GC phase 2 already had the right shape — `ref_count <= 0 AND NOT
EXISTS(manifest lists it) AND NOT EXISTS(file points at it)` — so unlike
phase 1 before 6dc045ea, a stale counter could only delay collection
there, never delete live bytes. What it did not have is any connection
to `BlobReferenceRegistry`: the two cross-checks named
`storage.chunk_manifests` and `storage.files` literally.
That is correct today and one source away from not being. Both
`content_derived_blobs` and `file_attached_blobs` return None at
RefLevel::Chunk, so the registry's chunk union is exactly manifests +
legacy files. The moment anything contributes at that level — a legacy
whole-file derived blob, or file_versions when versioning lands — phase
2 misses it and reaps referenced bytes. That is precisely the failure
the registry was built to prevent, and precisely what the phase 1
comment warns about while phase 2 sat unfixed.
## Why this is additive, not a swap
`no_reference_predicate` is assembled from fragments designed for
COUNTING, and FilesReferenceSource's chunk-level fragment deliberately
excludes files whose blob_hash has a manifest — otherwise a single-chunk
blob, whose file hash and lone chunk hash are the same BLAKE3, would be
counted at both levels. Correct for a recompute; too narrow for a reap
guard.
Concretely: a `storage.blobs` row keyed by a MULTI-chunk file's hash is
not a member of its own manifest's chunk_hashes, and such rows exist
transiently while `rechunk` migrates a legacy blob. Replacing the
hardcoded guards with the registry predicate would have satisfied
"unreferenced" for that row while a live storage.files row still pointed
at it — reaping it mid-migration. So the guards stay and the registry
predicate is ANDed on top. Adding a conjunct can only spare more rows,
never reap more, so this cannot regress; what it buys is that a future
chunk-level source is honoured automatically.
## Also: EXISTS instead of COUNT in the hot path
ChunksReferenceSource had no `ref_exists_sql` override, so the trait
default wrapped its counting fragment as `(SELECT COUNT(*) …) > 0`. That
now runs per candidate row inside the reap guard, and a
heavily-deduplicated chunk is exactly where counting every referrer is
most expensive and least necessary. FilesReferenceSource already carried
this override for the same reason; ChunksReferenceSource now does too.
Semantically identical, so no golden-test drift beyond the shape.
## Tests
`blob_reap_statement_is_stable` pins the assembled statement, and
`empty_registry_refuses_to_build_blob_reap_statement` mirrors the
manifest builder's loud failure on a wiring bug.
`a_new_chunk_level_source_reaches_the_blob_reap_statement` is the one
that earns its keep: since no shipped source contributes at chunk level,
a golden test alone would not notice the registry conjunct being dropped.
It registers a synthetic source and asserts the fragment appears.
Verified 921 passed / 0 failed on a clean database, and again on a second
consecutive run against the same one.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Edouard Vanbelle
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