Avoid listener blocking

[waterdrink]

close #1469

[mxinden]

@waterdrink do I understand this change correctly, that instead of sending all alerts to all iterators, including those that are already done, it only sends alerts to not-yet-done listeners and skips the others via case <-l.done:? I guess a listener can not be removed during that time, as Puts() holds the lock?

Not related, you are missing the sign-off line in your commit message, see Details on the DCO failure. Simply do a git commit --amend --no-edit -s and push the new commit.

[waterdrink]

@mxinden The listener do can not be removed when Puts() holds the lock, but listener can not receive alerts from Puts() when Subscribe is done but listener has not removed yet. So it will block at https://github.com/prometheus/alertmanager/blob/master/provider/mem/mem.go#L100

[waterdrink]

My fault. I messed up the pull request commit, and have no idea why ci is failed at config_test.go.I can close this one and start a new pull request if needed.

[mxinden]

and have no idea why ci is failed at config_test.go.

This is not related to your patch and will be fixed with #1486.

[stuartnelson3]

My fault. I messed up the pull request commit, and have no idea why ci is failed at config_test.go.I can close this one and start a new pull request if needed.

It's fine to keep working in this PR.

[waterdrink]

hi, I did rebase for ease of viewing.

[mxinden]

This looks good to me. #1503 adds a test failing without this fix.

Still leaving @stuartnelson3 and @simonpasquier some time to have a look before merging.

[stuartnelson3]

sorry for the delay, thanks for adding this

[mxinden]

@waterdrink great catch! Thanks for the contribution.