std.os.linux.setsid(): return raw syscall0 result

[blblack]

When not linking libc on 64-bit Linux and calling posix.setsid(), we get a type error at compile time inside of posix.errno(). This is because posix.errno()'s non-libc branch expects a usize-sized value, which is what all the error-returning os.linux syscalls return, and linux.setsid() instead returned a pid_t, which is only 32 bits wide.

This and the other 3 pid-related calls just below it (getpid(), getppid(), and gettid()) are the only Linux syscall examples here that are casting their return values to pid_t. For the other 3 this makes sense: those calls are documented to have no possible errors and always return a valid pid_t value.

However, setsid() actually can return the error EPERM, and therefore needs to return the raw value from syscall0 for posix.errno() to process like normal.

Additionally, posix.setsid() needs an @intcast(rc) for the success case as a result, like most other such cases.

[rootbeer]

Looks great to me.

Not required, but I'd vote for adding some comments in the non-standard (i.e., pid_t-casting wrappers) that says they can do this because the syscall can't fail. Might prevent future copypasta problems?

// Cast result directly to a pid_t because getppid() cannot fail.
return @bitCast(@as(u32, @truncate(syscall0(.getppid))));

Hmm, not really your change but shouldn't that be pid_t instead of u32 in the @as()?

(Again, feel free to leave this as-is, or ignore me in favor of whatever the Zig team members say.)

[blblack]

Hmm, not really your change but shouldn't that be pid_t instead of u32 in the @as()?

I think not, for the same reasons as #25205 (comment) . If these can't fail, the value should never be negative, even though pid_t technically can be.

[blblack]

Hmm, not really your change but shouldn't that be pid_t instead of u32 in the @as()?

I think not, for the same reasons as #25205 (comment) . If these can't fail, the value should never be negative, even though pid_t technically can be.

Hmmm no that was tired-brain thinking. The way it is now isn't any safer. Really if anything, these other calls should be asserting the positive-ness of the return value, perhaps like:

// Cast result to a pid_t, safety-checking positive-ness, because getppid() cannot fail.
return @intCast(@as(u32, @truncate(syscall0(.getppid))));

[rootbeer]

I like the new comments and the @intCast check (which I now notice is similar to how some syscalls above, like gitgid and getuid do it). Thanks! 👍