lightningd: save previous short_channel_ids during splice, and keep in db.

diff --git a/lightningd/channel.c b/lightningd/channel.c
index 40cd6cb6..e0377ac3 100644
--- a/lightningd/channel.c
+++ b/lightningd/channel.c
@@ -254,6 +254,19 @@ struct channel_type *desired_channel_type(const tal_t *ctx,
 	return channel_type_static_remotekey(ctx);
 }
 
+void channel_add_old_scid(struct channel *channel,
+			  struct short_channel_id old_scid)
+{
+	/* If this is not public, we skip */
+	if (!(channel->channel_flags & CHANNEL_FLAGS_ANNOUNCE_CHANNEL))
+		return;
+
+	if (!channel->old_scids)
+		channel->old_scids = tal_dup(channel, struct short_channel_id, &old_scid);
+	else
+		tal_arr_expand(&channel->old_scids, old_scid);
+}
+
 struct channel *new_unsaved_channel(struct peer *peer,
 				    u32 feerate_base,
 				    u32 feerate_ppm)
@@ -287,6 +300,7 @@ struct channel *new_unsaved_channel(struct peer *peer,
 	channel->last_htlc_sigs = NULL;
 	channel->remote_channel_ready = false;
 	channel->scid = NULL;
+	channel->old_scids = NULL;
 	channel->next_index[LOCAL] = 1;
 	channel->next_index[REMOTE] = 1;
 	channel->next_htlc_id = 0;
@@ -423,6 +437,7 @@ struct channel *new_channel(struct peer *peer, u64 dbid,
 			    bool remote_channel_ready,
 			    /* NULL or stolen */
 			    struct short_channel_id *scid,
+			    struct short_channel_id *old_scids TAKES,
 			    struct short_channel_id *alias_local TAKES,
 			    struct short_channel_id *alias_remote STEALS,
 			    struct channel_id *cid,
@@ -550,6 +565,7 @@ struct channel *new_channel(struct peer *peer, u64 dbid,
 	channel->our_funds = our_funds;
 	channel->remote_channel_ready = remote_channel_ready;
 	channel->scid = tal_steal(channel, scid);
+	channel->old_scids = tal_dup_talarr(channel, struct short_channel_id, old_scids);
 	channel->alias[LOCAL] = tal_dup_or_null(channel, struct short_channel_id, alias_local);
 	/* We always make sure this is set (historical channels from db might not) */
 	if (!channel->alias[LOCAL]) {
diff --git a/lightningd/channel.h b/lightningd/channel.h
index 4dfcf0ac..5cc1a774 100644
--- a/lightningd/channel.h
+++ b/lightningd/channel.h
@@ -217,6 +217,8 @@ struct channel {
 	bool remote_channel_ready;
 	/* Channel if locked locally. */
 	struct short_channel_id *scid;
+	/* Old scids if we were spliced */
+	struct short_channel_id *old_scids;
 
 	/* Alias used for option_zeroconf, or option_scid_alias, if
 	 * present. LOCAL are all the alias we told the peer about and
@@ -391,6 +393,7 @@ struct channel *new_channel(struct peer *peer, u64 dbid,
 			    bool remote_channel_ready,
 			    /* NULL or stolen */
 			    struct short_channel_id *scid STEALS,
+			    struct short_channel_id *old_scids TAKES,
 			    struct short_channel_id *alias_local STEALS,
 			    struct short_channel_id *alias_remote STEALS,
 			    struct channel_id *cid,
@@ -491,6 +494,10 @@ u32 channel_last_funding_feerate(const struct channel *channel);
 /* Only set completely_eliminate for never-existed channels */
 void delete_channel(struct channel *channel STEALS, bool completely_eliminate);
 
+/* Add a historic (public) short_channel_id to this channel */
+void channel_add_old_scid(struct channel *channel,
+			  struct short_channel_id old_scid);
+
 const char *channel_state_name(const struct channel *channel);
 const char *channel_state_str(enum channel_state state);
 
diff --git a/lightningd/channel_control.c b/lightningd/channel_control.c
index 1470dddf..c4fa0f3a 100644
--- a/lightningd/channel_control.c
+++ b/lightningd/channel_control.c
@@ -815,12 +815,16 @@ bool depthcb_update_scid(struct channel *channel,
 			lockin_has_completed(channel, false);
 
 	} else {
+		struct short_channel_id old_scid = *channel->scid;
+
 		/* We freaked out if required when original was
 		 * removed, so just update now */
 		log_info(channel->log, "Short channel id changed from %s->%s",
 			 fmt_short_channel_id(tmpctx, *channel->scid),
 			 fmt_short_channel_id(tmpctx, scid));
 		*channel->scid = scid;
+		/* In case we broadcast it before (e.g. splice!) */
+		channel_add_old_scid(channel, old_scid);
 		channel_gossip_scid_changed(channel);
 	}
 
