PageBuffer started a ProcessPoolExecutor(max_workers=4) and submitted page
renders to it. Every job failed. _render_page_worker returned
pickle.dumps(page), and a Page holds a live PIL canvas, which is not
picklable, so check_completed_renders swallowed a TypeError into a bare
print and cached nothing. The cost was paid in full for zero benefit:
four interpreter copies plus the whole block list shipped per job (~880KB
for a 200-block document).
Three further defects would have had to be fixed before it could ever
have worked: the worker built its BidirectionalLayouter without page_size
so it silently used the (800, 600) default; check_completed_renders
cached every result with is_backward=False, so backward renders landed in
the forward buffer; and both _queue_*_renders broke at the end of their
first loop body, queueing one page each despite looping buffer_size
times.
Two live defects go with it:
R1 - on Python 3.14 the default start method became forkserver, so
submit() reaches _check_not_importing_main() and raises unless the
caller sits inside an `if __name__ == "__main__"` guard.
EreaderLayoutManager.get_current_page() raised outright from ordinary
module-level script code.
R2 - PageBuffer.__del__ called executor.shutdown(wait=True). Blocking
on a process pool from a finaliser at interpreter teardown deadlocked;
the test suite finished in 11.5s and then never exited.
S12's measurement gate, on the tests/data Wikipedia fixture (411 blocks)
with text caches warm:
800x600 p50 8.8 ms p95 15.4 ms
1072x1448 p50 13.8 ms p95 56.1 ms
A page turn is cheaper than the IPC meant to hide it, so the gate says
delete rather than replace. The LRU buffers, position maps and
invalidation logic are kept unchanged; only the executor, worker,
pickling, prefetch queueing and the lock guarding the pending-render dict
are removed. If a slower device ever changes the numbers, the fallback is
a synchronous readahead() method or a single worker thread, not
processes.
EreaderLayoutManager.shutdown() becomes idempotent and its __del__ no
longer propagates exceptions - it was doing bookmark file I/O during
interpreter teardown.
Adds tests/layout/test_page_buffer.py, which the module had none of:
LRU eviction and position-map cleanup, cache hits, font-scale
invalidation, backward round-trip, and subprocess regressions for R1
(no __main__ guard) and R2 (exit without explicit shutdown).
870 passed, and the suite now exits in 12s wall instead of hanging.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
