JVM crashes on macOS when entering too many nested event loops

[Christopher Schnick]

Our code and some libraries do enter some nested event loops at a few 
places when it makes sense, but we didn't do anything to explicitly 
provoke this, this occurred naturally in our application. So it would be 
nice if JavaFX could somehow guard against this, especially since 
crashing the JVM is probably the worst thing that can happen.

I looked at the documentation, but it seems like the public API at 
Platform::enterNestedEventLoop does not mention this.
 From my understanding, the method Platform::canStartNestedEventLoop is 
potentially the right method to indicate to the caller that the limit is 
close by returning false.
And even if something like an exception is thrown when a nested event 
loop is started while it is close to the limit, that would still be much 
better than a direct crash.

Best
Christopher Schnick

On 10/03/2025 18:51, Andy Goryachev wrote:
>
> This looks to me like it might be hitting the (native) thread stack 
> size limit.
>
> c.s.glass.ui.Application::enterNestedEventLoop() even warns about it:
>
>      * An application may enter several nested loops recursively. 
> There's no
>
>      * limit of recursion other than that imposed by the native stack 
> size.
>
> -andy
>
> *From: *openjfx-dev <openjfx-dev-retn at openjdk.org> on behalf of Martin 
> Fox <martinfox656 at gmail.com>
> *Date: *Monday, March 10, 2025 at 10:10
> *To: *Christopher Schnick <crschnick at xpipe.io>
> *Cc: *OpenJFX <openjfx-dev at openjdk.org>
> *Subject: *Re: JVM crashes on macOS when entering too many nested 
> event loops
>
> Hi Christopher,
>
> I was able to reproduce this crash. I wrote a small routine that 
> recursively calls itself in a runLater block and then enters a nested 
> event loop. The program crashes when creating loop 254. I’m not sure 
> where that limit comes from so it’s possible that consuming some other 
> system resource could lower it. I couldn’t see any good way to 
> determine how many loops are active by looking at the crash report 
> since it doesn’t show the entire call stack.
> I did a quick trial on Linux and was able to create a lot more loops 
> (over 600) but then started seeing erratic behavior and errors coming 
> from the Java VM. The behavior was variable unlike on the Mac which 
> always crashes when creating loop 254.
>
> Martin
>
> > On Mar 7, 2025, at 6:24 AM, Christopher Schnick <crschnick at xpipe.io> 
> wrote:
> >
> > Hello,
> >
> > I have attached a JVM fatal error log that seemingly was caused by 
> our JavaFX application entering too many nested event loops, which 
> macOS apparently doesn't like.
> >
> > As far as I know, there is no upper limit defined on how often an 
> event loop can be nested, so I think this is a bug that can occur in 
> rare situations.
> >
> > Best
> > Christopher Schnick<hs_err_pid.txt>
>
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