Resolves the .skip-marker question. The table can say "here is the
artifact" but not "there is deliberately no artifact", and absence of a
row is ambiguous — it means both never-attempted and attempted-and-not-
worth-it, which destroys the only thing a negative cache holds.
Two live cases, not one. Transcodes drop a .skip marker when WebP is not
smaller. Thumbnails return empty Bytes for undecodable sources and ones
over MAX_DECODE_PIXELS — RAM-only today, so after moka evicts, a
60-megapixel upload has its full decode attempted again, forever. Same
gap, same table, so do both together.
Nullable rather than a sentinel hash: a sentinel stops blob_hash naming
a real blob and every future reader has to know the lie. Costs are one
`AND blob_hash IS NOT NULL` in ContentDerivedReferenceSource, the same
guard on the dangling-derived check, and dropping NOT NULL.
Permanent vs transient is the load-bearing split, and today's code
cannot tell them apart: generate_and_persist collapses every error into
empty Bytes, timeouts and semaphore closures included. Survivable while
the sentinel lives in moka, which evicts. Persist that same signal and a
thumbnail that timed out once under load is unrenderable forever. So the
renderer must return a typed outcome first, and only
permanently-unrenderable earns a row. The asymmetry sets the default —
a wrongly-cached transient is silent and permanent, a not-cached
permanent only costs repeated work — so unclassified errors are treated
as transient.
No TTL and no renderer-version term. Time is the wrong axis: the verdict
is deterministic in (content, encoder) and does not decay, so a TTL
re-attempts an OOMing decode on a schedule while still leaving staleness
for most of the window after a deploy. What removes the need for a
mechanism is that negative rows are disposable — they hold no data, so
a library upgrade invalidates them with a DELETE ... WHERE blob_hash IS
NULL in the same migration as the dependency bump. Recorded explicitly
so nobody later builds the expiry logic this replaces.
Edouard Vanbelle
647929ed11