graph/db: add test for SetSourceNode same timestamp behavior

This commit adds TestSetSourceNodeSameTimestamp to demonstrate the
current behavior when SetSourceNode is called with the same last update
timestamp. The test reveals a difference between the SQL and bbolt
implementations:

- SQL store returns sql.ErrNoRows when attempting to update with the
  same timestamp, as the upsert query's UPDATE clause requires the new
  timestamp to be strictly greater than the existing one
- bbolt store silently ignores stale updates and returns no error

This behavior is important to document because our own node
announcements may change quickly with the same timestamp, unlike
announcements from other nodes where same timestamp typically means
identical parameters.

graph/db/graph_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"crypto/sha256"
+	"database/sql"
 	"encoding/hex"
 	"errors"
 	"fmt"
@@ -396,6 +397,62 @@ func TestSourceNode(t *testing.T) {
 	compareNodes(t, testNode, sourceNode)
 }
 
+// TestSetSourceNodeSameTimestamp demonstrates that SetSourceNode can return an
+// error when called with the same last update timestamp. Calling SetSourceNode
+// with the same timestamp should be allowed (unlike AddNode), as it is
+// possible that our own node announcement may change quickly. This will be
+// fixed in an upcoming commit.
+func TestSetSourceNodeSameTimestamp(t *testing.T) {
+	t.Parallel()
+	ctx := t.Context()
+
+	graph := MakeTestGraph(t)
+
+	_, isSQLStore := graph.V1Store.(*SQLStore)
+
+	// Create and set the initial source node.
+	testNode := createTestVertex(t)
+	require.NoError(t, graph.SetSourceNode(ctx, testNode))
+
+	// Verify the source node was set correctly.
+	sourceNode, err := graph.SourceNode(ctx)
+	require.NoError(t, err)
+	compareNodes(t, testNode, sourceNode)
+
+	// Create a modified version of the node with the same timestamp but
+	// different parameters (e.g., different alias and color). This
+	// could well be the case for our own node announcement (unlike other
+	// announcements where same timestamp means same parameters).
+	modifiedNode := models.NewV1Node(
+		testNode.PubKeyBytes, &models.NodeV1Fields{
+			// Same timestamp.
+			LastUpdate: testNode.LastUpdate,
+			// Different alias.
+			Alias:        "different-alias",
+			Color:        color.RGBA{R: 100, G: 200, B: 50, A: 0},
+			Addresses:    testNode.Addresses,
+			Features:     testNode.Features.RawFeatureVector,
+			AuthSigBytes: testNode.AuthSigBytes,
+		},
+	)
+
+	// Attempt to set the source node with the same timestamp but
+	// different parameters.
+	err = graph.SetSourceNode(ctx, modifiedNode)
+
+	// The SQL store will return sql.ErrNoRows because the UPDATE clause
+	// in the upsert query requires the new timestamp to be strictly
+	// greater than the existing one. When this condition is not met, no
+	// rows are updated and the SQL query returns ErrNoRows. The bbolt KV
+	// store, on the other hand, silently ignores stale updates and returns
+	// no error.
+	if isSQLStore {
+		require.ErrorIs(t, err, sql.ErrNoRows)
+	} else {
+		require.NoError(t, err)
+	}
+}
+
 // TestEdgeInsertionDeletion tests the basic CRUD operations for channel edges.
 func TestEdgeInsertionDeletion(t *testing.T) {
 	t.Parallel()