Add command_start hook for rollback at statement level purpose.

src/backend/tcop/postgres.c

@@ -205,6 +205,8 @@ static void log_disconnections(int code, Datum arg);
 static void enable_statement_timeout(void);
 static void disable_statement_timeout(void);
 
+/* Hooks for plugins to get control at end of start_xact_command() */
+XactCommandStart_hook_type start_xact_command_hook = NULL;
 
 /* ----------------------------------------------------------------
  *		routines to obtain user input
@@ -1025,6 +1027,13 @@ exec_simple_query(const char *query_string)
 	 */
 	start_xact_command();
 
+	/*
+	 * Now give loadable modules a chance to execute code
+	 * before a transaction command is processed.
+	 */
+	if (start_xact_command_hook)
+		(*start_xact_command_hook) ();
+
 	/*
 	 * Zap any pre-existing unnamed statement.  (While not strictly necessary,
 	 * it seems best to define simple-Query mode as if it used the unnamed
@@ -1118,6 +1127,13 @@ exec_simple_query(const char *query_string)
 		/* Make sure we are in a transaction command */
 		start_xact_command();
 
+		/*
+		 * Now give loadable modules a chance to execute code
+		 * before a transaction command is processed.
+		 */
+		if (start_xact_command_hook)
+			(*start_xact_command_hook) ();
+
 		/*
 		 * If using an implicit transaction block, and we're not already in a
 		 * transaction block, start an implicit block to force this statement
@@ -1397,6 +1413,13 @@ exec_parse_message(const char *query_string,	/* string to execute */
 	 */
 	start_xact_command();
 
+	/*
+	 * Now give loadable modules a chance to execute code
+	 * before a transaction command is processed.
+	 */
+	if (start_xact_command_hook)
+		(*start_xact_command_hook) ();
+
 	/*
 	 * Switch to appropriate context for constructing parsetrees.
 	 *
@@ -1661,6 +1684,13 @@ exec_bind_message(StringInfo input_message)
 	 */
 	start_xact_command();
 
+	/*
+	 * Now give loadable modules a chance to execute code
+	 * before a transaction command is processed.
+	 */
+	if (start_xact_command_hook)
+		(*start_xact_command_hook) ();
+
 	/* Switch back to message context */
 	MemoryContextSwitchTo(MessageContext);
 
@@ -2154,6 +2184,13 @@ exec_execute_message(const char *portal_name, long max_rows)
 	 */
 	start_xact_command();
 
+	/*
+	 * Now give loadable modules a chance to execute code
+	 * before a transaction command is processed.
+	 */
+	if (start_xact_command_hook)
+		(*start_xact_command_hook) ();
+
 	/*
 	 * If we re-issue an Execute protocol request against an existing portal,
 	 * then we are only fetching more rows rather than completely re-executing
@@ -2560,6 +2597,13 @@ exec_describe_statement_message(const char *stmt_name)
 	 */
 	start_xact_command();
 
+	/*
+	 * Now give loadable modules a chance to execute code
+	 * before a transaction command is processed.
+	 */
+	if (start_xact_command_hook)
+		(*start_xact_command_hook) ();
+
 	/* Switch back to message context */
 	MemoryContextSwitchTo(MessageContext);
 
@@ -2654,6 +2698,13 @@ exec_describe_portal_message(const char *portal_name)
 	 */
 	start_xact_command();
 
+	/*
+	 * Now give loadable modules a chance to execute code
+	 * before a transaction command is processed.
+	 */
+	if (start_xact_command_hook)
+		(*start_xact_command_hook) ();
+
 	/* Switch back to message context */
 	MemoryContextSwitchTo(MessageContext);
 
@@ -4619,6 +4670,13 @@ PostgresMain(const char *dbname, const char *username)
 				/* start an xact for this function invocation */
 				start_xact_command();
 
+				/*
+				 * Now give loadable modules a chance to execute code
+				 * before a transaction command is processed.
+				 */
+				if (start_xact_command_hook)
+					(*start_xact_command_hook) ();
+
 				/*
 				 * Note: we may at this point be inside an aborted
 				 * transaction.  We can't throw error for that until we've

src/include/tcop/pquery.h

@@ -48,4 +48,8 @@ extern bool PlannedStmtRequiresSnapshot(struct PlannedStmt *pstmt);
 
 extern void EnsurePortalSnapshotExists(void);
 
+/* Hook for plugins to get control in start_xact_command() and finish_xact_command() */
+typedef void (*XactCommandStart_hook_type) (void);
+extern PGDLLIMPORT XactCommandStart_hook_type start_xact_command_hook;
+
 #endif							/* PQUERY_H */

src/tools/pgindent/typedefs.list

@@ -3052,6 +3052,7 @@ XPVIV
 XPVMG
 XactCallback
 XactCallbackItem
+XactCommandStart_hook_type
 XactEvent
 XactLockTableWaitInfo
 XidBoundsViolation