Revert "Move portal pinning from PL/pgSQL to SPI"

This reverts commit b3617cdfbb.

This broke returning unnamed cursors from PL/pgSQL functions.
Apparently, there are no test cases for this.

src/backend/executor/spi.c

@@ -1175,12 +1175,6 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
 	{
 		/* Use a random nonconflicting name */
 		portal = CreateNewPortal();
-
-		/*
-		 * Make sure the portal doesn't get closed by the user statements we
-		 * execute.
-		 */
-		PinPortal(portal);
 	}
 	else
 	{
@@ -1419,9 +1413,6 @@ SPI_cursor_close(Portal portal)
 	if (!PortalIsValid(portal))
 		elog(ERROR, "invalid portal in SPI cursor operation");
 
-	if (portal->portalPinned)
-		UnpinPortal(portal);
-
 	PortalDrop(portal, false);
 }
 

src/pl/plpgsql/src/pl_exec.c

@@ -5257,6 +5257,12 @@ exec_for_query(PLpgSQL_execstate *estate, PLpgSQL_stmt_forq *stmt,
 	/* Fetch loop variable's datum entry */
 	var = (PLpgSQL_variable *) estate->datums[stmt->var->dno];
 
+	/*
+	 * Make sure the portal doesn't get closed by the user statements we
+	 * execute.
+	 */
+	PinPortal(portal);
+
 	/*
 	 * Fetch the initial tuple(s).  If prefetching is allowed then we grab a
 	 * few more rows to avoid multiple trips through executor startup
@@ -5318,6 +5324,8 @@ loop_exit:
 	 */
 	SPI_freetuptable(tuptab);
 
+	UnpinPortal(portal);
+
 	/*
 	 * Set the FOUND variable to indicate the result of executing the loop
 	 * (namely, whether we looped one or more times). This must be set last so