[JENKINS-55361] DescriptorExtensionList not locking correctly, leading to deadlocks (#3828)

* Centralize Descriptor extension list retrieval in DescriptorExtensionList

* Switch to protected way of retrieving ExtensionList

* [JENKINS-55361] Correct DescriptorExtensionList lock to match Descriptor ExtensionList

* [JENKINS-55361] - Add comment to explain using the custom lock (and to retrigger CI)

core/src/main/java/hudson/DescriptorExtensionList.java

@@ -161,13 +161,13 @@ public class DescriptorExtensionList<T extends Describable<T>, D extends Descrip
     @Override
     public boolean add(D d) {
         boolean r = super.add(d);
-        hudson.getExtensionList(Descriptor.class).add(d);
+        getDescriptorExtensionList().add(d);
         return r;
     }
 
     @Override
     public boolean remove(Object o) {
-        hudson.getExtensionList(Descriptor.class).remove(o);
+        getDescriptorExtensionList().remove(o);
         return super.remove(o);
     }
 
@@ -176,7 +176,8 @@ public class DescriptorExtensionList<T extends Describable<T>, D extends Descrip
      */
     @Override
     protected Object getLoadLock() {
-        return this;
+        // Get a lock for the singleton extension list to prevent deadlocks (JENKINS-55361)
+        return getDescriptorExtensionList().getLoadLock();
     }
 
     /**
@@ -189,7 +190,7 @@ public class DescriptorExtensionList<T extends Describable<T>, D extends Descrip
             LOGGER.log(Level.WARNING, "Cannot load extension components, because Jenkins instance has not been assigned yet");
             return Collections.emptyList();
         }
-        return _load(jenkins.getExtensionList(Descriptor.class).getComponents());
+        return _load(getDescriptorExtensionList().getComponents());
     }
 
     @Override
@@ -211,6 +212,10 @@ public class DescriptorExtensionList<T extends Describable<T>, D extends Descrip
         return r;
     }
 
+    private ExtensionList<Descriptor> getDescriptorExtensionList() {
+        return ExtensionList.lookup(Descriptor.class);
+    }
+
     /**
      * Stores manually registered Descriptor instances. Keyed by the {@link Describable} type.
      */