Re: PG versus libxml2 2.12.x

I wrote:
> Peter Eisentraut <peter@eisentraut.org> writes:
>> In PL/Tcl, we used to have these CONST84 and CONST86 things, for similar 
>> reasons.  Maybe that would be another approach.

> Yeah, if the simple cast approach turns out to create warnings,
> we'll have to fall back on using actually different declarations.
> I'm hoping to not have to go there.

Actually ... what I really want to avoid is adding a configure test.
The alternative to that would be an #if test on LIBXML_VERSION,
which I'd initially not wanted to do ... but I now notice that
we already have one of those for a nearby purpose (coping with a
different change in libxml2's error APIs).  So adding another one
of those doesn't seem so bad after all.  I now like the attached
approach better.

            regards, tom lane

diff --git a/contrib/xml2/xpath.c b/contrib/xml2/xpath.c
index a967257546..b999b1f706 100644
--- a/contrib/xml2/xpath.c
+++ b/contrib/xml2/xpath.c
@@ -74,8 +74,6 @@ pgxml_parser_init(PgXmlStrictness strictness)
     /* Initialize libxml */
     xmlInitParser();

-    xmlLoadExtDtdDefaultValue = 1;
-
     return xmlerrcxt;
 }

diff --git a/src/backend/utils/adt/xml.c b/src/backend/utils/adt/xml.c
index f869c680af..3e24aba546 100644
--- a/src/backend/utils/adt/xml.c
+++ b/src/backend/utils/adt/xml.c
@@ -67,6 +67,16 @@
 #if LIBXML_VERSION >= 20704
 #define HAVE_XMLSTRUCTUREDERRORCONTEXT 1
 #endif
+
+/*
+ * libxml2 2.12 decided to insert "const" into the error handler API.
+ */
+#if LIBXML_VERSION >= 21200
+#define PgXmlErrorPtr const xmlError *
+#else
+#define PgXmlErrorPtr xmlErrorPtr
+#endif
+
 #endif                            /* USE_LIBXML */

 #include "access/htup_details.h"
@@ -124,7 +134,7 @@ static xmlParserInputPtr xmlPgEntityLoader(const char *URL, const char *ID,
                                            xmlParserCtxtPtr ctxt);
 static void xml_errsave(Node *escontext, PgXmlErrorContext *errcxt,
                         int sqlcode, const char *msg);
-static void xml_errorHandler(void *data, xmlErrorPtr error);
+static void xml_errorHandler(void *data, PgXmlErrorPtr error);
 static int    errdetail_for_xml_code(int code);
 static void chopStringInfoNewlines(StringInfo str);
 static void appendStringInfoLineSeparator(StringInfo str);
@@ -2024,7 +2034,7 @@ xml_errsave(Node *escontext, PgXmlErrorContext *errcxt,
  * Error handler for libxml errors and warnings
  */
 static void
-xml_errorHandler(void *data, xmlErrorPtr error)
+xml_errorHandler(void *data, PgXmlErrorPtr error)
 {
     PgXmlErrorContext *xmlerrcxt = (PgXmlErrorContext *) data;
     xmlParserCtxtPtr ctxt = (xmlParserCtxtPtr) error->ctxt;