The OutOfOrderBuffer had a deadlock issue where:
1. Buffer fills with 8 pages from workers
2. Next expected page (e.g., page 0) is missing
3. All workers block trying to push more pages
4. Deadlock because no one can push page 0
Fix: Implement smarter backpressure that:
- Blocks when buffer is full AND next expected page is missing
- Allows push if we're pushing the missing next expected page
- Allows push if next expected page is already in buffer
Also add pop_next_in_order_blocking() for multi-threaded scenarios.
Acceptance criteria:
- Unit test: push pages 3,1,4,1,5,9,2,6 -> pop in 0..=9 order PASS
- Backpressure test: 9th push blocks until page 0 arrives PASS
- Concurrency stress test: 8 workers + 1 consumer, 1000 pages PASS
- finish() test: producer finished, heap drained -> pop returns None PASS
Closes: pdftract-31bum
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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