The v3.3.102 log (real 224-job batch) confirmed History virtualization works and
steady-state uploads are pristine (fps=32, event-loop mean 11.8ms). The remaining
residual was a ~6s spin-up burst at batch start: the main event loop blocked for
336ms with cpu=0%core (i.e. blocked on I/O, not computing) and the renderer janked
196-391ms, then everything settled clean.
A multi-agent investigation plus an adversarial review corrected the obvious-looking
hypothesis. The renderer's uncapped progress-batch drain is NOT the cause:
handleProgress only mutates plain JS state and schedules already-coalesced renders
(one per frame), and main coalesces progress to ~50 latest-per-job entries per
100ms. Chunking that drain would fix nothing — and the reviewer showed it would
REGRESS correctness: requestAnimationFrame throttles to ~0 when the window is
minimized (the common state for a background uploader), so a rAF-chunked drain would
grow an unbounded backlog and defer persistQueueStateSoon for every buffered item,
losing terminal 'done' events on close (the queue-persistence ghost-fix class). So
that path is deliberately not taken.
The real cause (cpu=0%core = blocked on I/O) is a synchronous fs.statSync storm in
UploadManager.startBatch: the dedup loop ran up to DEDUP_CHUNK=200 synchronous
fs.statSync calls in a single tick before yielding (200 x ~1.68ms on the user's VM
= the exact 336ms), on a disk already saturated by the 1MB read-ahead.
Fix — make the batch-start stats non-blocking:
- The dedup loop now dedupes synchronously (cheap Map work) and then stats the
unique files in parallel via await Promise.all(fs.promises.stat ...) per chunk, so
the stat I/O runs on the libuv threadpool and the main thread never blocks. The
results-Map shape ({name,size,results:[]}) and dedup semantics (size 0 on failure)
are unchanged.
- The per-job statSync fallback is converted to await fs.promises.stat for
consistency (it sits in an async function before the first real await; the cached
size from dedup already lets nearly every job skip it).
Tests: the upload-manager mocks override fs.statSync; they now also override
fs.promises.stat with the same fake sizes (upload-manager.test.js x2,
suspect-reject-alternates.test.js). 407 tests pass; clean Electron boot.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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