diff --git a/lightningd/opening_control.c b/lightningd/opening_control.c
index 43163835..adcb3f6b 100644
--- a/lightningd/opening_control.c
+++ b/lightningd/opening_control.c
@@ -188,6 +188,7 @@ wallet_commit_channel(struct lightningd *ld,
 			      local_funding,
 			      false, /* !remote_channel_ready */
 			      NULL, /* no scid yet */
+			      NULL, /* no old scids */
 			      NULL, /* assign random local alias */
 			      NULL, /* They haven't told us an alias yet */
 			      cid,
@@ -1587,6 +1588,7 @@ static struct channel *stub_chan(struct command *cmd,
 			      AMOUNT_SAT(0),
 			      true, /* remote_channel_ready */
 			      scid,
+			      NULL,
 			      scid,
 			      scid,
 			      &cid,
diff --git a/wallet/db.c b/wallet/db.c
index 17f0d337..15c54eb0 100644
--- a/wallet/db.c
+++ b/wallet/db.c
@@ -1043,6 +1043,7 @@ static struct migration dbmigrations[] = {
     {SQL("ALTER TABLE channel_funding_inflights ADD locked_scid BIGINT DEFAULT 0;"), NULL},
     {NULL, migrate_initialize_channel_htlcs_wait_indexes_and_fixup_forwards},
     {SQL("ALTER TABLE channel_funding_inflights ADD i_sent_sigs INTEGER DEFAULT 0"), NULL},
+    {SQL("ALTER TABLE channels ADD old_scids BLOB DEFAULT NULL;"), NULL},
 };
 
 /**
diff --git a/wallet/test/run-db.c b/wallet/test/run-db.c
index a8552932..09ecb5b3 100644
--- a/wallet/test/run-db.c
+++ b/wallet/test/run-db.c
@@ -188,6 +188,7 @@ struct channel *new_channel(struct peer *peer UNNEEDED, u64 dbid UNNEEDED,
 			    bool remote_channel_ready UNNEEDED,
 			    /* NULL or stolen */
 			    struct short_channel_id *scid STEALS UNNEEDED,
+			    struct short_channel_id *old_scids TAKES UNNEEDED,
 			    struct short_channel_id *alias_local STEALS UNNEEDED,
 			    struct short_channel_id *alias_remote STEALS UNNEEDED,
 			    struct channel_id *cid UNNEEDED,
diff --git a/wallet/test/run-wallet.c b/wallet/test/run-wallet.c
index 3084da4d..6cfe7f83 100644
--- a/wallet/test/run-wallet.c
+++ b/wallet/test/run-wallet.c
@@ -2026,7 +2026,8 @@ static bool test_channel_inflight_crud(struct lightningd *ld, const tal_t *ctx)
 			   &outpoint,
 			   funding_sats, AMOUNT_MSAT(0),
 			   our_sats,
-			   0, false,
+			   0, NULL,
+			   NULL, /* old scids */
 			   NULL, /* alias[LOCAL] */
 			   NULL, /* alias[REMOTE] */
 			   &cid,
diff --git a/wallet/wallet.c b/wallet/wallet.c
index 8ff8592b..7d9f2d36 100644
--- a/wallet/wallet.c
+++ b/wallet/wallet.c
@@ -1779,7 +1779,7 @@ static struct channel *wallet_stmt2channel(struct wallet *w, struct db_stmt *stm
 	struct channel_info channel_info;
 	struct fee_states *fee_states;
 	struct height_states *height_states;
-	struct short_channel_id *scid, *alias[NUM_SIDES];
+	struct short_channel_id *scid, *alias[NUM_SIDES], *old_scids;
 	struct channel_id cid;
 	struct channel *chan;
 	u64 peer_dbid;
@@ -1818,6 +1818,7 @@ static struct channel *wallet_stmt2channel(struct wallet *w, struct db_stmt *stm
 	}
 
 	scid = db_col_optional_scid(tmpctx, stmt, "scid");
+	old_scids = db_col_short_channel_id_arr(tmpctx, stmt, "old_scids");
 	alias[LOCAL] = db_col_optional_scid(tmpctx, stmt, "alias_local");
 	alias[REMOTE] = db_col_optional_scid(tmpctx, stmt, "alias_remote");
 
@@ -2032,6 +2033,7 @@ static struct channel *wallet_stmt2channel(struct wallet *w, struct db_stmt *stm
 			   our_funding_sat,
 			   db_col_int(stmt, "funding_locked_remote") != 0,
 			   scid,
+			   old_scids,
 			   alias[LOCAL],
 			   alias[REMOTE],
 			   &cid,
@@ -2246,6 +2248,7 @@ static bool wallet_channels_load_active(struct wallet *w)
 					"  id"
 					", peer_id"
 					", scid"
+					", old_scids"
 					", full_channel_id"
 					", channel_config_local"
 					", channel_config_remote"
@@ -2526,6 +2529,7 @@ void wallet_channel_save(struct wallet *w, struct channel *chan)
 	stmt = db_prepare_v2(w->db, SQL("UPDATE channels SET"
 					"  shachain_remote_id=?,"
 					"  scid=?,"
+					"  old_scids=?,"
 					"  full_channel_id=?,"
 					"  state=?,"
 					"  funder=?,"
@@ -2585,6 +2589,7 @@ void wallet_channel_save(struct wallet *w, struct channel *chan)
 	else
 		db_bind_null(stmt);
 
+	db_bind_short_channel_id_arr(stmt, chan->old_scids);
 	db_bind_channel_id(stmt, &chan->cid);
 	db_bind_int(stmt, channel_state_in_db(chan->state));
 	db_bind_int(stmt, chan->opener);