RFR: 8351733: [macos] Crash when creating too many nested event loops [v5]

[Kevin Rushforth]

On Thu, 27 Mar 2025 15:21:09 GMT, Martin Fox <mfox at openjdk.org> wrote:

>> There is an undocumented limit on nesting calls to CFRunLoopRun (or the equivalent wrapper NSRunLoop methods). When the limit is hit the OS terminates the Java app. The situation arises when a JavaFX app creates too many nested event loops from within  Platform.runLater runnables.
>> 
>> This PR doesn't change the limit (which is 250+ nested loops) but it does throw an exception just before the limit is reached so a JavaFX developer will get a useful Java stack trace instead of an OS crash log.
>> 
>> On the Mac the nested event loop has two stages: first we ask the run loop to run, then we pull an event out and process it. A Platform.runLater runnable is executed in the first stage so if the runnable starts a new nested event loop the system will re-enter CFRunLoopRun. The same isn't true if an input event handler starts a new nested event loop; at that point we're in stage two and are past the call to CFRunLoopRun.
>
> Martin Fox has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Removed unnecessary import
>  - The max nested event loop constant is no longer public

The implementation looks good.

I left one question / suggestion on the type of Exception throws.

I think the suggestion made by @mstr2 to recommend calling `canStartNestedLoop` if the app wants to know whether or not this call will succeed is a good one.

I think we should update the docs for `Stage::showAndWait` documenting the same exception, since the implementation of that method also spins up a nested event loop.

Since this fix has been expanded to all platforms, let's remove `[macos]` from the title of the JBS issue and PR.

modules/javafx.graphics/src/main/java/javafx/application/Platform.java line 305:

> 303:      * other than the JavaFX Application Thread.
> 304:      *
> 305:      * @throws RuntimeException if this call would exceed the maximum

I wonder if there is a more specific subclass of `RuntimeException` that we could throw? Maybe `IllegalStateException`, which seems the closest in meaning and has the advantage that we already throw it in other cases.

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PR Review: https://git.openjdk.org/jfx/pull/1741#pullrequestreview-2722832212
PR Comment: https://git.openjdk.org/jfx/pull/1741#issuecomment-2758937412
PR Review Comment: https://git.openjdk.org/jfx/pull/1741#discussion_r2017254207