channeldb: add new ChannelView method to the ChannelGraph

This commit introduces a new method to the ChannelGraph struct:
ChannelView. This struct returns all the outpoints that represent the
set of active channels within the network. The set of items returned by
this new method will possibly shrink with each call to `PruneGraph`,
and possibly be expanded by each call to `AddChannelEdge`.

The graph pruning tests have been updated to ensure the description
above holds true.

channeldb/graph_test.go

@@ -683,7 +683,28 @@ func asserNumChans(t *testing.T, graph *ChannelGraph, n int) {
 	}
 	if numChans != n {
 		_, _, line, _ := runtime.Caller(1)
-		t.Fatalf("line %v: expected %v chans instead have %v", line, n, numChans)
+		t.Fatalf("line %v: expected %v chans instead have %v", line,
+			n, numChans)
+	}
+}
+
+func assertChanViewEqual(t *testing.T, a []wire.OutPoint, b []*wire.OutPoint) {
+	if len(a) != len(b) {
+		_, _, line, _ := runtime.Caller(1)
+		t.Fatalf("line %v: chan views dont match", line)
+	}
+
+	chanViewSet := make(map[wire.OutPoint]struct{})
+	for _, op := range a {
+		chanViewSet[op] = struct{}{}
+	}
+
+	for _, op := range b {
+		if _, ok := chanViewSet[*op]; !ok {
+			_, _, line, _ := runtime.Caller(1)
+			t.Fatalf("line %v: chanPoint(%v) not found in first view",
+				line, op)
+		}
 	}
 }
 
@@ -768,6 +789,14 @@ func TestGraphPruning(t *testing.T) {
 		}
 	}
 
+	// With all the channel points added, we'll consult the graph to ensure
+	// it has the same channel view as the one we just constructed.
+	channelView, err := graph.ChannelView()
+	if err != nil {
+		t.Fatalf("unable to get graph channel view: %v", err)
+	}
+	assertChanViewEqual(t, channelView, channelPoints)
+
 	// Now with our test graph created, we can test the pruning
 	// capabilities of the channel graph.
 
@@ -793,6 +822,13 @@ func TestGraphPruning(t *testing.T) {
 	// should be remaining.
 	asserNumChans(t, graph, 2)
 
+	// Those channels should also be missing from the channel view.
+	channelView, err = graph.ChannelView()
+	if err != nil {
+		t.Fatalf("unable to get graph channel view: %v", err)
+	}
+	assertChanViewEqual(t, channelView, channelPoints[2:])
+
 	// Next we'll create a block that doesn't close any channels within the
 	// graph to test the negative error case.
 	fakeHash := sha256.Sum256([]byte("test prune"))
@@ -833,12 +869,22 @@ func TestGraphPruning(t *testing.T) {
 			2, len(prunedChans))
 	}
 
-	// TODO(roasbeef): asser that proper chans have been closed
-
 	// The prune tip should be updated, and no channels should be found
 	// within the current graph.
 	assertPruneTip(t, graph, &blockHash, blockHeight)
 	asserNumChans(t, graph, 0)
+
+	// Finally, the channel view at this point in the graph should now be
+	// completely empty.
+	// Those channels should also be missing from the channel view.
+	channelView, err = graph.ChannelView()
+	if err != nil {
+		t.Fatalf("unable to get graph channel view: %v", err)
+	}
+	if len(channelView) != 0 {
+		t.Fatalf("channel view should be empty, instead have: %v",
+			channelView)
+	}
 }
 
 // compareNodes is used to compare two LightningNodes while excluding the