48 lines
2.3 KiB
PL/PgSQL
48 lines
2.3 KiB
PL/PgSQL
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-- Garbage-collection safety for orphaned blobs.
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--
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-- The dedup GC deletes a blob row (committed) and then unlinks the backing
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-- file. A concurrent uploader of identical content can re-reference a chunk in
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-- that window. Two mechanisms make the sweep safe:
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-- (a) garbage_collect() never collects a blob still referenced by a manifest
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-- (chunk) or a file (legacy whole-file blob) — cross-checks backed by
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-- idx_chunk_manifests_chunk_hashes_gin and idx_files_blob_hash. A stale
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-- ref_count = 0 on live content can then only delay collection, never
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-- delete it.
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-- (b) garbage_collect() never collects a blob that became unreferenced only
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-- moments ago — the grace period below, mirroring git's gc.pruneExpire,
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-- so a writer about to pin a just-orphaned chunk cannot race the sweep.
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--
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-- `orphaned_at` records when ref_count last reached 0. NULL means the row is
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-- referenced (ref_count > 0) or predates this column.
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ALTER TABLE storage.blobs ADD COLUMN IF NOT EXISTS orphaned_at TIMESTAMPTZ;
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-- Existing orphans start their grace window now, so applying this migration
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-- never triggers an immediate sweep of content a writer might still be racing.
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UPDATE storage.blobs
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SET orphaned_at = now()
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WHERE ref_count <= 0 AND orphaned_at IS NULL;
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-- GC scan index: orphan rows ordered by when they became collectible. Replaces
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-- the old ref_count-only partial index (the GC now also filters on orphaned_at).
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DROP INDEX IF EXISTS storage.idx_blobs_orphaned;
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CREATE INDEX IF NOT EXISTS idx_blobs_gc_eligible
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ON storage.blobs (orphaned_at) WHERE ref_count = 0;
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-- Stamp orphaned_at when a file delete drops a blob's ref_count to 0, so the
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-- grace window starts at the moment of orphaning. No-op for multi-chunk files
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-- whose file_hash is not itself a storage.blobs row.
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CREATE OR REPLACE FUNCTION storage.decrement_blob_ref()
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RETURNS trigger AS $$
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BEGIN
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UPDATE storage.blobs
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SET ref_count = GREATEST(ref_count - 1, 0),
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orphaned_at = CASE WHEN GREATEST(ref_count - 1, 0) = 0 THEN now() ELSE orphaned_at END
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WHERE hash = OLD.blob_hash;
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RETURN OLD;
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END;
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$$ LANGUAGE plpgsql;
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COMMENT ON COLUMN storage.blobs.orphaned_at IS
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'When ref_count last reached 0; GC waits a grace period past this before deleting (NULL = referenced or pre-migration)';
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