fail_point! does nothing unless a FailScenario exists about fail-rs

[sourcefrog]

Looking at the code, I think the problem is that the environment variable is only read by the FailScenario constructor, so binaries that just directly have failpoints won't ever fail.

I guess I would have expected that you could just embed fail_point! and then exercise it interactively or from tests that call the binary, by setting $FAILPOINTS: at least, the docs somewhat give this impression.

Maybe the macro could lazily load the environment variable? I got the idea that FailScenario was a convenience for test code.

At least, maybe this could be more clear in the docs. But, perhaps the division of responsibility between them could be reconsidered...

The reason I hit this is that I want to exercise failpoints in code run as a subprocess of a test. I thought setting the variable would do it, but it seems that the code under test needs to do FailScenario::setup from its main function for them to have any effect. With that added, my reproduction case passes.

It seems like it would be better to separate

1. Run a test using failpoints, holding a mutex to prevent any other changes to global failpoint configuration, and allowing the test to reconfigure failpoints
2. Prepare code-under-test to respond to failpoints set in the environment or by tests: ideally this would be implicit by just using fail_point!