Every BlockGroup frame back-patched its element size via ebml::end_master, which
does two stream_position() calls and two real seeks. BufWriter does not override
Seek::stream_position, so each position query is seek(Current(0)) = flush_buf +
lseek — and each of those flushed the 4 MiB BufWriter while every position-moving
seek reset WritebackPipeline::last_flush_pos. The buffer never got to do its job.
An MPEG-2 title takes this path for EVERY frame (the parser stamps a per-frame
duration, so I, P and B all become BlockGroups) — roughly 350,000 per feature.
A BlockGroup's size is knowable before writing, so there is no need to back-patch
at all. New seek-free twins start_master_buf / end_master_buf patch a placeholder
by buffer INDEX instead of file offset, and build_block_group assembles the whole
element into a persistent buffer that write_block_group and its MVC sibling take,
fill, write once, and hand back — including on the error path — so the allocation
is made once rather than per frame.
Measured with a counting writer that, like BufWriter, does not override
stream_position, over 200 frames:
seek calls position-moving
plain before 912 451
plain after 112 51
MVC before 2516 1253
MVC after 116 53
Per-frame cost is now zero; the residual is the header, the per-cluster
back-patch and Cues. For 350k BlockGroups that is 1.4M seek calls removed on the
plain path, 4.2M on an MVC title.
end_master is deliberately NOT changed for its other callers. The Cluster master
genuinely streams — frames are appended to an open cluster over time, so its body
cannot be buffered without holding a whole cluster in memory — and the rest
(EBML header, Tracks, Info, Chapters, Cues) run once, not per frame.
Byte-identity is the safety property, and it is structural: end_master always
patches a FIXED-width 8-byte VINT, and the buffered pair writes and patches
exactly that same placeholder, so the encodings cannot differ. Verified by
capture-then-compare over 200 frames (17 keyframes / 183 non-keyframes, multiple
clusters, BlockDuration present and absent, both reference branches) — output
byte-for-byte identical.
Three tests now pin it permanently: buffered vs seeking output byte-for-byte for
empty/tiny/multi-byte bodies, the same for NESTED masters (the MVC path nests
BlockAdditions > BlockMore, where an index-arithmetic slip would surface), and
end_master_buf erroring rather than panicking on a position outside the buffer.
All three verified red against a deliberately divergent placeholder width.
A note on verifying this kind of change: comparing emitted MKV across two
worktrees at different commits shows a spurious 14-byte difference, because
MuxingApp/WritingApp embed the build's git SHA twice. Compare at the same base.
Matthew Jackson
e3676e7cdf