Don't panic if /var/run/utmp is missing

[pothos]

• Don't panic if /var/run/utmp is missing
  In a container as set up by Podman there is no utmp file in (/var)/run.
  When no file is present, we can return NULL.
• Document requirements for runtime environment. (Based on the above this is only about getent.)

[pothos]

Tested it and it doesn't panic anymore as wanted. Reading the glibc code I don't see errno being set up for this case. I will remove set_errno(Errno(libc::ENOENT)); and maybe I think we should also skip setting errno. Or we could use set_errno(Errno(0)) to make this explicit.

[sunfishcode]

Thanks for the patch!

I tried a testcase in glibc:

 cat t.c
#include <utmp.h>
#include <utmpx.h>
#include <errno.h>
#include <stdio.h>

int main() {
	errno = 0;
	struct utmpx* p = getutxent();
	printf("%p; %m\n", p);
	return 0;
}

on a system where /var/run/utmp doesn't exist and it set errno to ENOENT. Probably it does an open which does this implicitly.

[sunfishcode]

My sense is it's not necessary to list all the getent subcommands here. And this list will grow over time if c-ward adds support for more of the NSS APIs; we can just say it needs getent as provided on glibc and musl systems.

[pothos]

Good, removed the subcommand list.

[pothos]

Thanks for the test! I've switched back to my initial version which was to set ENOENT. The man page also says: On failure, these functions errno set to indicate the error.

[sunfishcode]

Thanks!

[sunfishcode]

This is now released in c-gull 0.16.1.