Fix memory leak in XMLSERIALIZE(... INDENT).

xmltotext_with_options sometimes tries to replace the existing
root node of a libxml2 document.  In that case xmlDocSetRootElement
will unlink and return the old root node; if we fail to free it,
it's leaked for the remainder of the session.  The amount of memory
at stake is not large, a couple hundred bytes per occurrence, but
that could still become annoying in heavy usage.

Our only other xmlDocSetRootElement call is not at risk because
it's working on a just-created document, but let's modify that
code too to make it clear that it's dependent on that.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Discussion: https://postgr.es/m/1358967.1747858817@sss.pgh.pa.us
Backpatch-through: 16

diff --git a/src/backend/utils/adt/xml.c b/src/backend/utils/adt/xml.c
index cbab8308d1ce03310a2e48d1c1519c1ad153442b..55154e0261ef9aefa7a74455e7c8fad85538ae75 100644 (file)
--- a/src/backend/utils/adt/xml.c
+++ b/src/backend/utils/adt/xml.c
@@ -733,6 +733,7 @@ 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");
@@ -740,8 +741,14 @@ 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);
 
            /*
@@ -1829,6 +1836,7 @@ xml_parse(text *data, XmlOptionType xmloption_arg,
        else
        {
            xmlNodePtr  root;
+           xmlNodePtr  oldroot PG_USED_FOR_ASSERTS_ONLY;
 
            /* set up document with empty root node to be the context node */
            doc = xmlNewDoc(version);
@@ -1847,8 +1855,13 @@ xml_parse(text *data, XmlOptionType xmloption_arg,
            if (root == NULL || xmlerrcxt->err_occurred)
                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;
+            * and there can't yet be any old root to free.
+            */
+           oldroot = xmlDocSetRootElement(doc, root);
+           Assert(oldroot == NULL);
 
            /* allow empty content */
            if (*(utf8string + count))