make stop channel work properly for both maintainCerts and maintainOCSP

[tomokinat]

Current implementation passes stop channel to both maintainCerts and maintainOCSP functions. It won't work as expected unless stop is buffered (e.g. make(chan struct{}, 2)) and the value is sent twice, which will result in a arcane parameter precondition of an exported function CertCache.Init.

This PR fixes that problem in another way. PoC

[tomokinat]

CI's failure seems due to this flaky test :(
https://github.com/tomokinat/amppackager/blob/fa4e932ef896aab256f89a155bf150928b2dbd85/packager/certcache/certcache_test.go#L227

[twifkak]

Good find! I think a simpler solution may be possible, per https://blog.golang.org/pipelines#TOC_6%2e:

We need a way to tell an unknown and unbounded number of goroutines to stop sending their values downstream. In Go, we can do this by closing a channel, because a receive operation on a closed channel can always proceed immediately, yielding the element type's zero value.

So can you try changing the code in main() to close the channel instead of sending a value?

If that doesn't work, then please extract this channel-forking code into a separate function and leave a comment explaining it.

Sorry about the flake! Restarted the build.

[tomokinat]

Yes it works :) Thanks for the suggestion.

Yet I still feel like "pass to me stop chan struct{} and close it if you want to stop me" isn't a good I/F.
How about returning a function, similar to https://golang.org/pkg/context/#WithCancel ?

code

[twifkak]

I'm okay changing the interface. Rather than returning a func from Init(), what about adding func (this *CertCache) Stop()? Init() could store the channel(s) in the CertCache struct.

[tomokinat]

I just realized that if CertCache.Init is assumed to be callable even after once stopped for re-starting, stop should be initialized in Init, not in New. Should it be?

[twifkak]

No, I think we only designed Init to be called once, and to be the first call after New. Should I merge now, or do you want to make any other changes?

[tomokinat]

No, now this should be ready to merge :)

[twifkak]

Thanks! (On second thought, I guess make would make sense inside New, but it's no big deal either way.)