chanevents+db: add scid-to-peer index

Add ScidToPeerMap, which materialises a snapshot of every short channel
id paired with the pubkey of the channel's remote peer. Forwarding-data
sources index events by short channel id, but downstream analyses need
to attribute behaviour to the peer, not the channel. The map skips
channels whose short channel id is still zero (unconfirmed), so callers
see only fully advertised channels. Coverage extends TestStore with a
two-channel fixture pinning the join.

chanevents/store.go

@@ -49,6 +49,8 @@ type Queries interface {
 		sqlc.ChannelEvent,
 		error,
 	)
+
+	GetChannels(ctx context.Context) ([]sqlc.GetChannelsRow, error)
 }
 
 // Store provides access to the db for channel events.
@@ -263,6 +265,29 @@ func (s *Store) GetChannelEvents(ctx context.Context, channelID, afterID int64,
 	return events, nil
 }
 
+// ScidToPeerMap returns the historic scid→peer index, including channels that
+// have since closed. Unconfirmed channels (scid still zero) are not part of
+// the contract.
+func (s *Store) ScidToPeerMap(ctx context.Context) (map[uint64]string, error) {
+	dbChannels, err := s.db.GetChannels(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	scidToPeer := make(map[uint64]string, len(dbChannels))
+	for _, dbChannel := range dbChannels {
+		// The short channel ID can be zero if it's not known yet. We
+		// should just ignore those.
+		if dbChannel.ShortChannelID == 0 {
+			continue
+		}
+
+		scidToPeer[uint64(dbChannel.ShortChannelID)] = dbChannel.Pubkey
+	}
+
+	return scidToPeer, nil
+}
+
 // GetLatestChannelUpdateBefore returns the latest channel event before a given
 // time (exclusive). If no event is found, it returns (nil, nil).
 func (s *Store) GetLatestChannelUpdateBefore(ctx context.Context,

chanevents/store_test.go

@@ -100,6 +100,13 @@ func TestStore(t *testing.T) {
 	require.NoError(t, err)
 	require.NotZero(t, channel2ID)
 
+	// Get the historic channel to peer map.
+	chanToPeer, err := store.ScidToPeerMap(ctx)
+	require.NoError(t, err)
+	require.Len(t, chanToPeer, 2)
+	require.Equal(t, testPubKey, chanToPeer[testShortChanID1])
+	require.Equal(t, testPubKey, chanToPeer[testShortChanID2])
+
 	// Add an online event for the channel.
 	onlineEvent := &ChannelEvent{
 		ChannelID: channelID,

db/sqlc/chanevents.sql.go

@@ -98,6 +98,41 @@ func (q *Queries) GetChannelEvents(ctx context.Context, arg GetChannelEventsPara
 	return items, nil
 }
 
+const getChannels = `-- name: GetChannels :many
+SELECT c.id, c.short_channel_id, p.pubkey
+FROM channels c
+JOIN peers p ON c.peer_id = p.id
+`
+
+type GetChannelsRow struct {
+	ID             int64
+	ShortChannelID int64
+	Pubkey         string
+}
+
+func (q *Queries) GetChannels(ctx context.Context) ([]GetChannelsRow, error) {
+	rows, err := q.db.QueryContext(ctx, getChannels)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []GetChannelsRow
+	for rows.Next() {
+		var i GetChannelsRow
+		if err := rows.Scan(&i.ID, &i.ShortChannelID, &i.Pubkey); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Close(); err != nil {
+		return nil, err
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const getLatestChannelEventBefore = `-- name: GetLatestChannelEventBefore :one
 SELECT id, channel_id, event_type, timestamp, local_balance_sat, remote_balance_sat, is_sync FROM channel_events
 WHERE channel_id = $1 AND event_type = $2 AND timestamp < $3

db/sqlc/querier.go

@@ -12,6 +12,7 @@ type Querier interface {
 	GetChannelByChanPoint(ctx context.Context, channelPoint string) (Channel, error)
 	GetChannelByShortChanID(ctx context.Context, shortChannelID int64) (Channel, error)
 	GetChannelEvents(ctx context.Context, arg GetChannelEventsParams) ([]ChannelEvent, error)
+	GetChannels(ctx context.Context) ([]GetChannelsRow, error)
 	GetLatestChannelEventBefore(ctx context.Context, arg GetLatestChannelEventBeforeParams) (ChannelEvent, error)
 	GetPeerByPubKey(ctx context.Context, pubkey string) (Peer, error)
 	InsertChannel(ctx context.Context, arg InsertChannelParams) (int64, error)

db/sqlc/queries/chanevents.sql

@@ -33,3 +33,8 @@ SELECT * FROM channel_events
 WHERE channel_id = $1 AND event_type = $2 AND timestamp < $3
 ORDER BY timestamp DESC, id DESC
 LIMIT 1;
+
+-- name: GetChannels :many
+SELECT c.id, c.short_channel_id, p.pubkey
+FROM channels c
+JOIN peers p ON c.peer_id = p.id;