graph/db: move Topology client management to ChannelGraph

We plan to later on add an option for a remote graph source which will
be managed from the ChannelGraph. In such a set-up, a node would rely on
the remote graph source for graph updates instead of from gossip sync.
In this scenario, however, our topology subscription logic should still
notify clients of all updates and so it makes more sense to have the
logic as part of the ChannelGraph so that we can send updates we receive
from the remote graph.

graph/db/graph_test.go

@@ -972,6 +972,23 @@ func randEdgePolicy(chanID uint64, db kvdb.Backend) *models.ChannelEdgePolicy {
 	return newEdgePolicy(chanID, db, update)
 }
 
+func copyEdgePolicy(p *models.ChannelEdgePolicy) *models.ChannelEdgePolicy {
+	return &models.ChannelEdgePolicy{
+		SigBytes:                  p.SigBytes,
+		ChannelID:                 p.ChannelID,
+		LastUpdate:                p.LastUpdate,
+		MessageFlags:              p.MessageFlags,
+		ChannelFlags:              p.ChannelFlags,
+		TimeLockDelta:             p.TimeLockDelta,
+		MinHTLC:                   p.MinHTLC,
+		MaxHTLC:                   p.MaxHTLC,
+		FeeBaseMSat:               p.FeeBaseMSat,
+		FeeProportionalMillionths: p.FeeProportionalMillionths,
+		ToNode:                    p.ToNode,
+		ExtraOpaqueData:           p.ExtraOpaqueData,
+	}
+}
+
 func newEdgePolicy(chanID uint64, db kvdb.Backend,
 	updateTime int64) *models.ChannelEdgePolicy {
 
@@ -2937,6 +2954,7 @@ func TestChannelEdgePruningUpdateIndexDeletion(t *testing.T) {
 	if err := graph.UpdateEdgePolicy(edge1); err != nil {
 		t.Fatalf("unable to update edge: %v", err)
 	}
+	edge1 = copyEdgePolicy(edge1) // Avoid read/write race conditions.
 
 	edge2 := randEdgePolicy(chanID.ToUint64(), graph.db)
 	edge2.ChannelFlags = 1
@@ -2945,6 +2963,7 @@ func TestChannelEdgePruningUpdateIndexDeletion(t *testing.T) {
 	if err := graph.UpdateEdgePolicy(edge2); err != nil {
 		t.Fatalf("unable to update edge: %v", err)
 	}
+	edge2 = copyEdgePolicy(edge2) // Avoid read/write race conditions.
 
 	// checkIndexTimestamps is a helper function that checks the edge update
 	// index only includes the given timestamps.
@@ -4052,6 +4071,7 @@ func TestGraphCacheForEachNodeChannel(t *testing.T) {
 		253, 217, 3, 8, 0, 0, 0, 10, 0, 0, 0, 20,
 	}
 	require.NoError(t, graph.UpdateEdgePolicy(edge1))
+	edge1 = copyEdgePolicy(edge1) // Avoid read/write race conditions.
 
 	directedChan := getSingleChannel()
 	require.NotNil(t, directedChan)