From: Tom Lane <tgl@sss.pgh.pa.us>
To: pgsql-hackers@lists.postgresql.org
Subject: PG versus libxml2 2.12.x
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Date: Sat, 27 Jan 2024 14:04:22 -0500
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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

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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 =3D 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 =3D 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 *errc=
xt,
  * Error handler for libxml errors and warnings
  */
 static void
-xml_errorHandler(void *data, xmlErrorPtr error)
+xml_errorHandler(void *data, const xmlError *error)
 {
 	PgXmlErrorContext *xmlerrcxt =3D (PgXmlErrorContext *) data;
 	xmlParserCtxtPtr ctxt =3D (xmlParserCtxtPtr) error->ctxt;
@@ -4803,7 +4804,8 @@ XmlTableFetchRow(TableFuncScanState *state)
 	xtCxt =3D 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 =3D=3D NULL)
 	{
@@ -4857,7 +4859,8 @@ XmlTableGetValue(TableFuncScanState *state, int coln=
um,
 		   xtCxt->xpathobj->nodesetval !=3D NULL);
 =

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

 	*isnull =3D false;
 =

@@ -5000,7 +5003,8 @@ XmlTableDestroyOpaque(TableFuncScanState *state)
 	xtCxt =3D 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 !=3D NULL)
 	{

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