[lworld] RFR: 8372515: [lworld] Plumb in javac flags for compiling with preview mode

[David Beaumont]

On Tue, 25 Nov 2025 15:14:33 GMT, David Beaumont <duke at openjdk.org> wrote:

>> Plumbing for javac flags, mostly inspired by/copied from test commits made by @lahodaj .
>> 
>> There are several things here, mostly entangled, so it's a bit tricky to try splitting this out, but it would be possible if people wanted.
>> 
>> The biggest "refactoing" part of this PR is "src/jdk.compiler/share/classes/com/sun/tools/javac/file/JRTIndex.java" which now has a properly controlled lifecycle and disposed its resources correctly. Prior to this, the class used a non-closeable JRT file-system reference, which leads to "persistent open file" issues such as JDK-8357249.
>> 
>> This *does* mean that if compilation and the runtime have the same preview mode, then a 2nd JRT file system to the same jimage file is "opened", *but* the file system itself is lightweight, non-caching and both of them will use the underlying SharedImageReader (which is where nodes are cached etc.) so it really shouldn't be an issue (I will make sure javac benchmarks are checked however).
>> 
>> The benefit of this is that now, the shared index (which does do some caching) is correctly tracked across all users, and will be closed when the last user closes the lightweight wrapper instance.
>> 
>> A lot of the smaller "spot fix" changes in this PR were just copied by me, or at least inspired directly by Jan's work, so I may have missed some semantic subtlety in the code I'm not familiar with. Please evaluate that carefully.
>
> src/jdk.compiler/share/classes/com/sun/tools/javac/file/JRTIndex.java line 317:
> 
>> 315:     public void close() throws IOException {
>> 316:         // Release is atomic and succeeds at most once.
>> 317:         if (!sharedResources.release(this)) {
> 
> This exception may be contentious. Please speak up if you don't like it.
> It's here to track any accidental double-closing of the index, and did catch one case already.
> Since this is an internal class it feels like we should be able to track and eliminate any double close events, but it is a runtime exception in complex code, so I'm happy to remove it or maybe replace it with logging of some kind.
> Please speak up if you have opinions.

Also, nulling out "sharedResources" during close() for better GC cleanup is doable, but not completely trivial, since it adds "post-closure" failure modes that didn't exist before.
I do think that *somewhere* we should be detaching the index for garbage collection, but maybe all its users are sufficiently well scoped that it's unlikely to matter.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1761#discussion_r2560446777