devtools: create conversion tool for old gossip stores.

diff --git a/devtools/Makefile b/devtools/Makefile
index a80ebe4a..60627914 100644
--- a/devtools/Makefile
+++ b/devtools/Makefile
@@ -1,4 +1,4 @@
-DEVTOOLS := devtools/bolt11-cli devtools/decodemsg devtools/onion devtools/dump-gossipstore devtools/gossipwith devtools/create-gossipstore devtools/mkcommit devtools/mkfunding devtools/mkclose devtools/mkgossip devtools/mkencoded devtools/mkquery devtools/lightning-checkmessage devtools/topology devtools/route devtools/bolt12-cli devtools/encodeaddr devtools/features devtools/fp16 devtools/rune devtools/gossmap-compress devtools/bip137-verifysignature
+DEVTOOLS := devtools/bolt11-cli devtools/decodemsg devtools/onion devtools/dump-gossipstore devtools/gossipwith devtools/create-gossipstore devtools/mkcommit devtools/mkfunding devtools/mkclose devtools/mkgossip devtools/mkencoded devtools/mkquery devtools/lightning-checkmessage devtools/topology devtools/route devtools/bolt12-cli devtools/encodeaddr devtools/features devtools/fp16 devtools/rune devtools/gossmap-compress devtools/bip137-verifysignature devtools/convert-gossmap
 ifeq ($(HAVE_SQLITE3),1)
 DEVTOOLS += devtools/checkchannels
 endif
@@ -72,6 +72,9 @@ devtools/dump-gossipstore: $(DEVTOOLS_COMMON_OBJS) $(JSMN_OBJS) $(BITCOIN_OBJS)
 
 devtools/dump-gossipstore.o: gossipd/gossip_store_wiregen.h
 
+devtools/convert-gossmap: $(DEVTOOLS_COMMON_OBJS) $(JSMN_OBJS) $(BITCOIN_OBJS) wire/fromwire.o wire/towire.o devtools/convert-gossmap.o
+devtools/convert-gossmap.o: gossipd/gossip_store_wiregen.h
+
 devtools/create-gossipstore: $(DEVTOOLS_COMMON_OBJS) $(JSMN_OBJS) $(BITCOIN_OBJS) wire/fromwire.o wire/towire.o devtools/create-gossipstore.o gossipd/gossip_store_wiregen.o
 devtools/create-gossipstore.o: gossipd/gossip_store_wiregen.h
 
diff --git a/devtools/convert-gossmap.c b/devtools/convert-gossmap.c
new file mode 100644
index 00000000..dee2b1f1
--- /dev/null
+++ b/devtools/convert-gossmap.c
@@ -0,0 +1,97 @@
+/* Tool we can use to convert our testing gossip_store files */
+#include "config.h"
+#include <ccan/crc32c/crc32c.h>
+#include <ccan/err/err.h>
+#include <ccan/opt/opt.h>
+#include <ccan/read_write_all/read_write_all.h>
+#include <common/gossip_store.h>
+#include <fcntl.h>
+#include <gossipd/gossip_store_wiregen.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <wire/peer_wire.h>
+
+/* Current versions we support */
+#define GSTORE_MAJOR 0
+#define GSTORE_MINOR 15
+
+/* Obsolete ZOMBIE bit */
+#define GOSSIP_STORE_ZOMBIE_BIT_V13 0x1000U
+
+static bool upgrade_field(u8 oldversion,
+			  be16 *hdr_flags,
+			  u8 **msg)
+{
+	int type = fromwire_peektype(*msg);
+
+	switch (oldversion) {
+	case 10:
+		/* Remove old channel_update with no htlc_maximum_msat */
+		if (type == WIRE_CHANNEL_UPDATE
+		    && tal_bytelen(*msg) == 130) {
+			*msg = tal_free(*msg);
+			return true;
+		}
+		/* fall thru */
+	case 11:
+	case 12:
+		/* Remove private entries */
+		if (type == WIRE_GOSSIP_STORE_PRIVATE_CHANNEL_OBS) {
+			*msg = tal_free(*msg);
+			return true;
+		} else if (type == WIRE_GOSSIP_STORE_PRIVATE_UPDATE_OBS) {
+			*msg = tal_free(*msg);
+			return true;
+		}
+		/* fall thru */
+	case 13:
+		/* Discard any zombies */
+		if (be16_to_cpu(*hdr_flags) & GOSSIP_STORE_ZOMBIE_BIT_V13) {
+			*msg = tal_free(*msg);
+			return true;
+		}
+	case 14:
+		/* Add completed field */
+		*hdr_flags |= CPU_TO_BE16(GOSSIP_STORE_COMPLETED_BIT);
+		/* fall thru */
+	case 15:
+		/* Noop */
+		return true;
+	}
+
+	return false;
+}
+
+int main(int argc, char *argv[])
+{
+	u8 oldversion, version;
+	struct gossip_hdr hdr;
+
+	setup_locale();
+	if (!read_all(STDIN_FILENO, &oldversion, sizeof(oldversion)))
+		errx(1, "Empty file");
+
+	if (GOSSIP_STORE_MAJOR_VERSION(oldversion) != GSTORE_MAJOR)
+		errx(1, "Unsupported major gossip_version %u (expected %u)",
+		      GOSSIP_STORE_MAJOR_VERSION(oldversion), GSTORE_MAJOR);
+
+	version = ((GSTORE_MAJOR << 5) | GSTORE_MINOR);
+	if (!write_all(STDOUT_FILENO, &version, sizeof(version)))
+		err(1, "Write error");
+
+	while (read_all(STDIN_FILENO, &hdr, sizeof(hdr))) {
+		u8 *msg;
+		msg = tal_arr(NULL, u8, be16_to_cpu(hdr.len));
+		if (!read_all(STDIN_FILENO, msg, tal_bytelen(msg)))
+			err(1, "truncated file");
+		if (!upgrade_field(oldversion, &hdr.flags, &msg))
+			errx(1, "Cannot upgrade from version %u", oldversion);
+		if (msg) {
+			if (!write_all(STDOUT_FILENO, &hdr, sizeof(hdr))
+			    || !write_all(STDOUT_FILENO, msg, tal_bytelen(msg)))
+				err(1, "Write error");
+			tal_free(msg);
+		}
+	}
+	return 0;
+}