fix/non-determinism-in-aggregator-order

Author: mchain0

pkg/capabilities/consensus/ocr3/aggregators/reduce_aggregator.go

@@ -468,16 +468,17 @@ func mode(items []values.Value) (values.Value, int, error) {
 		}
 	}
 
-	var modes []values.Value
-	for _, ctr := range counts {
+	var tied [][32]byte
+	for sha, ctr := range counts {
 		if ctr.count == maxCount {
-			modes = append(modes, ctr.fullObservation)
+			tied = append(tied, sha)
 		}
 	}
-
-	// If more than one mode found, choose first
-
-	return modes[0], maxCount, nil
+	slices.SortFunc(tied, func(a, b [32]byte) int {
+		return bytes.Compare(a[:], b[:])
+	})
+	// If more than one mode ties for max count, pick the one with smallest content hash (stable across nodes).
+	return counts[tied[0]].fullObservation, maxCount, nil
 }
 
 func modeHasQuorum(quorumType string, count int, f int) error {

pkg/capabilities/consensus/ocr3/aggregators/reduce_aggregator_mode_test.go

@@ -0,0 +1,40 @@
+package aggregators
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/proto"
+
+	"github.com/smartcontractkit/chainlink-protos/cre/go/values"
+	"github.com/smartcontractkit/chainlink-protos/cre/go/values/pb"
+)
+
+// TestMode_twoWayTieIsDeterministicAcrossRepeatedCalls targets a bimodal tie: two
+// distinct values each appear the same number of times (here, 3× "tie-a" and 3× "tie-b").
+// The winner must not depend on map iteration order, so repeated calls with the same
+// multiset must return an observation equal under protobuf semantics every time.
+func TestMode_twoWayTieIsDeterministicAcrossRepeatedCalls(t *testing.T) {
+	t.Parallel()
+
+	a, err := values.Wrap("tie-a")
+	require.NoError(t, err)
+	b, err := values.Wrap("tie-b")
+	require.NoError(t, err)
+	items := []values.Value{a, a, a, b, b, b}
+
+	var baseline *pb.Value
+	for i := range 400 {
+		got, maxCount, err := mode(items)
+		require.NoError(t, err)
+		require.Equal(t, 3, maxCount)
+
+		gotProto := values.Proto(got)
+		if i == 0 {
+			baseline = proto.Clone(gotProto).(*pb.Value)
+			continue
+		}
+		require.True(t, proto.Equal(baseline, gotProto),
+			"iteration %d: mode() must pick the same tied winner on every call", i)
+	}
+}