Add a PlannerBuilder

[torkleyy]

This allows sharing the thread pool, which is pretty important for highly parallel applications in order to allow a good performance.

Sorry I've done so bad in my previous PR, I hope this one is better.

[kvark]

I wonder why you want to have a builder at all? There is only 2 ways to create a planner: with given thread pool or without. So it sounds like Planner could just have 2 different constructors, let's say new and with_thread_pool. Did you consider this?

[kvark]

what is the point of having an Option here in the first place?

[torkleyy]

Yes, I thought about using with_thread_pool, but I thought it's not descriptive enough because you still have another parameter. Yes, you are right I could have asked for world already in the builder constructor, but I think it is nicer to use. Of course it's not compile-time enforced then.

[torkleyy]

Plus, now the pool size defaults to the number of cpus, which should be pretty common. I do have another solution in mind, though:

enum PoolOptions {
    Pool(Arc<ThreadPool>),
    NumThreads(usize),
    NumCpus,
}

impl Planner {
    pub fn from_options(options: PoolOptions) -> Self {
        
    }
}

[kvark]

How about we have just new going the NumCpu path and from_pool accepting a thread pool? that would be simple enough and covers all use-cases.

[torkleyy]

I found a non-breaking way, using a trait PoolSize.

[kvark]

@torkleyy I don't think it's worth it. Let's not have a fancy API, let's have a simple one, with new() and from_pool(...) constructors.

[torkleyy]

So you mean I should just take a plain usize?

[kvark]

No, I mean that we don't even need a constructor that takes usize. A most common case would be to use the number cpus, and that's what I propose for the new() constructor to do. A less common case would be to receive the Arc<ThreadPool>, which covers a case where you want a specific number of threads.

[nchashch]

Maybe add a Planner::new, taking a usize and Planner::new_num_cpus. There are use cases where you don't need an Arc<ThreadPool> and you need to set the number of threads (for example to benchmark performance with and without multithreading).

And from the user's perspective it isn't obvious that the way you get a Planner with a set number of threads is call Planner::from_pool.

[torkleyy]

Closing because title does not match the content anymore. #106 replaces this PR