RFR: JDK-8269921 Text in Textflow and listeners on bounds can cause endless loop/crash and other performance issues

[Kevin Rushforth]

On Tue, 6 Jul 2021 14:53:24 GMT, Florian Kirmaier <fkirmaier at openjdk.org> wrote:

> It's "a bit" complicated.
> In some situations, getRuns get's called because listeners on bounds are set.
> This causes TextFlow to layout to compute the runs.
> Afterward, the bounds of the parents get updated. 
> This triggers a call to compute bounds - which cascades up to the children.
> When the geometry of the previous Text gets computed in this big stack - it throws an nullpointer.
> The Text doesn't have its runs, and calling TextFlow.layout is now a noop (it detects repeated calls in the same stack)
> 
> In the case it happened - it didn't repair and the application kinda crashed.
> This bug most likely can also be triggered by ScenicView or similar tools, which sets listeners to the bounds.
> It also can cause unpredictable performance issues.
> 
> Unit test and example stacktrace are in the ticket.
> 
> The suggested fix makes sure that recomputing the geometry of the Text, doesn't trigger the layout of the TextFlow. 
> The Textflow should be layouting by the Parent.
> This might change the behavior in some cases, but as far as I've tested it works without issues in TextFlow Heavy applications.
> 
> Benefits:
>  * Better Tooling Support For ScenicView etc.
>  * Fixes complicated but reproducible crashes
>  * Might fix some rare crashes, which are hard to reproduce
>  * Likely improves performance - might fix some edge cases with unpredictable bad performance

modules/javafx.graphics/src/main/java/javafx/scene/text/Text.java line 386:

> 384:     private GlyphList[] getRuns() {
> 385:         if (textRuns != null) return textRuns;
> 386:         if (!isSpan()) {

Removing the call to `getParent().layout()` here seems very likely to cause a regression in behavior. This will need some further analysis and lots of testing.

tests/system/src/test/java/test/javafx/scene/text/TextFlowCrashTest.java line 1:

> 1: package test.javafx.scene.text;

This needs a standard copyright header block.

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PR: https://git.openjdk.java.net/jfx/pull/564