[JENKINS-45892] Enhanced diagnostics (#2997)

* [JENKINS-45892] Enhanced diagnostics.

* Refined fix which should avoid a needless warning when called from MultiBranchProject.onLoad.

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

@@ -120,6 +120,7 @@ public final class XmlFile {
     private final XStream xs;
     private final File file;
     private static final Map<Object, Void> beingWritten = Collections.synchronizedMap(new IdentityHashMap<>());
+    private static final ThreadLocal<File> writing = new ThreadLocal<>();
 
     public XmlFile(File file) {
         this(DEFAULT_XSTREAM,file);
@@ -175,10 +176,12 @@ public final class XmlFile {
         try {
             w.write("<?xml version='1.0' encoding='UTF-8'?>\n");
             beingWritten.put(o, null);
+            writing.set(file);
             try {
                 xs.toXML(o, w);
             } finally {
                 beingWritten.remove(o);
+                writing.set(null);
             }
             w.commit();
         } catch(StreamException e) {
@@ -200,11 +203,11 @@ public final class XmlFile {
      * @since 2.74
      */
     public static Object replaceIfNotAtTopLevel(Object o, Supplier<Object> replacement) {
-        if (beingWritten.containsKey(o)) {
+        File currentlyWriting = writing.get();
+        if (beingWritten.containsKey(o) || currentlyWriting == null) {
             return o;
         } else {
-            // Unfortunately we cannot easily tell which XML file is actually being saved here, at least without implementing a custom Converter.
-            LOGGER.log(Level.WARNING, "JENKINS-45892: reference to {0} being saved but not at top level", o);
+            LOGGER.log(Level.WARNING, "JENKINS-45892: reference to " + o + " being saved from unexpected " + currentlyWriting, new IllegalStateException());
             return replacement.get();
         }
     }