Deprecate runAsyncAndBlock API #35911
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We don't want people using this as a means of jumping from synchronous code to async code. It blocks the thread that it's running on and spawns a new thread to run the async code. This has a couple of drawbacks. First, if it blocks a thread that is needed by the async code (i.e, calling something on the main actor while blocking the main thread), it is an immediate deadlock. This is a bad day for everyone. Second, it can easily lead to thread-explosions. The proper entry-point for async code is to use something of the following form: ``` @main struct Main { static func main() async { // ... } } ``` This should take care of most new code. There are still places where folks will need to jump from synchronous code to async code. That still needs to be designed and implemented, but we don't want people to come to rely on `runAsyncAndBlock`.
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@swift-ci please smoke test |
DougGregor
approved these changes
Feb 11, 2021
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Will top level code in a script also be async eventually? |
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@swift-ci please smoke test linux platform |
Yes, top-level code will have async capabilities eventually. |
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I'm throwing a deprecation warning on
runAsyncAndBlockto keep people from starting to rely on it in the wild.We don't want to use
runAsyncAndBlockfor a couple reasons.First, because it blocks the thread it's on, which can cause deadlocks. If you try to run
@MainActorcode after blocking the main thread, you're going to have a bad day.Second, because it spawns a new thread, it can contribute to thread-explosions, which are also bad news.
For new code, the correct entry-point is
Remember that for single-file tests, you'll need to pass
-parse-as-libraryor it will complain about@mainbeing used with top-level code.