Add ErrRecordExists sentinel and IsAlreadyExists predicate - #2
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Insert is specified to fail when a key already exists, but there was no exported way to tell that failure apart from any other insert error — the mirror-image gap of ErrRecordNotFound/IsNotFound. Downstream consumers (sneat-co/slugs, bookius) currently must treat every insert failure as "slug taken", so a transient backend blip is misreported to users as a name collision. ErrRecordExists/IsAlreadyExists follow the exact ErrRecordNotFound/IsNotFound idiom: a wrapped sentinel plus an errors.Is-based predicate. IsAlreadyExists positively identifies a confirmed conflict; false is not proof of the opposite, because an adapter that hasn't adopted the sentinel yet will never return it. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Summary
ErrRecordExistsandIsAlreadyExists, the mirror image of the existingErrRecordNotFound/IsNotFoundpair, so an adapter that can reliably tell "insert failed because the key already exists" from any other insert failure has an exported sentinel/predicate to use.errors.Newsentinel plus anerrors.Is-based predicate, documented with the caveat thatfalseis not proof of "not a conflict" — only adapters that adopt the sentinel reporttrue.Why
Insertis specified to fail when a key already exists, and adapters already implement that, but there is no way for a caller to distinguish that failure from any other insert error. This has already caused a real defect:sneat-co/slugsclaims a unique slug viatx.Insertand, lacking any way to tell "already exists" apart, treats every insert failure asErrSlugTaken— so a transient backend blip is misreported to a user as their chosen name being taken.sneat-co/bookiushas the identical issue.This is step 1 of a 3-repo rollout:
dal-go/record(this PR) →dal-go/dalgo(adapters + conformance suite) →dal-go/dalgo2firestore(the adapter that actually fixes the live defect, since Firestore is what Sneat runs on).Test plan
go build ./...go vet ./...go test -race ./... -covergofmt -l .(clean except a pre-existing, untouchedupdate/update.gofinding)golangci-lint run ./...(0 issues)errors_test.gocovers both predicates: true on the sentinel (bare and wrapped), false onnil, the other sentinel, and an unrelated error🤖 Generated with Claude Code