Don't crash if cursor_to_xmlschema is used on a non-data-returning Portal.

tglsfdc · tglsfdc · commit f73fa5a470b4 · 2023-09-18T14:28:17.000-04:00
cursor_to_xmlschema() assumed that any Portal must have a tupDesc, which is not so. Add a defensive check. It's plausible that this mistake occurred because of the rather poorly chosen name of the lookup function SPI_cursor_find(), which in such cases is returning something that isn't very much like a cursor. Add some documentation to try to forestall future errors of the same ilk. Report and patch by Boyu Yang (docs changes by me). Back-patch to all supported branches. Discussion: https://postgr.es/m/dd343010-c637-434c-a8cb-418f53bda3b8.yangboyu.yby@alibaba-inc.com
diff --git a/doc/src/sgml/spi.sgml b/doc/src/sgml/spi.sgml
@@ -2727,6 +2727,19 @@ Portal SPI_cursor_find(const char * <parameter>name</parameter>)
    <symbol>NULL</symbol> if none was found
   </para>
  </refsect1>
+
+ <refsect1>
+  <title>Notes</title>
+
+  <para>
+   Beware that this function can return a <type>Portal</type> object
+   that does not have cursor-like properties; for example it might not
+   return tuples.  If you simply pass the <type>Portal</type> pointer
+   to other SPI functions, they can defend themselves against such
+   cases, but caution is appropriate when directly inspecting
+   the <type>Portal</type>.
+  </para>
+ </refsect1>
 </refentry>
 
 <!-- *********************************************** -->
diff --git a/src/backend/utils/adt/xml.c b/src/backend/utils/adt/xml.c
@@ -3012,6 +3012,10 @@ cursor_to_xmlschema(PG_FUNCTION_ARGS)
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_CURSOR),
 				 errmsg("cursor \"%s\" does not exist", name)));
+	if (portal->tupDesc == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_CURSOR_STATE),
+				 errmsg("portal \"%s\" does not return tuples", name)));
 
 	xmlschema = _SPI_strdup(map_sql_table_to_xmlschema(portal->tupDesc,
 													   InvalidOid, nulls,
