[swift-corelibs-dev] DispatchQueue and Threads

[Pierre Habouzit]

> On Dec 21, 2017, at 10:57 AM, Geordie J via swift-corelibs-dev <swift-corelibs-dev at swift.org> wrote:
> 
> Hi,
> 
> We are having some issues with threading / dispatch queues this week. Basically I’m wondering why the following “async” code has the output it does:
> 
> print(Thread.isMainThread) // true
> DispatchQueue.main.async {
>      print(Thread.isMainThread) // false (???)
> }

This is wrong.

> 
> Whereas this same code but with “sync" prints “true” “true”:
> 
> print(Thread.isMainThread) // true
> DispatchQueue.main.sync {
>      print(Thread.isMainThread) // true
> }

This is expected, because dispatch_sync() on the main thread is... difficult.

> 
> This goes against what I thought I understood about Dispatch. Are there any threading guarantees made by DispatchQueue at all? Especially on the main queue. And if not, is there another construct we can use?
> 
> The issue is that we’re on Android (where the threading situation is cloudy at best to begin with) and the JNI (Java-Native Interface) has specific requirements about which threads its methods are run on. Let alone our graphics rendering code.
> 
> Basically, initialising and using the JNI in separate DispatchQueue.main.async blocks apparently does not end up on the same thread (and apparently neither of them are the main thread). I’m quite confused by this.
> 
> Are there any resources or docs to get up to scratch with the internals of Dispatch a bit better?

THe problem here is not Dispatch, it's how it's (mis-)ported to Android for now.

On other platforms, either you call dispatch_main() and there's no main thread anymore, or from the main thread you are supposed to drain the Main CFRunloop which in turn is integrated with dispatch (or rather the other way around: dispatch is integrated with the main CFRunloop) and the main queue acts as a source on the main CFRunloop.

Last I heard, Android has not done that. I tried to explain this on https://github.com/apple/swift-corelibs-libdispatch/pull/299 <https://github.com/apple/swift-corelibs-libdispatch/pull/299> which atm is still not resolved.

-Pierre
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