libcnb-test: Images not cleaned up when pack build unexpectedly suceeeds

[edmorley]

When libcnb-test invokes pack build, it passes an explicit Docker image name that is then docker rmi'd once the associated TestContext struct is dropped:

libcnb.rs/libcnb-test/src/test_context.rs

Lines 248 to 264 in d9f0bc1

	impl<'a> Drop for TestContext<'a> {
	fn drop(&mut self) {
	// We do not care if image removal succeeded or not. Panicking here would result in
	// SIGILL since this function might be called in a Tokio runtime.
	let _image_delete_result =
	self.runner
	.tokio_runtime
	.block_on(self.runner.docker.remove_image(
	&self.image_name,
	Some(RemoveImageOptions {
	force: true,
	..RemoveImageOptions::default()
	}),
	None,
	));
	}
	}

However, there is one scenario where this clean-up is not taking place: When .expected_pack_result(PackResult::Failure) has been used, and then the pack build unexpectedly succeeds.

For example, during this the unexpected_pack_success testcase:

libcnb.rs/libcnb-test/tests/integration_test.rs

Lines 161 to 174 in d9f0bc1

	#[test]
	#[ignore = "integration test"]
	#[should_panic(expected = "pack command unexpectedly succeeded with exit-code 0!
	pack stdout:
	")]
	fn unexpected_pack_success() {
	TestRunner::default().build(
	BuildConfig::new("heroku/builder:22", "test-fixtures/procfile")
	.buildpacks(vec![BuildpackReference::Other(String::from(PROCFILE_URL))])
	.expected_pack_result(PackResult::Failure),
	|_| {},
	);
	}

In this scenario, the panic occurs in run_pack_command before the TestContext has been created:

libcnb.rs/libcnb-test/src/pack.rs

Lines 179 to 214 in d9f0bc1

	/// Runs the given pack command, panicking with user-friendly error messages when errors occur.
	pub(crate) fn run_pack_command<C: Into<Command>>(
	command: C,
	expected_result: &PackResult,
	) -> Output {
	let output = command.into()
	.output()
	.unwrap_or_else(|io_error| {
	if io_error.kind() == std::io::ErrorKind::NotFound {
	panic!("External `pack` command not found. Install Pack CLI and ensure it is on PATH: https://buildpacks.io/docs/install-pack");
	} else {
	panic!("Could not spawn external `pack` process: {io_error}");
	};
	});
	if (expected_result == &PackResult::Success && !output.status.success())
	|| (expected_result == &PackResult::Failure && output.status.success())
	{
	panic!(
	"pack command unexpectedly {} with exit-code {}!\n\npack stdout:\n{}\n\npack stderr:\n{}",
	if output.status.success() {
	"succeeded"
	} else {
	"failed"
	},
	output
	.status
	.code()
	.map_or(String::from("<unknown>"), |exit_code| exit_code.to_string()),
	String::from_utf8_lossy(&output.stdout),
	String::from_utf8_lossy(&output.stderr)
	);
	}
	output
	}

This means there is no TestContext to be dropped, so no remove image command is invoked.

This causes a leak of one image each time the libcnb.rs repo's integration tests are run, eg:

$ docker images
REPOSITORY                                  TAG       IMAGE ID       CREATED        SIZE
...
libcnbtest_bwgvfgtzszywyveaqjfnhhknfycclp   latest    d35f71755225   43 years ago   650MB

GUS-W-13903181.