Contrary to what the documentation might seem to imply [1], Bats' 'fail'
helper only aborts a test case under certain circumstances. eg., when
called from setup_suite(), but not from within a child function, and a
@test case, but not from within the 'run' helper.
If 'fail' is called from within 'run', then the code after it will
continue to execute. The test case will only fail if 'run' eventually
catches a non-zero exit code that's caught by 'assert_success' [2].
Similarly, it doesn't abort if called from within a child function in
setup_suite().
Currently, _pull_and_cache_distro_image() is a child function called
from setup_suite(). So 'fail' won't abort if an invalid distribution is
specified.
Fortunately, pull_distro_image() is being called from within @test
cases, but outside 'run'. So, there's no problem with it now. However,
some future code changes can unknowingly alter this reality and it too
can run into unexpected behaviour.
Therefore, it's better to be safe, and explicitly specify a non-zero
exit code after 'fail'. It will ensure that it works as expected under
all circumstances.
[1] https://github.com/bats-core/bats-support
[2] https://github.com/bats-core/bats-asserthttps://github.com/containers/toolbox/pull/1375
e1745ef9c2