feat: repair matcher candidates through redis projection

server/store/candidate_projection_test.go

@@ -46,3 +46,28 @@ func TestCandidateProjectionDoesNotReturnCacheWhenRepairSourceFails(t *testing.T
 		t.Fatal("cache projection succeeded without a usable Redis/index source")
 	}
 }
+
+func TestCandidateProjectionRepairsEmptyIndexFromDurableSource(t *testing.T) {
+	mini, err := miniredis.Run()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer mini.Close()
+	client := redis.NewClient(&redis.Options{Addr: mini.Addr()})
+	defer client.Close()
+	now := time.Unix(1000, 0).UTC()
+	candidate := domain.Candidate{TicketID: "miss-ticket", PlayerID: "miss-player", EnqueuedAt: now}
+	projection := CandidateProjection{
+		Index: RedisCandidateIndex{Client: client, Prefix: "miss", TTL: time.Minute},
+		Source: func(context.Context, time.Time) ([]domain.Candidate, error) {
+			return []domain.Candidate{candidate}, nil
+		},
+	}
+	got, err := projection.Snapshot(context.Background(), now)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(got) != 1 || got[0].TicketID != candidate.TicketID {
+		t.Fatalf("empty-index repair = %+v", got)
+	}
+}

server/store/redis_candidates.go

@@ -49,6 +49,15 @@ func (p CandidateProjection) Snapshot(ctx context.Context, now time.Time) ([]dom
 	}
 	candidates, err := p.Index.Snapshot(ctx, now)
 	if err == nil {
+		// An empty index is indistinguishable from a Redis restart or a lost
+		// keyspace. Rebuild from PostgreSQL before returning so queued players
+		// are not hidden until the next enqueue mutation.
+		if len(candidates) == 0 {
+			if err := p.Repair(ctx, now); err != nil {
+				return nil, err
+			}
+			return p.Index.Snapshot(ctx, now)
+		}
 		return candidates, nil
 	}
 	if err := p.Repair(ctx, now); err != nil {