wallet: Call db_stmt_free from the db_stmt destructor automatically

This is much more in line with the rest of our memory management.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

wallet/db.c

@@ -554,8 +554,15 @@ sqlite3_stmt *db_select_prepare_(const char *location, struct db *db, const char
 	return stmt;
 }
 
+static void db_stmt_free(struct db_stmt *stmt)
+{
+	if (stmt->inner_stmt)
+		stmt->db->config->stmt_free_fn(stmt);
+	assert(stmt->inner_stmt == NULL);
+}
+
 struct db_stmt *db_prepare_v2_(const char *location, struct db *db,
-			       const char *query_id)
+				     const char *query_id)
 {
 	struct db_stmt *stmt = tal(db, struct db_stmt);
 	size_t num_slots;
@@ -586,12 +593,11 @@ struct db_stmt *db_prepare_v2_(const char *location, struct db *db,
 	stmt->location = location;
 	stmt->error = NULL;
 	stmt->db = db;
-	return stmt;
-}
+	stmt->inner_stmt = NULL;
 
-void db_stmt_free(struct db_stmt *stmt)
-{
-	stmt->db->config->stmt_free_fn(stmt);
+	tal_add_destructor(stmt, db_stmt_free);
+
+	return stmt;
 }
 
 #define db_prepare_v2(db,query) \
@@ -882,7 +888,7 @@ static void db_migrate(struct lightningd *ld, struct db *db, struct log *log)
 			struct db_stmt *stmt =
 			    db_prepare_v2(db, dbmigrations[current].sql);
 			db_exec_prepared_v2(stmt);
-			db_stmt_free(stmt);
+			tal_free(stmt);
 		}
 		if (dbmigrations[current].func)
 			dbmigrations[current].func(ld, db);
@@ -1281,7 +1287,7 @@ void migrate_pr2342_feerate_per_channel(struct lightningd *ld, struct db *db)
 	db_bind_int(stmt, 1, ld->config.fee_per_satoshi);
 
 	db_exec_prepared_v2(stmt);
-	db_stmt_free(stmt);
+	tal_free(stmt);
 }
 
 void sqlite3_bind_timeabs(sqlite3_stmt *stmt, int col, struct timeabs t)

wallet/db.h

@@ -249,7 +249,6 @@ void db_bind_blob(struct db_stmt *stmt, int pos, u8 *val, size_t len);
 void db_bind_text(struct db_stmt *stmt, int pos, const char *val);
 bool db_exec_prepared_v2(struct db_stmt *stmt TAKES);
 
-void db_stmt_free(struct db_stmt *stmt);
 
 struct db_stmt *db_prepare_v2_(const char *location, struct db *db,
 			       const char *query_id);

wallet/db_sqlite3.c

@@ -128,7 +128,7 @@ static void db_sqlite3_stmt_free(struct db_stmt *stmt)
 {
 	if (stmt->inner_stmt)
 		sqlite3_finalize(stmt->inner_stmt);
-	tal_free(stmt);
+	stmt->inner_stmt = NULL;
 }
 
 struct db_config db_sqlite3_config = {