Expose after_start and before_stop in runtime::Builder

[bkchr]

This new function is called by tokio around each thread that is started.
This functionality is useful, when the user needs to setup certain
conditions for each new thread.

Motivation

Hey, I require to setup each thread that is started by tokio. In my case I want to call to Java and that requires that a thread is attached to the JVM. By using the new around_thread function, I'm able to setup each thread easily.

Solution

Added a function to runtime::Builder. The function accepts a closure that will be given another closure that represents the thread. The outer closure is required to call the inner closure, otherwise the thread would not do any work.

[bkchr]

Hmm the test failure looks like another error, maybe restarting the job could solve it?

[tobz]

So I can't restart AppVeyor runs myself, unfortunately, but had a question: any reason you opted to create a new mechanism for this instead of threading through to tokio_threadpool::Builder::around_worker?

[bkchr]

Yeah, if you look here you see that tokio already itself uses the functionality.
And there was already an issue, saying that it does not work: #321
Which makes clearly sense, if you look into the code :)

[carllerche]

@bkchr do before and after hooks work for you? Those are more conservative and would be easier to add.

[bkchr]

@carllerche I tested another solution and that should work with before and after hooks. I would update this pull request to forward before and after to the threadpool used by the builder. Would that be okay?

[carllerche]

@bkchr That would be a good solution as long as no tokio-threadpool API (types) leak out. The goal is to be able to bump the version of tokio-threadpool internally w/o a breaking change.

[bkchr]

@carllerche I updated the pr to expose after_start and before_stop in runtime::Builder and removed the around_thread stuff.

[carllerche]

Docs and a test 👍 Looks great to me.