Allow ALTER EXTENSION UPDATE inside a transaction block

Opening a transaction with BEGIN or START TRANSACTION used to load the
extension. That made ALTER EXTENSION timescaledb UPDATE fail inside a
transaction block because the old version was already loaded before the
update ran.

The loader now skips loading on BEGIN and START TRANSACTION, so the
update can run as the first extension command in the transaction.

Author: svenklemm

.unreleased/allow-alter-extension-in-transaction

@@ -0,0 +1 @@
+Fixes: #XXXX Allow running ALTER EXTENSION timescaledb UPDATE inside a transaction block

src/loader/loader.c

@@ -392,6 +392,27 @@ should_load_on_create_extension(Node const *const utility_stmt, TsExtension cons
 	return false;
 }
 
+static bool
+should_load_on_transaction(Node const *const utility_stmt, TsExtension const *const /*ext*/)
+{
+	TransactionStmt *stmt = (TransactionStmt *) utility_stmt;
+
+	/*
+	 * Do not load the extension just to open a transaction. This lets ALTER
+	 * EXTENSION ... UPDATE run as the first extension-touching command inside
+	 * a transaction block instead of failing because BEGIN already loaded the
+	 * old version.
+	 */
+	switch (stmt->kind)
+	{
+		case TRANS_STMT_BEGIN:
+		case TRANS_STMT_START:
+			return false;
+		default:
+			return true;
+	}
+}
+
 static bool
 load_utility_cmd(Node const *const utility_stmt, TsExtension const *const ext)
 {
@@ -403,6 +424,8 @@ load_utility_cmd(Node const *const utility_stmt, TsExtension const *const ext)
 			return should_load_on_alter_extension(utility_stmt, ext);
 		case T_CreateExtensionStmt:
 			return should_load_on_create_extension(utility_stmt, ext);
+		case T_TransactionStmt:
+			return should_load_on_transaction(utility_stmt, ext);
 		case T_DropStmt:
 			return !drop_statement_drops_extension((DropStmt *) utility_stmt, ext);
 		default:

test/expected/loader-tsl.out

@@ -191,6 +191,28 @@ SELECT * FROM test.extension;
  plpgsql     | 1.0     | pg_catalog | PL/pgSQL procedural language
  timescaledb | mock-2  | public     | Enables scalable inserts and complex queries for time-series data
 
+--ALTER EXTENSION UPDATE works inside a transaction block; BEGIN must not
+--load the old version before UPDATE runs as the first command.
+DROP EXTENSION timescaledb;
+\c :TEST_DBNAME_2 :ROLE_SUPERUSER
+CREATE EXTENSION timescaledb VERSION 'mock-1';
+WARNING:  mock init "mock-1"
+\c :TEST_DBNAME_2 :ROLE_SUPERUSER
+BEGIN;
+ALTER EXTENSION timescaledb UPDATE TO 'mock-2';
+WARNING:  mock init "mock-2"
+COMMIT;
+SELECT 1;
+ ?column? 
+----------
+        1
+
+SELECT * FROM test.extension;
+    Name     | Version |   Schema   |                            Description                            
+-------------+---------+------------+-------------------------------------------------------------------
+ plpgsql     | 1.0     | pg_catalog | PL/pgSQL procedural language
+ timescaledb | mock-2  | public     | Enables scalable inserts and complex queries for time-series data
+
 --drop extension
 DROP EXTENSION timescaledb;
 SELECT 1;

test/sql/loader.sql.in

@@ -93,6 +93,18 @@ ALTER EXTENSION timescaledb UPDATE TO 'mock-2';
 SELECT 1;
 SELECT * FROM test.extension;
 
+--ALTER EXTENSION UPDATE works inside a transaction block; BEGIN must not
+--load the old version before UPDATE runs as the first command.
+DROP EXTENSION timescaledb;
+\c :TEST_DBNAME_2 :ROLE_SUPERUSER
+CREATE EXTENSION timescaledb VERSION 'mock-1';
+\c :TEST_DBNAME_2 :ROLE_SUPERUSER
+BEGIN;
+ALTER EXTENSION timescaledb UPDATE TO 'mock-2';
+COMMIT;
+SELECT 1;
+SELECT * FROM test.extension;
+
 --drop extension
 DROP EXTENSION timescaledb;
 SELECT 1;