RFR: 8294594: Fix cast-function-type warnings in signal handling code [v2]

[Aleksey Shipilev]

On Sun, 9 Oct 2022 01:40:29 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> src/hotspot/os/posix/signals_posix.cpp line 1727:
>> 
>>> 1725:   // Set up signal handler for suspend/resume
>>> 1726:   act.sa_flags = SA_RESTART|SA_SIGINFO;
>>> 1727:   act.sa_handler = CAST_FROM_FN_PTR(sa_handler_t, SR_handler);
>> 
>> The underlying cast in the macro seems a little odd for this situation but if it appeases the compiler then that is fine.
>
> No, don't do this.  The code is just wrong, and should be fixed.
> 
> We have a struct sigaction.  We've set the sa_flags member to a value that
> includes SA_SIGINFO.  That means the 3 argument sa_sigaction handler is
> supposed to be invoked.
> 
> We're storing the SR_handler (cast to a value matching the type of the
> sa_handler member of the sigaction). This is "working" purely by accident.
> Apparently on all POSIX platforms we support, the sa_handler and sa_sigaction
> occupy the same memory (being in a union, which is permitted by the spec).
> 
> The signature for SR_handler should be changed from
> void SR_handler(int, siginfo_t*, ucontext_t*)
> to
> void SR_handler(int, siginfo_t*, void*)
> and adjust the code to deal with the change to the type of the 3rd parameter.
> 
> Then we can just assign the sa_sigaction member to SR_handler without any casts.

This is an excellent catch, @kimbarrett! I think I missed it the same way David did: I completely missed there is `sa_handler` and `sa_sigaction`! Re-done this in new commits.

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PR: https://git.openjdk.org/jdk/pull/10494