Ability to not create 64 threads when booting vapor.

[John-Connolly]

Describe the bug

On iOS platforms the default threadpool size of 64 is a bit aggressive. Sometimes iOS will not want to create that many threads leading to a crash in ThreadOpsPosix.run (ThreadPosix.swift:109). Even when swapping the NIOThreadPool to one with only one thread doesn't prevent the default one from creating 64 threads.

To Reproduce

Boot a vapor app.

Steps to reproduce the behavior:
n/a this is the default behaviour.

Expected behavior

app.threadPool = NIOThreadPool(numberOfThreads: 1) should replace the default threadpool before it's started.

Environment

• Vapor Framework version: 4.76.0
• Vapor Toolbox version: N/A
• OS version: N/A

Additional context

I'm happy to provide a patch for this.

[0xTim]

Yes this seems sensible and has come up in the past - happy to accept a patch for this change in behaviour

[dfed]

Adding a +1 to this. When deploying Vapor on iOS we're creating way too many threads. Would love to be able to cap this in the initializer.

[gwynne]

Addressed by #3092 and the preexisting ability to replace the default thread pool.