`content_derived_blobs.variant` held the size alone, so one source could
hold exactly one artifact per size regardless of codec. That surfaced
when the read order flipped in 10c: a JPEG request matched the WebP row
and would have been served the wrong codec — hidden previously because
the .jpg sidecar won first. The flip had to be gated to WebP, which
meant JPEG clients could never leave the sidecar, which meant the
sidecar could never be deleted.
It blocks transcodes harder: those are multi-format by nature, so two
output codecs of one source collide on the primary key without a format
term.
The axis goes inside the string rather than into a fourth PK column,
per the column's own rule — "new axes go inside this string, never into
new columns". Shape is {size}.{ext}: preview.webp, icon.jpg, later
720p.webp.
The backfill is deterministic, not a guess: store_derived_blob has only
ever written "image/webp" for thumbnails. content_type is checked anyway
rather than assumed — a row that fails the assumption is left alone and
counted in a warning, because the read path then simply misses it and
falls back to the sidecar, whereas guessing a codec would serve wrong
bytes. Idempotent via NOT LIKE '%.%', so a re-apply cannot produce
preview.webp.webp; verified on a scratch PG by applying it twice.
One helper builds the string, because it is a primary-key component:
a writer and reader that disagree do not fail loudly, they just never
find each other's rows and the derived tier silently looks empty. It
lives on the service's ThumbnailSize, not the port's — they are distinct
types, which the compiler pointed out after I put it on the wrong one.
The WebP gate on the read path is now removed: each codec has its own
row, so JPEG can finally reach the derived tier — the prerequisite for
deleting the sidecar for those clients.
file_attached_blobs keeps a bare size: store_external_thumbnail
re-encodes everything to JPEG, so it is single-format by construction
and a format term would cost a migration for nothing.
Edouard Vanbelle
86d0d65583