RFR: 8366149: JNI exception pending in Java_sun_awt_X11GraphicsDevice_pGetBounds of awt_GraphicsEnv.c:1484

[Phil Race]

On Fri, 5 Sep 2025 21:46:22 GMT, Damon Nguyen <dnguyen at openjdk.org> wrote:

>> src/java.desktop/unix/native/libawt_xawt/awt/awt_GraphicsEnv.c line 1289:
>> 
>>> 1287:             AWT_UNLOCK ();
>>> 1288: 
>>> 1289:             if ((*env)->ExceptionCheck(env)) {
>> 
>> Is the problem that we are here because bounds == null and one way this is possible is that the call at line 1265 failed? Meaning threw an unlikely exception.
>> If so, maybe that is where we should just "return NULL".
>
> My understanding of the issue is that there is a possible `pendingException` on line 1287. This is possible by `AWT_NOFLUSH_UNLOCK_IMPL()` in `awt.h` as you previously pointed me to. Seems like setting bounds by the code that was previously on lines 1289-1290 was unsafe due to the `AWT_UNLOCK` possibly throwing an exception right before it.
> 
> I don't think bounds can be null in this area because line 1279 checks for this. But if the `AWT_UNLOCK` here is throwing a `pendingException`, I think returning null is what should be done here since the same is done on line 1297, except this won't be reached in this case until after bounds is unsafely set.
> 
> I can just `return NULL` instead near line 1265 as you suggested, but from what I read, the exception would be at line 1287 (which is after).

You are right - there is a check for !bounds. Although that is likely there because we might not be in the Xinerama case, not a concern about a pending exception, although it would also help in that case too.
However if NewObject fails then bounds will be null and there will be a pending exception. So I think we should return immediately in that case.

But my point is that this bug report is a result of static analysis, so we should be able to follow that same logic.
Meaning AWT_UNLOCK isn't the source of the exception.
It is rethrowing one it previously copied and cleared
But it only does all of that IF there was such a pending exception.

So I am looking back at where the exception might have come from.

I missed this line
                    (*env)->ThrowNew(env, exceptionClass, "Illegal screen index");

that seems likely to be it. If we are throwing an exception because of an illegal screen index, there is absolutely no point in returning a new object - the caller won't be able to receive it because they'll get an exception. And clearing it would be wrong - it is meant to be thrown.

So there should also be a return after that ThrowNew.

Once you've done that we can statically prove there are no possible pending exceptions and this call to ExceptionCheck isn't necessary.

Basically a fix for this is to return immediately in both cases - failure of NewObject and ThrowNew.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27110#discussion_r2334443764