[FIXED JENKINS-4525] XML encoding sniffing can fail with old JAXP.

Modified to assume UTF-8 if we fail to find it.

changelog.html

@@ -55,6 +55,9 @@ Upcoming changes</a>
 <!-- Record your changes in the trunk here. -->
 <div id="trunk" style="display:none"><!--=TRUNK-BEGIN=-->
 <ul class=image>
+  <li class=bug>
+    Fixed the XML encoding sniffing problem in environments that have old JAXP
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-4525">issue 4525</a>)
   <li class=bug>
     Suggestions for search do not work since 1.433
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-11208">issue 11208</a>)

core/src/main/java/hudson/XmlFile.java

@@ -261,8 +261,7 @@ public final class XmlFile {
                 @Override
                 public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {
                     attempt();
-                    // if we still haven't found it at the first start element,
-                    // there's something wrong.
+                    // if we still haven't found it at the first start element, then we are not going to find it.
                     throw new Eureka(null);
                 }
 
@@ -279,9 +278,11 @@ public final class XmlFile {
             // can't reach here
             throw new AssertionError();
         } catch (Eureka e) {
-            if(e.encoding==null)
-                throw new IOException("Failed to detect encoding of "+file);
-            return e.encoding;
+            if(e.encoding!=null)
+                return e.encoding;
+            // the environment can contain old version of Xerces and others that do not support Locator2
+            // in such a case, assume UTF-8 rather than fail, since Jenkins internally always write XML in UTF-8
+            return "UTF-8";
         } catch (SAXException e) {
             throw new IOException2("Failed to detect encoding of "+file,e);
         } catch (ParserConfigurationException e) {