compilemessages reports errors only once

[Balazs Endresz]

When calling compilemessages for a locale that has invalid entries it prints out the errors correctly first.
But any subsequent calls just say that it's already compiled and up to date.
This is because msgfmt updates the last modified date of the mo file even if it's invalid, and then django will skip that locale afterwards:
​https://github.com/django/django/blob/main/django/core/management/commands/compilemessages.py#L153

This can reproduced by calling compilemessages twice in django/tests/i18n/test_compilation.py:

class CompilationErrorHandling(MessageCompilationTests):
    def test_error_reported_by_msgfmt(self):
        # po file contains wrong po formatting.
        with self.assertRaises(CommandError):
            call_command("compilemessages", locale=["ja"], verbosity=0)

        # subsequent calls should still fail
        with self.assertRaises(CommandError):
            call_command("compilemessages", locale=["ja"], verbosity=0)  # currently this doesn't raise CommandError

One way of fixing this is perhaps doing the validation separately first, e.g. with mo_path = "NUL" if os.name == "nt" else "/dev/null" and then calling msgfmt again with the actual mo_path only if that passes.

[Claude Paroz]

Good catch! I think your suggested resolution strategy makes sense.