lightningd: maintain a hash table of short_channel_id, for faster lookup.

diff --git a/lightningd/channel.c b/lightningd/channel.c
index 2ea68021..ff673df1 100644
--- a/lightningd/channel.c
+++ b/lightningd/channel.c
@@ -254,6 +254,60 @@ struct channel_type *desired_channel_type(const tal_t *ctx,
 	return channel_type_static_remotekey(ctx);
 }
 
+static void chanmap_remove(struct lightningd *ld,
+			   const struct channel *channel,
+			   struct short_channel_id scid)
+{
+	struct scid_to_channel *scc = channel_scid_map_get(ld->channels_by_scid, scid);
+	assert(scc->channel == channel);
+	tal_free(scc);
+}
+
+static void destroy_scid_to_channel(struct scid_to_channel *scc,
+				    struct lightningd *ld)
+{
+	if (!channel_scid_map_del(ld->channels_by_scid, scc))
+		abort();
+}
+
+static void chanmap_add(struct lightningd *ld,
+			struct channel *channel,
+			struct short_channel_id scid)
+{
+	struct scid_to_channel *scc = tal(channel, struct scid_to_channel);
+	scc->channel = channel;
+	scc->scid = scid;
+	channel_scid_map_add(ld->channels_by_scid, scc);
+	tal_add_destructor2(scc, destroy_scid_to_channel, ld);
+}
+
+static void channel_set_random_local_alias(struct channel *channel)
+{
+	assert(channel->alias[LOCAL] == NULL);
+	channel->alias[LOCAL] = tal(channel, struct short_channel_id);
+	randombytes_buf(channel->alias[LOCAL], sizeof(struct short_channel_id));
+	/* We don't check for uniqueness.  We would crash on a clash, but your machine is
+	 * probably broken beyond repair if it gets two equal 64 bit numbers */
+	chanmap_add(channel->peer->ld, channel, *channel->alias[LOCAL]);
+}
+
+void channel_set_scid(struct channel *channel, const struct short_channel_id *new_scid)
+{
+	struct lightningd *ld = channel->peer->ld;
+
+	/* Get rid of old one (if any) */
+	if (channel->scid != NULL) {
+		chanmap_remove(ld, channel, *channel->scid);
+		channel->scid = tal_free(channel->scid);
+	}
+
+	/* Add new one (if any) */
+	if (new_scid) {
+		channel->scid = tal_dup(channel, struct short_channel_id, new_scid);
+		chanmap_add(ld, channel, *new_scid);
+	}
+}
+
 void channel_add_old_scid(struct channel *channel,
 			  struct short_channel_id old_scid)
 {
@@ -265,6 +319,8 @@ void channel_add_old_scid(struct channel *channel,
 		channel->old_scids = tal_dup(channel, struct short_channel_id, &old_scid);
 	else
 		tal_arr_expand(&channel->old_scids, old_scid);
+
+	chanmap_add(channel->peer->ld, channel, old_scid);
 }
 
 struct channel *new_unsaved_channel(struct peer *peer,
@@ -312,10 +368,8 @@ struct channel *new_unsaved_channel(struct peer *peer,
 		= CLOSING_FEE_NEGOTIATION_STEP_UNIT_PERCENTAGE;
 	channel->shutdown_wrong_funding = NULL;
 	channel->closing_feerate_range = NULL;
-	channel->alias[REMOTE] = NULL;
-	/* We don't even bother checking for clashes. */
-	channel->alias[LOCAL] = tal(channel, struct short_channel_id);
-	randombytes_buf(channel->alias[LOCAL], sizeof(struct short_channel_id));
+	channel->alias[REMOTE] = channel->alias[LOCAL] = NULL;
+	channel_set_random_local_alias(channel);
 
