5.10.2: NPE in batch multi-sheet processing — parallel task lambda returns null, unboxed as Boolean in reachBookStep

[recrack]

Summary

Since 5.10.x, batch-mode transcription of any multi-sheet book fails with an NPE when sheet-parallel processing is enabled (omr.OmrExecutors.useParallelism default). Single-sheet books are unaffected.

WARN  Book 2073 | Error in performing PAGE
java.lang.NullPointerException: Cannot invoke "java.lang.Boolean.booleanValue()" because the return value of "java.util.concurrent.Future.get()" is null
	at org.audiveris.omr.sheet.Book.reachBookStep(Book.java:1981)
	at org.audiveris.omr.sheet.Book.transcribe(Book.java:2626)
	at org.audiveris.omr.sheet.Book.export(Book.java:593)
	at org.audiveris.omr.CLI$ProcessingTask.processBook(CLI.java:927)

Root cause

In Book.reachBookStep (parallel branch), the Callable<Boolean> lambda returns null:

tasks.add( () -> {
    LogUtil.start(stub);
    try {
        stub.reachStep(maxStep, force);
        if (OMR.gui == null) {
            stub.swapSheet();
        }
        return null;          // <-- here
    } finally { ... }
});

while the consuming loop unboxes the result:

for (Future<Boolean> future : futures) {
    if (!future.get()) {      // NPE: null.booleanValue()
        someFailure = true;
    }
}

In 5.8.1 the lambda returned true, so this is a regression. Still present on current master (ec7350e).

Reproduction

Any book with 2+ sheets, batch mode:

Audiveris -batch -transcribe -export some-multipage.pdf

→ every sheet transcribes fine, then Error in performing PAGE + Could not export since transcription did not complete successfully. With useParallelism=false the same book succeeds.

Fix + measured result

Restoring the 5.8.1 behavior (one line, return null; → return true;) fixes it. With that patch applied on top of 5.10.2 we ran our regression corpus (piano scores, OpenScore Lieder ground truth) — 5.10.2 then works and is a clear improvement over 5.8.1:

corpus item	5.8.1 pitch/note F1	5.10.2+fix pitch/note F1	time
lieder-volkslied (2 pages)	0.8174 / 0.7565	0.9652 / 0.8957	16.4s → 14.2s
lieder-an-laura	0.99 / 0.92	0.99 / 0.92	18.7s → 13.0s
lieder-der-winter	1.0 / 0.9913	1.0 / 0.9913	13.5s → 12.6s
4-page book	(no ground truth)	works, no NPE	34.4s → 24.5s

Happy to open a PR with the one-line fix if useful.

Environment: OpenJDK 25, macOS 14 (also reproduced in Debian bookworm container).

[hbitteur]

@recrack

Oops!
Fix published today on development branch, commit 81fe6b8
Thanks a lot.