remove PENDING_CLOSE flag

[wei-lee]

We have come across a problem when try to use this module. We found that the circuit breaker will be kept open if it's going to the "halfOpen" state the second time. There will be no way to close the circuit breaker afterwards.

After a little of debugging, it looks like the problem is caused by the PENDING_CLOSE flag. I can't really tell why we need the flag, and removing it fix the problem.

The case is being shown in the test.

@lance can you please review this and see if I am right?

Thanks.

[lance]

@wei-lee thanks for making the contribution. But we actually do need PENDING_CLOSE.

It's a real edge case, but the reason we have PENDING_CLOSE is because the circuit should only ever attempt to execute once when in the HALF_OPEN state. In a scenario where lots and lots of requests are occurring in parallel, many could execute while the circuit is HALF_OPEN. By checking the PENDING_CLOSE state, we ensure this doesn't happen.

You did discover a bug, however. So thank you! The PENDING_CLOSE flag was never being reset. I've fixed that, and used your test case in my latest commit.