 	channel->shutdown_scriptpubkey[REMOTE] = NULL;
 	channel->last_was_revoke = false;
@@ -567,11 +621,19 @@ struct channel *new_channel(struct peer *peer, u64 dbid,
 	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);
+	/* All these possible short_channel_id variants go in the lookup table! */
+	/* Stub channels all have the same scid though, *and* get loaded from db! */
+	if (channel->scid && !is_stub_scid(*channel->scid))
+		chanmap_add(peer->ld, channel, *channel->scid);
+	if (channel->alias[LOCAL])
+		chanmap_add(peer->ld, channel, *channel->alias[LOCAL]);
+	for (size_t i = 0; i < tal_count(channel->old_scids); i++)
+		chanmap_add(peer->ld, channel, channel->old_scids[i]);
+
 	/* We always make sure this is set (historical channels from db might not) */
-	if (!channel->alias[LOCAL]) {
-		channel->alias[LOCAL] = tal(channel, struct short_channel_id);
-		randombytes_buf(channel->alias[LOCAL], sizeof(struct short_channel_id));
-	}
+	if (!channel->alias[LOCAL])
+		channel_set_random_local_alias(channel);
+
 	channel->alias[REMOTE] = tal_steal(channel, alias_remote);  /* Haven't gotten one yet. */
 	channel->cid = *cid;
 	channel->our_msat = our_msat;
diff --git a/lightningd/channel.h b/lightningd/channel.h
index 5cc1a774..82889071 100644
--- a/lightningd/channel.h
+++ b/lightningd/channel.h
@@ -1,6 +1,7 @@
 #ifndef LIGHTNING_LIGHTNINGD_CHANNEL_H
 #define LIGHTNING_LIGHTNINGD_CHANNEL_H
 #include "config.h"
+#include <ccan/htable/htable_type.h>
 #include <common/channel_config.h>
 #include <common/channel_id.h>
 #include <common/channel_type.h>
@@ -844,6 +845,35 @@ struct channel *peer_any_channel_bystate(struct peer *peer,
 
 struct channel *channel_by_dbid(struct lightningd *ld, const u64 dbid);
 
+struct scid_to_channel {
+	struct short_channel_id scid;
+	struct channel *channel;
+};
+
+static inline const struct short_channel_id scid_to_channel_key(const struct scid_to_channel *scidchan)
+{
+	return scidchan->scid;
+}
+
+static inline bool scid_to_channel_eq_scid(const struct scid_to_channel *scidchan,
+					   struct short_channel_id scid)
+{
+	return short_channel_id_eq(scidchan->scid, scid);
+}
+
+/* Define channel_scid_map */
+HTABLE_DEFINE_NODUPS_TYPE(struct scid_to_channel,
+			  scid_to_channel_key,
+			  short_channel_id_hash,
+			  scid_to_channel_eq_scid,
+			  channel_scid_map);
+
+/* The only allowed way to set channel->scid */
+void channel_set_scid(struct channel *channel, const struct short_channel_id *new_scid);
+
+/* The only allowed way to set channel->alias[LOCAL] */
+void channel_set_local_alias(struct channel *channel, struct short_channel_id alias_scid);
+
 /* Includes both real scids and aliases.  If !privacy_leak_ok, then private
  * channels' real scids are not included. */
 struct channel *any_channel_by_scid(struct lightningd *ld,
diff --git a/lightningd/channel_control.c b/lightningd/channel_control.c
index c4fa0f3a..3e341e9d 100644
--- a/lightningd/channel_control.c
+++ b/lightningd/channel_control.c
@@ -803,7 +803,7 @@ bool depthcb_update_scid(struct channel *channel,
 	if (!channel->scid) {
 		wallet_annotate_txout(ld->wallet, outpoint,
 				      TX_CHANNEL_FUNDING, channel->dbid);
-		channel->scid = tal_dup(channel, struct short_channel_id, &scid);
+		channel_set_scid(channel, &scid);
 
 		/* If we have a zeroconf channel, i.e., no scid yet
 		 * but have exchange `channel_ready` messages, then we
@@ -822,7 +822,7 @@ bool depthcb_update_scid(struct channel *channel,
 		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;
+		channel_set_scid(channel, &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/dual_open_control.c b/lightningd/dual_open_control.c
index 7835dcf6..8e106c1b 100644
--- a/lightningd/dual_open_control.c
+++ b/lightningd/dual_open_control.c
@@ -1038,7 +1038,7 @@ static enum watch_result opening_depth_cb(struct lightningd *ld,
 	if (!inflight->channel->scid) {
 		wallet_annotate_txout(ld->wallet, &inflight->funding->outpoint,
 				      TX_CHANNEL_FUNDING, inflight->channel->dbid);
-		inflight->channel->scid = tal_dup(inflight->channel, struct short_channel_id, &scid);
+		channel_set_scid(inflight->channel, &scid);
 		wallet_channel_save(ld->wallet, inflight->channel);
 	} else if (!short_channel_id_eq(*inflight->channel->scid, scid)) {
 		/* We freaked out if required when original was
@@ -1046,7 +1046,7 @@ static enum watch_result opening_depth_cb(struct lightningd *ld,
 		log_info(inflight->channel->log, "Short channel id changed from %s->%s",
 			 fmt_short_channel_id(tmpctx, *inflight->channel->scid),
 			 fmt_short_channel_id(tmpctx, scid));
-		*inflight->channel->scid = scid;
+		channel_set_scid(inflight->channel, &scid);
 		wallet_channel_save(ld->wallet, inflight->channel);
 	}
 
