you like to finish this PR? The CI failure seems to be irrelevant, rebasing your branch should resolve it :)

Sorry for the delay, I thought it was already merged