io: phase 2.5 — writer thread + bounded ring + active-cluster buffer

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WritebackFile now offloads all File I/O to a dedicated writer thread.
Muxer's Write/Seek/sync_all push into a bounded byte ring; the writer
drains the ring and runs the syscalls.

- Commands: Write(Vec<u8>), Seek(SeekFrom), SyncAll(oneshot),
  Finish(oneshot). One ring carries all four; ordering preserved.
- ActiveClusterBuffer mirrors the last ACTIVE_CLUSTER_WINDOW_BYTES
  written. Seek-back within window → in-memory patch + re-emit
  (no forced drain). Seek-back outside window → real drain+seek.
  Wins for MKV: cluster size patches almost always land inside the
  active window; only the end-of-mux Cues + segment header backpatch
  fall outside.
- Ring capacity RING_CAPACITY_BYTES = 128 MiB. Backpressure on full
  blocks the muxer (correct semantics for archival workflows).
- All syscalls in writer thread wrapped in bounded_syscall(60s) so
  a wedged NFS doesn't pin the thread forever.
- Drop drains via Finish + joins the writer thread; sync_all blocks
  until ring drained AND underlying fsync completes.

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This commit is contained in:

matthew

2026-05-13 20:17:29 -07:00

parent baa789852d

commit 4c47a243ea

1 changed files with 887 additions and 64 deletions

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src/io/writeback_file/mod.rs

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