From 6eb8518de5c3d767bb6b58426bb04b2173166f2c Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 12 Mar 2026 15:43:45 +0900
Subject: [PATCH] xml2: Fix undeterministic result with xslt_process()

---
 contrib/xml2/expected/xml2.out   | 10 ++++++++++
 contrib/xml2/expected/xml2_1.out |  6 ++++++
 contrib/xml2/sql/xml2.sql        |  6 ++++++
 contrib/xml2/xslt_proc.c         |  8 +++++++-
 4 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/contrib/xml2/expected/xml2.out b/contrib/xml2/expected/xml2.out
index 3d97b14c3a1e..1906fcf33e2a 100644
--- a/contrib/xml2/expected/xml2.out
+++ b/contrib/xml2/expected/xml2.out
@@ -261,3 +261,13 @@ $$<xsl:stylesheet version="1.0"
   </xsl:template>
 </xsl:stylesheet>$$);
 ERROR:  failed to apply stylesheet
+-- empty output
+select xslt_process('<aaa/>',
+$$<xsl:stylesheet version="1.0"
+      xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
+</xsl:stylesheet>$$);
+ xslt_process 
+--------------
+ 
+(1 row)
+
diff --git a/contrib/xml2/expected/xml2_1.out b/contrib/xml2/expected/xml2_1.out
index 31700040a604..9a2144d58f57 100644
--- a/contrib/xml2/expected/xml2_1.out
+++ b/contrib/xml2/expected/xml2_1.out
@@ -205,3 +205,9 @@ $$<xsl:stylesheet version="1.0"
   </xsl:template>
 </xsl:stylesheet>$$);
 ERROR:  xslt_process() is not available without libxslt
+-- empty output
+select xslt_process('<aaa/>',
+$$<xsl:stylesheet version="1.0"
+      xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
+</xsl:stylesheet>$$);
+ERROR:  xslt_process() is not available without libxslt
diff --git a/contrib/xml2/sql/xml2.sql b/contrib/xml2/sql/xml2.sql
index ef99d164f272..510d18a36799 100644
--- a/contrib/xml2/sql/xml2.sql
+++ b/contrib/xml2/sql/xml2.sql
@@ -153,3 +153,9 @@ $$<xsl:stylesheet version="1.0"
     </sax:output>
   </xsl:template>
 </xsl:stylesheet>$$);
+
+-- empty output
+select xslt_process('<aaa/>',
+$$<xsl:stylesheet version="1.0"
+      xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
+</xsl:stylesheet>$$);
diff --git a/contrib/xml2/xslt_proc.c b/contrib/xml2/xslt_proc.c
index 2be87bec0cdf..23e5509b99ad 100644
--- a/contrib/xml2/xslt_proc.c
+++ b/contrib/xml2/xslt_proc.c
@@ -145,7 +145,13 @@ xslt_process(PG_FUNCTION_ARGS)
 		resstat = xsltSaveResultToString((xmlChar **) &resstr, &reslen,
 										 restree, stylesheet);
 
-		if (resstat >= 0)
+		/*
+		 * If an empty string has been returned, resstr would be NULL.
+		 * In this case, assume that the result is an empty string.
+		 */
+		if (reslen == 0)
+			result = cstring_to_text_with_len("", reslen);
+		else if (resstat >= 0)
 			result = cstring_to_text_with_len((char *) resstr, reslen);
 	}
 	PG_CATCH();
-- 
2.53.0