chanevents: add short-chan-id store lookup

Add GetChannelByShortChanID, the inverse of AddChannel. The
forwarding-ability analyzer receives scids from lnd's forwarding history
and must map them back to the chanevents store's internal channel id to
query events.

chanevents/store.go

@@ -192,6 +192,30 @@ func (s *Store) GetChannel(ctx context.Context, channelPoint string) (*Channel,
 	}, nil
 }
 
+// GetChannelByShortChanID retrieves a channel by its short channel ID,
+// returning ErrUnknownChannel if no row matches.
+func (s *Store) GetChannelByShortChanID(ctx context.Context,
+	shortChannelID uint64) (*Channel, error) {
+
+	dbChannel, err := s.db.GetChannelByShortChanID(
+		ctx, scidToInt64(shortChannelID),
+	)
+	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			return nil, ErrUnknownChannel
+		}
+
+		return nil, err
+	}
+
+	return &Channel{
+		ID:             dbChannel.ID,
+		ChannelPoint:   dbChannel.ChannelPoint,
+		ShortChannelID: int64ToSCID(dbChannel.ShortChannelID),
+		PeerID:         dbChannel.PeerID,
+	}, nil
+}
+
 // AddChannelEvent adds a new channel event.
 func (s *Store) AddChannelEvent(ctx context.Context,
 	event *ChannelEvent) error {

chanevents/store_test.go

@@ -93,6 +93,20 @@ func TestStore(t *testing.T) {
 	require.Equal(t, testShortChanID1, dbChannel.ShortChannelID)
 	require.Equal(t, peerID, dbChannel.PeerID)
 
+	// Look up the same channel by its scid; the analyzer relies on this
+	// inverse of AddChannel.
+	dbChannel, err = store.GetChannelByShortChanID(ctx, testShortChanID1)
+	require.NoError(t, err)
+	require.Equal(t, channelID, dbChannel.ID)
+	require.Equal(t, testChanPoint1, dbChannel.ChannelPoint)
+	require.Equal(t, testShortChanID1, dbChannel.ShortChannelID)
+	require.Equal(t, peerID, dbChannel.PeerID)
+
+	// An unknown scid surfaces the typed sentinel, not raw sql.ErrNoRows.
+	dbChannel, err = store.GetChannelByShortChanID(ctx, 9999)
+	require.ErrorIs(t, err, ErrUnknownChannel)
+	require.Nil(t, dbChannel)
+
 	// Add a second channel for the same peer.
 	channel2ID, err := store.AddChannel(
 		ctx, testChanPoint2, testShortChanID2, peerID,