Fix deadlock in concurrent tracker mapping

The tracker-mapping worker runs many Process calls in parallel. In
Phase 4 a single transaction locked the worker's own claimed pattern
row via UpdateMapping and then locked sibling rows on the same banner
via the re-enqueue. Two workers mapping sibling patterns on one banner
each held their own row and waited on the other's, forming a lock cycle
that Postgres aborted with deadlock detected (40P01).

Split the sibling re-enqueue into its own short transaction that runs
after the mapping commits, so the claimed-row lock is released before
any sibling row is locked. Also take the sibling UPDATE row locks in a
deterministic id order through an ORDER BY id ... FOR UPDATE subquery,
so overlapping re-enqueues can no longer invert lock order between
themselves. The re-enqueue only flags siblings, so deferring it past
the commit is safe and lets reprocessed siblings observe committed data.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/cookiebanner/tracker_mapping_worker.go

@@ -289,7 +289,9 @@ func (h *trackerMappingHandler) Process(ctx context.Context, tp coredata.Tracker
 	// Phase 4: persist the pattern mapping in a short transaction. The
 	// unmatched fallback keeps catalog coverage complete even when no
 	// vendor was resolved.
-	return h.pg.WithTx(
+	mapped := true
+
+	if err := h.pg.WithTx(
 		ctx,
 		func(ctx context.Context, tx pg.Tx) error {
 			if commonPatternID == nil {
@@ -331,6 +333,8 @@ func (h *trackerMappingHandler) Process(ctx context.Context, tp coredata.Tracker
 						log.String("tracker_pattern_id", tp.ID.String()),
 					)
 
+					mapped = false
+
 					return nil
 				}
 
@@ -344,20 +348,39 @@ func (h *trackerMappingHandler) Process(ctx context.Context, tp coredata.Tracker
 				log.String("tracker_pattern_id", tp.ID.String()),
 			)
 
-			// This run newly resolved a catalog third party, so
+			return nil
-			// same-banner siblings that share an initiator domain but
+		},
-			// were processed earlier and left unmatched can now match
+	); err != nil {
-			// against it. Re-arm their mapping so the worker revisits
+		return err
-			// them; the guards keep already-mapped siblings untouched.
+	}
-			if commonThirdPartyID != nil && !det.commonThirdPartyPreexisted {
+
-				if err := h.reenqueueUnmappedSiblings(ctx, tx, tp, det.domains); err != nil {
+	// Phase 5: re-arm same-banner siblings in a separate short
+	// transaction, after the mapping above has committed. This run newly
+	// resolved a catalog third party, so siblings that share an initiator
+	// domain but were processed earlier and left unmatched can now match
+	// against it. Re-arm their mapping so the worker revisits them; the
+	// guards keep already-mapped siblings untouched.
+	//
+	// The re-enqueue must not run inside the Phase 4 transaction: that
+	// transaction holds the row lock on tp, and the sibling UPDATE then
+	// takes locks on other tracker_patterns rows while holding it. Two
+	// workers mapping sibling patterns on the same banner would acquire
+	// those row locks in opposite orders and deadlock. Committing Phase 4
+	// first releases tp's lock, and RequestMappingForUnmappedSiblings
+	// takes its locks in a deterministic id order, so the two can no
+	// longer cycle.
+	if mapped && commonThirdPartyID != nil && !det.commonThirdPartyPreexisted {
+		if err := h.pg.WithTx(
+			ctx,
+			func(ctx context.Context, tx pg.Tx) error {
+				return h.reenqueueUnmappedSiblings(ctx, tx, tp, det.domains)
+			},
+		); err != nil {
 			return err
 		}
 	}
 
 	return nil
-		},
-	)
 }
 
 // deterministicResult carries the outcome of the pure-SQL catalog

pkg/coredata/tracker_pattern.go

@@ -1286,11 +1286,19 @@ func (tps *TrackerPatterns) RequestMappingForUnmappedSiblings(
 		return 0, nil
 	}
 
+	// The target rows are locked through an ORDER BY id ... FOR UPDATE
+	// subquery so concurrent re-enqueues over overlapping sibling sets
+	// always acquire their row locks in the same ascending id order. Two
+	// workers mapping sibling patterns on the same banner would otherwise
+	// lock the shared rows in opposite orders and deadlock (40P01).
 	q := `
 UPDATE tracker_patterns
 SET
 	mapping_requested_at = NOW(),
 	updated_at = NOW()
+WHERE id IN (
+	SELECT id
+	FROM tracker_patterns
 	WHERE
 		%[1]s
 		AND cookie_banner_id = @cookie_banner_id
@@ -1306,6 +1314,9 @@ WHERE
 				AND initiator_domain = ANY(@domains)
 				AND tracker_pattern_id IS NOT NULL
 		)
+	ORDER BY id
+	FOR UPDATE
+)
 `
 
 	q = fmt.Sprintf(q, scope.SQLFragment())