Revise memory management for libxml calls. Instead of keeping libxml's data

in whichever context happens to be current during a call of an xml.c function,
use a dedicated context that will not go away until we explicitly delete it
(which we do at transaction end or subtransaction abort).  This makes recovery
after an error much simpler --- we don't have to individually delete the data
structures created by libxml.  Also, we need to initialize and cleanup libxml
only once per transaction (if there's no error) instead of once per function
call, so it should be a bit faster.  We'll need to keep an eye out for
intra-transaction memory leaks, though.  Alvaro and Tom.

src/backend/access/transam/xact.c

@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.256 2008/01/03 21:23:15 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.257 2008/01/15 18:56:59 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -45,6 +45,7 @@
 #include "utils/inval.h"
 #include "utils/memutils.h"
 #include "utils/relcache.h"
+#include "utils/xml.h"
 
 
 /*
@@ -1671,6 +1672,7 @@ CommitTransaction(void)
 
 	AtEOXact_GUC(true, 1);
 	AtEOXact_SPI(true);
+	AtEOXact_xml();
 	AtEOXact_on_commit_actions(true);
 	AtEOXact_Namespace(true);
 	/* smgrcommit already done */
@@ -1880,6 +1882,7 @@ PrepareTransaction(void)
 	/* PREPARE acts the same as COMMIT as far as GUC is concerned */
 	AtEOXact_GUC(true, 1);
 	AtEOXact_SPI(true);
+	AtEOXact_xml();
 	AtEOXact_on_commit_actions(true);
 	AtEOXact_Namespace(true);
 	/* smgrcommit already done */
@@ -2021,6 +2024,7 @@ AbortTransaction(void)
 
 	AtEOXact_GUC(false, 1);
 	AtEOXact_SPI(false);
+	AtEOXact_xml();
 	AtEOXact_on_commit_actions(false);
 	AtEOXact_Namespace(false);
 	smgrabort();
@@ -3851,6 +3855,7 @@ AbortSubTransaction(void)
 
 		AtEOXact_GUC(false, s->gucNestLevel);
 		AtEOSubXact_SPI(false, s->subTransactionId);
+		AtEOXact_xml();
 		AtEOSubXact_on_commit_actions(false, s->subTransactionId,
 									  s->parent->subTransactionId);
 		AtEOSubXact_Namespace(false, s->subTransactionId,

src/include/utils/xml.h

@@ -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/include/utils/xml.h,v 1.22 2008/01/01 19:45:59 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/utils/xml.h,v 1.23 2008/01/15 18:57:00 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -75,6 +75,8 @@ extern char *map_sql_identifier_to_xml_name(char *ident, bool fully_escaped, boo
 extern char *map_xml_name_to_sql_identifier(char *name);
 extern char *map_sql_value_to_xml_value(Datum value, Oid type);
 
+extern void AtEOXact_xml(void);
+
 typedef enum
 {
 	XMLBINARY_BASE64,