common: add mkdatastorekey helper to make string arrays.

diff --git a/common/Makefile b/common/Makefile
index 62ceb252..7c762487 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -68,6 +68,7 @@ COMMON_SRC_NOGEN :=				\
 	common/keyset.c				\
 	common/lease_rates.c			\
 	common/memleak.c			\
+	common/mkdatastorekey.c			\
 	common/msg_queue.c			\
 	common/node_id.c			\
 	common/onion_decode.c			\
diff --git a/common/mkdatastorekey.c b/common/mkdatastorekey.c
new file mode 100644
index 00000000..3f331a5c
--- /dev/null
+++ b/common/mkdatastorekey.c
@@ -0,0 +1,18 @@
+#include "config.h"
+#include <ccan/tal/str/str.h>
+#include <common/mkdatastorekey.h>
+#include <common/utils.h>
+
+const char **mkdatastorekey_(const tal_t *ctx, ...)
+{
+	va_list ap;
+	const char *s;
+	const char **key = tal_arr(ctx, const char *, 0);
+
+	va_start(ap, ctx);
+	while ((s = va_arg(ap, const char *)) != NULL)
+		tal_arr_expand(&key, tal_strdup(key, s));
+	va_end(ap);
+
+	return key;
+}
diff --git a/common/mkdatastorekey.h b/common/mkdatastorekey.h
new file mode 100644
index 00000000..01f9dac5
--- /dev/null
+++ b/common/mkdatastorekey.h
@@ -0,0 +1,15 @@
+#ifndef LIGHTNING_COMMON_MKDATASTOREKEY_H
+#define LIGHTNING_COMMON_MKDATASTOREKEY_H
+#include "config.h"
+#include <ccan/compiler/compiler.h>
+#include <ccan/tal/tal.h>
+
+/* Generate an array of strings from these values: great for making
+ * keys for datastore operations */
+#define mkdatastorekey(ctx, ...) \
+	mkdatastorekey_(ctx, __VA_ARGS__, NULL)
+
+LAST_ARG_NULL
+const char **mkdatastorekey_(const tal_t *ctx, ... TAKES);
+
+#endif /* LIGHTNING_COMMON_MKDATASTOREKEY_H */
diff --git a/wallet/wallet.c b/wallet/wallet.c
index 2b080247..d2a3e215 100644
--- a/wallet/wallet.c
+++ b/wallet/wallet.c
@@ -8,6 +8,7 @@
 #include <channeld/channeld_wiregen.h>
 #include <common/clock_time.h>
 #include <common/memleak.h>
+#include <common/mkdatastorekey.h>
 #include <common/onionreply.h>
 #include <common/randbytes.h>
 #include <common/trace.h>
@@ -6364,13 +6365,9 @@ void wallet_datastore_save_utxo_description(struct db *db,
 					    const struct bitcoin_outpoint *outpoint,
 					    const char *desc)
 {
-	const char **key;
-
-	key = tal_arr(tmpctx, const char *, 4);
-	key[0] = "bookkeeper";
-	key[1] = "description";
-	key[2] = "utxo";
-	key[3] = fmt_bitcoin_outpoint(key, outpoint);
+	const char **key = mkdatastorekey(tmpctx,
+					  "bookkeeper", "description", "utxo",
+					  take(fmt_bitcoin_outpoint(NULL, outpoint)));
 
 	/* In case it's a duplicate, remove first */
 	db_datastore_remove(db, key);
@@ -6381,13 +6378,9 @@ void wallet_datastore_save_payment_description(struct db *db,
 					       const struct sha256 *payment_hash,
 					       const char *desc)
 {
-	const char **key;
-
-	key = tal_arr(tmpctx, const char *, 4);
-	key[0] = "bookkeeper";
-	key[1] = "description";
-	key[2] = "payment";
-	key[3] = fmt_sha256(key, payment_hash);
+	const char **key = mkdatastorekey(tmpctx,
+					  "bookkeeper", "description", "payment",
+					  take(fmt_sha256(NULL, payment_hash)));
 
 	/* In case it's a duplicate, remove first */
 	db_datastore_remove(db, key);
@@ -6740,12 +6733,8 @@ void migrate_datastore_commando_runes(struct lightningd *ld, struct db *db)
 {
 	const char **startkey;
 
-	/* datastore routines expect a tal_arr */
-	startkey = tal_arr(tmpctx, const char *, 2);
-
 	/* We deleted this from the datastore on migration. */
-	startkey[0] = "commando";
-	startkey[1] = "rune_counter";
+	startkey = mkdatastorekey(tmpctx, "commando", "rune_counter");
 	if (db_datastore_get(tmpctx, db, startkey, NULL))
 		db_fatal(db, "Commando runes still present?  Migration removed in v25.02: call Rusty!");
 }