pasta: Save errno on signal handler entry, restore on return when needed

Ed reported this:

  # Error: pasta failed with exit code 1:
  # Couldn't drop cap 3 from bounding set
  # : No child processes

in a Podman CI run with tests being run in parallel. The error message
itself, by the way, is fixed by commit 1cd773081f ("log: Drop
newlines in the middle of the perror()-like messages"), but how can we
possibly get ECHILD as failure code for prctl()?

Well, we don't, but if we exit early enough, pasta_child_handler()
might run before we're even done with isolation steps, and it calls
waitid(), which sets errno. We need to restore it before returning
from the signal handler (if we return after calling functions that
might set it), as signal-safety(7) also implies:

       Fetching and setting the value of errno is async-signal-safe
       provided that the signal handler saves errno on entry and
       restores its value before returning.

Eventually, we'll probably need to switch to signalfd(2) the day we
want to implement multithreading, but this will do for the moment.

Reported-by: Ed Santiago <santiago@redhat.com>
Link: https://github.com/containers/podman/issues/23478
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: Paul Holzinger <pholzing@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

pasta.c

@@ -61,6 +61,7 @@ int pasta_child_pid;
  */
 void pasta_child_handler(int signal)
 {
+	int errno_save = errno;
 	siginfo_t infop;
 
 	(void)signal;
@@ -85,6 +86,8 @@ void pasta_child_handler(int signal)
 
 	waitid(P_ALL, 0, NULL, WEXITED | WNOHANG);
 	waitid(P_ALL, 0, NULL, WEXITED | WNOHANG);
+
+	errno = errno_save;
 }
 
 /**