diff --git a/lightningd/lightningd.c b/lightningd/lightningd.c
index 0d7faa0d..d76d5b60 100644
--- a/lightningd/lightningd.c
+++ b/lightningd/lightningd.c
@@ -209,6 +209,10 @@ static struct lightningd *new_lightningd(const tal_t *ctx)
 	ld->peers_by_dbid = tal(ld, struct peer_dbid_map);
 	peer_dbid_map_init(ld->peers_by_dbid);
 
+	/*~ This speeds lookups for short_channel_ids to their channels. */
+	ld->channels_by_scid = tal(ld, struct channel_scid_map);
+	channel_scid_map_init(ld->channels_by_scid);
+
 	/*~ For multi-part payments, we need to keep some incoming payments
 	 * in limbo until we get all the parts, or we time them out. */
 	ld->htlc_sets = tal(ld, struct htlc_set_map);
diff --git a/lightningd/lightningd.h b/lightningd/lightningd.h
index 34c8a158..44a76e18 100644
--- a/lightningd/lightningd.h
+++ b/lightningd/lightningd.h
@@ -215,6 +215,8 @@ struct lightningd {
 	struct peer_node_id_map *peers;
 	/* And those in database by dbid */
 	struct peer_dbid_map *peers_by_dbid;
+	/* Here are all our channels and their aliases */
+	struct channel_scid_map *channels_by_scid;
 
 	/* Outstanding connect commands. */
 	struct list_head connects;
diff --git a/lightningd/memdump.c b/lightningd/memdump.c
index c999db71..b2f32a10 100644
--- a/lightningd/memdump.c
+++ b/lightningd/memdump.c
@@ -11,6 +11,7 @@
 #include <gossipd/gossipd_wiregen.h>
 #include <hsmd/hsmd_wiregen.h>
 #include <lightningd/chaintopology.h>
+#include <lightningd/channel.h>
 #include <lightningd/closed_channel.h>
 #include <lightningd/hsm_control.h>
 #include <lightningd/jsonrpc.h>
@@ -202,6 +203,7 @@ static bool lightningd_check_leaks(struct command *cmd)
 	memleak_scan_htable(memtable, &ld->htlc_sets->raw);
 	memleak_scan_htable(memtable, &ld->peers->raw);
 	memleak_scan_htable(memtable, &ld->peers_by_dbid->raw);
+	memleak_scan_htable(memtable, &ld->channels_by_scid->raw);
 	memleak_scan_htable(memtable, &ld->closed_channels->raw);
 	wallet_memleak_scan(memtable, ld->wallet);
 
diff --git a/lightningd/opening_control.c b/lightningd/opening_control.c
index adcb3f6b..521fd833 100644
--- a/lightningd/opening_control.c
+++ b/lightningd/opening_control.c
@@ -1589,8 +1589,8 @@ static struct channel *stub_chan(struct command *cmd,
 			      true, /* remote_channel_ready */
 			      scid,
 			      NULL,
-			      scid,
-			      scid,
+			      NULL,
+			      NULL,
 			      &cid,
 			      /* The three arguments below are msatoshi_to_us,
 			       * msatoshi_to_us_min, and msatoshi_to_us_max.
diff --git a/lightningd/peer_control.c b/lightningd/peer_control.c
index ae944a69..35b88089 100644
--- a/lightningd/peer_control.c
+++ b/lightningd/peer_control.c
@@ -2225,7 +2225,7 @@ static enum watch_result funding_depth_cb(struct lightningd *ld,
 			/* That's not entirely unexpected in early states */
 			log_debug(channel->log, "Funding tx %s reorganized out!",
 				  fmt_bitcoin_txid(tmpctx, txid));
-			channel->scid = tal_free(channel->scid);
+			channel_set_scid(channel, NULL);
 			return KEEP_WATCHING;
 
 		/* But it's often Bad News in later states */