db: Track the data_version in the database

This increments the `data_version` upon committing dirty transactions, reads
the last data_version upon startup, and tracks the number in memory in
parallel to the DB (see next commit for rationale).

Changelog-Changed: JSON-RPC: Added a `data_version` field to the `db_write` hook which returns a numeric transaction counter.

lightningd/plugin_hook.c

@@ -175,6 +175,8 @@ void plugin_hook_db_sync(struct db *db)
 	ph_req->hook = hook;
 	ph_req->db = db;
 
+	json_add_num(req->stream, "data_version", db_data_version_get(db));
+
 	json_array_start(req->stream, "writes");
 	for (size_t i = 0; i < tal_count(changes); i++)
 		json_add_string(req->stream, NULL, changes[i]);

wallet/db.c

@@ -802,11 +802,27 @@ void db_begin_transaction_(struct db *db, const char *location)
 	db->in_transaction = location;
 }
 
+static void db_data_version_incr(struct db *db)
+{
+       struct db_stmt *stmt = db_prepare_v2(
+	       db, SQL("UPDATE vars "
+		       "SET intval = intval + 1 "
+		       "WHERE name = 'data_version'"));
+       db_exec_prepared_v2(stmt);
+       tal_free(stmt);
+       db->data_version++;
+}
+
 void db_commit_transaction(struct db *db)
 {
 	bool ok;
 	assert(db->in_transaction);
 	db_assert_no_outstanding_statements(db);
+
+	/* Increment before reporting changes to an eventual plugin. */
+	if (db->dirty)
+		db_data_version_incr(db);
+
 	db_report_changes(db, NULL, 0);
 	ok = db->config->commit_tx_fn(db);
 
@@ -954,7 +970,18 @@ static void db_migrate(struct lightningd *ld, struct db *db)
 		db_exec_prepared_v2(stmt);
 		tal_free(stmt);
 	}
+}
 
+u32 db_data_version_get(struct db *db)
+{
+	struct db_stmt *stmt;
+	u32 version;
+	stmt = db_prepare_v2(db, SQL("SELECT intval FROM vars WHERE name = 'data_version'"));
+	db_query_prepared(stmt);
+	db_step(stmt);
+	version = db_column_int(stmt, 0);
+	tal_free(stmt);
+	return version;
 }
 
 struct db *db_setup(const tal_t *ctx, struct lightningd *ld)
@@ -966,6 +993,7 @@ struct db *db_setup(const tal_t *ctx, struct lightningd *ld)
 
 	db_migrate(ld, db);
 
+	db->data_version = db_data_version_get(db);
 	db_commit_transaction(db);
 	return db;
 }

wallet/db.h

@@ -229,4 +229,7 @@ struct db_stmt *db_prepare_v2_(const char *location, struct db *db,
  */
 const char **db_changes(struct db *db);
 
+/* Get the current data version. */
+u32 db_data_version_get(struct db *db);
+
 #endif /* LIGHTNING_WALLET_DB_H */

wallet/db_common.h

@@ -34,6 +34,10 @@ struct db {
 	/* Were there any modifying statements in the current transaction?
 	 * Used to bump the data_version in the DB.*/
 	bool dirty;
+
+	/* The current DB version we expect to update if changes are
+	 * committed. */
+	u32 data_version;
 };
 
 struct db_query {

wallet/test/run-db.c

@@ -63,6 +63,7 @@ static struct db *create_test_db(void)
 
 	dsn = tal_fmt(NULL, "sqlite3://%s", filename);
 	db = db_open(NULL, dsn);
+	db->data_version = 0;
 	tal_free(dsn);
 	return db;
 }
@@ -107,6 +108,10 @@ static bool test_primitives(void)
 	CHECK_MSG(db_err, "Failing SQL command");
 	tal_free(stmt);
 	db_err = tal_free(db_err);
+
+	/* We didn't migrate the DB, so don't have the vars table. Pretend we
+	 * didn't change anything so we don't bump the data_version. */
+	db->dirty = false;
 	db_commit_transaction(db);
 	CHECK(!db->in_transaction);
 	tal_free(db);

wallet/test/run-wallet.c

@@ -749,6 +749,7 @@ static struct wallet *create_test_wallet(struct lightningd *ld, const tal_t *ctx
 	CHECK_MSG(w->db, "Failed opening the db");
 	db_begin_transaction(w->db);
 	db_migrate(ld, w->db);
+	w->db->data_version = 0;
 	db_commit_transaction(w->db);
 	CHECK_MSG(!wallet_err, "DB migration failed");
 	w->max_channel_dbid = 0;