PG versus libxml2 2.12.x

Buildfarm member caiman has been failing build for a couple weeks now.
The reason turns out to be that recent libxml2 has decided to throw
a "const" into the signature required for custom error handlers.
(API compatibility?  What's that?)

I don't mind adopting the "const" --- it's a good idea in isolation.
The trouble is in fixing our code to work with both old and new
libxml2 versions.  We could thrash around with a configure test or
something, but I think the most expedient answer is just to insert
some explicit casts, as shown in the attached.  It's possible though
that some compilers will throw a cast-away-const warning.  I'm
not seeing any, but ...

Also, I'm seeing a deprecation warning in contrib/xml2/xpath.c
for

    xmlLoadExtDtdDefaultValue = 1;

I'm not sure why that's still there, given that we disabled external
DTD access ages ago.  I propose we just remove it.

In short, I suggest the attached.

            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..a6734f3550 100644
--- a/src/backend/utils/adt/xml.c
+++ b/src/backend/utils/adt/xml.c
@@ -124,7 +124,7 @@ 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, const xmlError *error);
 static int    errdetail_for_xml_code(int code);
 static void chopStringInfoNewlines(StringInfo str);
 static void appendStringInfoLineSeparator(StringInfo str);
@@ -1196,7 +1196,8 @@ pg_xml_init(PgXmlStrictness strictness)
     errcxt->saved_errcxt = xmlGenericErrorContext;
 #endif

-    xmlSetStructuredErrorFunc((void *) errcxt, xml_errorHandler);
+    xmlSetStructuredErrorFunc((void *) errcxt,
+                              (xmlStructuredErrorFunc) xml_errorHandler);

     /*
      * Verify that xmlSetStructuredErrorFunc set the context variable we
@@ -2024,7 +2025,7 @@ 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, const xmlError *error)
 {
     PgXmlErrorContext *xmlerrcxt = (PgXmlErrorContext *) data;
     xmlParserCtxtPtr ctxt = (xmlParserCtxtPtr) error->ctxt;
@@ -4803,7 +4804,8 @@ XmlTableFetchRow(TableFuncScanState *state)
     xtCxt = GetXmlTableBuilderPrivateData(state, "XmlTableFetchRow");

     /* Propagate our own error context to libxml2 */
-    xmlSetStructuredErrorFunc((void *) xtCxt->xmlerrcxt, xml_errorHandler);
+    xmlSetStructuredErrorFunc((void *) xtCxt->xmlerrcxt,
+                              (xmlStructuredErrorFunc) xml_errorHandler);

     if (xtCxt->xpathobj == NULL)
     {
@@ -4857,7 +4859,8 @@ XmlTableGetValue(TableFuncScanState *state, int colnum,
            xtCxt->xpathobj->nodesetval != NULL);

     /* Propagate our own error context to libxml2 */
-    xmlSetStructuredErrorFunc((void *) xtCxt->xmlerrcxt, xml_errorHandler);
+    xmlSetStructuredErrorFunc((void *) xtCxt->xmlerrcxt,
+                              (xmlStructuredErrorFunc) xml_errorHandler);

     *isnull = false;

@@ -5000,7 +5003,8 @@ XmlTableDestroyOpaque(TableFuncScanState *state)
     xtCxt = GetXmlTableBuilderPrivateData(state, "XmlTableDestroyOpaque");

     /* Propagate our own error context to libxml2 */
-    xmlSetStructuredErrorFunc((void *) xtCxt->xmlerrcxt, xml_errorHandler);
+    xmlSetStructuredErrorFunc((void *) xtCxt->xmlerrcxt,
+                              (xmlStructuredErrorFunc) xml_errorHandler);

     if (xtCxt->xpathscomp != NULL)
     {