api,core: Adds an Executor field to NameResolver.Args.

[groakley]

Adds an Executor field to NameResolver.Args, which is optionally set on ManagedChannelBuilder. This allows NameResolver implementations to avoid creating their own thread pools if the application already manages its own pools.

Addresses #3703.

[groakley]

Some additional context: Before this change, there is no way to prevent the Channel returned by the OkHttpChannelBuilder from creating its own thread pools (even if executor() is used to override the default).

From the user's perspective, this means that even though the executor() override was used, "default-grpc-executor*" threads will still be started.

After this change, the NameResolver executor can be overridden using the channelbuilder, e.g.

OkHttpChannelBuilder.forAddress("foo", 123)
    .executor(executor1)
    .transportExecutor(executor2)
    .nameResolverExecutor(executor3)
    .build();

[ejona86]

I don't know why the discussion on #3703 was ignored.

We don't want an executor just for the name resolver on ManagedChannelBuilder. The reason the NameResolver needs an executor is that it does blocking calls. We have a few cases that need an executor for blocking reasons. Please change it so blockingExecutor() is the exposed API.

Also, in the Args API it should be clear the purpose of the Executor: that it is for blocking operations. The API should be getBlockingExecutor()/setBlockingExecutor().

[ejona86]

See previous comment

[groakley]

Renamed NameExecutorResolver->BlockingExecutor.

I use git/github about once a year to contribute to this project. Looks like I screwed up the branch trying to rebase from head. (Looks good in my fork project, totally screwed up in the remote project). Might be something that is easy to resolve, but I can also recreate this PR if that is the easiest path forward.

[ejona86]

Shoot. Did it a bit differently than I wanted. One moment.

[ejona86]

Looks pretty good. Just one comment, which I think will be fairly easy to address.

[ejona86]

Thank you! I'm happy this is done.