channeldb: PruneGraph now returns the channels closed

This commit alters the return value of PrunedGraph to me a bit more
useful: the function now returns all the channels that were closed when
processing the block (slice of spent outpoints). With this information,
callers gain greater visibility into exactly which channels were
closed. This can be used in higher levels to present detailed summaries
of how blocks affect closed channels.

channeldb/graph.go

@@ -509,12 +509,12 @@ const (
 // funding output of any known channels within he graph will be deleted.
 // Additionally, the "prune tip", or the last block which has been used to
 // prune the graph is stored so callers can ensure the graph is fully in sync
-// with the current UTXO state. An integer is returned which reflects the
-// number of channels pruned due to the new incoming block.
+// with the current UTXO state. A slice of channels that have been closed by
+// the target block are returned if the function succeeds without error.
 func (c *ChannelGraph) PruneGraph(spentOutputs []*wire.OutPoint,
-	blockHash *chainhash.Hash, blockHeight uint32) (uint32, error) {
+	blockHash *chainhash.Hash, blockHeight uint32) ([]*ChannelEdgeInfo, error) {
 
-	var numChans uint32
+	var chansClosed []*ChannelEdgeInfo
 
 	err := c.db.Update(func(tx *bolt.Tx) error {
 		// First grab the edges bucket which houses the information
@@ -541,16 +541,35 @@ func (c *ChannelGraph) PruneGraph(spentOutputs []*wire.OutPoint,
 			// TODO(roasbeef): load channel bloom filter, continue
 			// if NOT if filter
 
-			// Attempt to delete the channel, and ErrEdgeNotFound
+			var opBytes bytes.Buffer
+			if err := writeOutpoint(&opBytes, chanPoint); err != nil {
+				return nil
+			}
+
+			// First attempt to see if the channel exists within
+			// the database, if not, then we can exit early.
+			chanID := chanIndex.Get(opBytes.Bytes())
+			if chanID == nil {
+				continue
+			}
+
+			// However, if it does, then we'll read out the full
+			// version so we can add it to the set of deleted
+			// channels.
+			edgeInfo, err := fetchChanEdgeInfo(edgeIndex, chanID)
+			if err != nil {
+				return err
+			}
+			chansClosed = append(chansClosed, edgeInfo)
+
+			// Attempt to delete the channel, an ErrEdgeNotFound
 			// will be returned if that outpoint isn't known to be
 			// a channel. If no error is returned, then a channel
 			// was successfully pruned.
-			err := delChannelByEdge(edges, edgeIndex, chanIndex,
+			err = delChannelByEdge(edges, edgeIndex, chanIndex,
 				chanPoint)
 			if err != nil && err != ErrEdgeNotFound {
 				return err
-			} else if err == nil {
-				numChans += 1
 			}
 		}
 
@@ -560,7 +579,7 @@ func (c *ChannelGraph) PruneGraph(spentOutputs []*wire.OutPoint,
 		}
 
 		// With the graph pruned, update the current "prune tip" which
-		// can eb used to check if the graph is fully synced with the
+		// can be used to check if the graph is fully synced with the
 		// current UTXO state.
 		var newTip [pruneTipBytes]byte
 		copy(newTip[:], blockHash[:])
@@ -569,10 +588,10 @@ func (c *ChannelGraph) PruneGraph(spentOutputs []*wire.OutPoint,
 		return metaBucket.Put(pruneTipKey, newTip[:])
 	})
 	if err != nil {
-		return 0, err
+		return nil, err
 	}
 
-	return numChans, nil
+	return chansClosed, nil
 }
 
 // PruneTip returns the block height and hash of the latest block that has been
@@ -905,7 +924,6 @@ func (c *ChannelGraph) HasLightningNode(pub *btcec.PublicKey) (time.Time, bool,
 // boltdb transaction, then it should be passed as the first argument.
 // Otherwise the first argument should be nil and a fresh transaction will be
 // created to execute the graph traversal.
-func (l *LightningNode) ForEachChannel(tx *bolt.Tx, cb func(*ChannelEdge) error) error {
 func (l *LightningNode) ForEachChannel(tx *bolt.Tx, cb func(*ChannelEdgeInfo, *ChannelEdgePolicy) error) error {
 	// TODO(roasbeef): remove the option to pass in a transaction after
 	// all?