Reading the tags of a file materialised every attachment, so a fast read of
a file with cover art cost as much as the art is large, even though the
caller may never ask for it. Measured with a counting IOStream over
tests/data/no-tags.mka with an attachment behind its seek head, read with
AudioProperties::Fast:
attachment | read before | read now
----------------------------------
128 KiB | 131350 B | 278 B
512 KiB | 524544 B | 256 B
4 MiB | 4194563 B | 259 B
The number of read calls is unchanged (183, 164, 164 before; 182, 163, 163
now), so this is volume, not round trips, and what remains does not grow
with the payload.
MkAttachedFileData is now a DeferredBinaryElement, which registers the offset
of its data and skips over it. Matroska::File::attachments() reads the data
before handing the attachments out, so callers see no difference.
save() renders the attachments from the attached files, which would write an
empty attachment for data that was never requested. It therefore loads the
data before writing.
Co-authored-by: Claude Opus 5 <[email protected]>
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