gemini: explicit return of values after mutations

The mutation routines now return the used values to the generators
that reuse these for the "old" pks that validators use.

cmd/gemini/jobs.go

@@ -183,6 +183,11 @@ func mutation(ctx context.Context, schema *gemini.Schema, _ *gemini.SchemaConfig
 	}
 	mutateQuery := mutateStmt.Query
 	mutateValues := mutateStmt.Values()
+	defer func() {
+		v := make(gemini.Value, len(table.PartitionKeys))
+		copy(v, mutateValues)
+		source.GiveOld(v)
+	}()
 	if w := logger.Check(zap.DebugLevel, "validation statement"); w != nil {
 		w.Write(zap.String("pretty_cql", mutateStmt.PrettyCQL()))
 	}

generator.go

@@ -19,16 +19,7 @@ type Source struct {
 }
 
 func (s *Source) Get() (Value, bool) {
-	v := s.pick()
-	values := make([]interface{}, len(v))
-	// Make a copy to allow callers to work with the slice directly
-	copy(values, v)
-	select {
-	case s.oldValues <- v:
-	default:
-		// Old source is full, just drop the value
-	}
-	return values, true
+	return s.pick(), true
 }
 
 func (s *Source) GetOld() (Value, bool) {
@@ -41,6 +32,14 @@ func (s *Source) GetOld() (Value, bool) {
 	}
 }
 
+func (s *Source) GiveOld(v Value) {
+	select {
+	case s.oldValues <- v:
+	default:
+		// Old source is full, just drop the value
+	}
+}
+
 func (s *Source) pick() Value {
 	return s.values[s.idxFunc()]
 }