It turns out the LIBXML_TEST_VERSION macro calls xmlInitParser().

Therefore we must xmlCleanupParser(), or we risk leaving behind
dangling pointers to whatever memory context is current when xml_init()
is called.  This seems to fix bug #3860, though we might still want
the more invasive solution being worked on by Alvaro.

src/backend/utils/adt/xml.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/utils/adt/xml.c,v 1.66 2008/01/12 10:50:03 neilc Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/adt/xml.c,v 1.67 2008/01/12 21:14:08 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -958,6 +958,9 @@ xml_init(void)
 		/* Check library compatibility */
 		LIBXML_TEST_VERSION;
 
+		/* The above calls xmlInitParser(); must clean up dangling pointers */
+		xmlCleanupParser();
+
 		first_time = false;
 	}
 	else