[Tom Lane <tgl@sss.pgh.pa.us>]

Jim Jones <jim.jones@uni-muenster.de> writes:
> In my environment (libxml2 v2.9.10 and Ubuntu 22.04) I couldn't 
> reproduce this memory leak.

Just when you thought it was safe to go back in the water ...

Experimenting with the improved valgrind leak detection code at [1],
I discovered that XMLSERIALIZE(... INDENT) has yet a different memory
leak problem.  It turns out that xmlDocSetRootElement() doesn't
merely install the given root node: it unlinks the document's old
root node and returns it to you.  If you don't free it, it's leaked
(for the session, since this is a malloc not palloc).  The amount of
leakage isn't that large, seems to be a couple hundred bytes per
iteration, which may explain why this escaped our notice in the
previous testing.  Still, it could add up under extensive usage.
So I think we need to apply the attached, back to PG 16.

			regards, tom lane

[1] https://www.postgresql.org/message-id/1295385.1747847681%40sss.pgh.pa.us

Attachments:

fix-leakage-in-xmlserialize-indent.patch (text/x-diff, 1.1K) download
diff --git a/src/backend/utils/adt/xml.c b/src/backend/utils/adt/xml.c
index db8d0d6a7e8..73fd4fa090c 100644
--- a/src/backend/utils/adt/xml.c
+++ b/src/backend/utils/adt/xml.c
@@ -754,6 +754,7 @@ xmltotext_with_options(xmltype *data, XmlOptionType xmloption_arg, bool indent)
 			 * content nodes, and then iterate over the nodes.
 			 */
 			xmlNodePtr	root;
+			xmlNodePtr	oldroot;
 			xmlNodePtr	newline;
 
 			root = xmlNewNode(NULL, (const xmlChar *) "content-root");
@@ -761,8 +762,14 @@ xmltotext_with_options(xmltype *data, XmlOptionType xmloption_arg, bool indent)
 				xml_ereport(xmlerrcxt, ERROR, ERRCODE_OUT_OF_MEMORY,
 							"could not allocate xml node");
 
-			/* This attaches root to doc, so we need not free it separately. */
-			xmlDocSetRootElement(doc, root);
+			/*
+			 * This attaches root to doc, so we need not free it separately...
+			 * but instead, we have to free the old root if there was one.
+			 */
+			oldroot = xmlDocSetRootElement(doc, root);
+			if (oldroot != NULL)
+				xmlFreeNode(oldroot);
+
 			xmlAddChildList(root, content_nodes);
 
 			/*