XWIKI-19504: VFS Api: fails to start due to missing service

* Missing a func tests ATM to prove that it works, it's not very easy to write as it means adding func tests to the module but even more complex, to pre-install an extension (and its deps) before starting XWiki.

xwiki-platform-core/xwiki-platform-vfs/xwiki-platform-vfs-api/src/main/java/org/xwiki/vfs/internal/VfsAttachDriverRegistrationListener.java

@@ -69,9 +69,10 @@ public List<Event> getEvents()
         // We don't listen to any event because all we're interested in is that we initialize TrueVFS when
         // this listener is initialized. This allows to fulfill the two uses cases we need:
         // 1) When the VFS API extension is installed at runtime, the Extension Manager handles the listeners found in
-        // in it by instantiating them and registering them against the Observation Manager. Thus the initialize()
+        // it by instantiating them and registering them against the Observation Manager. Thus the initialize()
         // method below is called and TrueVFS initialized.
-        // 2) When XWiki starts, all found listeners are also instantiated and registered.
+        // 2) When XWiki starts, all found listeners are also instantiated and registered and thus the initialize()
+        // method below is also called.
         return Collections.emptyList();
     }
 
@@ -84,11 +85,24 @@ public void onEvent(Event event, Object source, Object data)
     @Override
     public void initialize()
     {
-        // Register our Attach VFS Driver and inject a Component Manager in it.
-        TConfig config = TConfig.current();
-        // Note: Make sure we add our own Archive Detector to the existing Detector so that all archive formats
-        // supported by TrueVFS are handled properly.
-        config.setArchiveDetector(new TArchiveDetector(config.getArchiveDetector(), "attach",
-            new AttachDriver(this.componentManagerProvider.get())));
+        // This class is loaded using a NamespaceURLClassLoader by the Extension API. However the call to
+        // TConfig.current() will use the context CL to load an FsManagerFactory implementation. Since at this point
+        // (when EventListener are instantiated) we haven't set any context CL, the main servlet container CL is used
+        // and thus doesn't have access to dependent JARs of the VFS API. Thus we set it here and put everything back
+        // as it was before to be sure to not cause any problems with other Event Listeners and other code down the
+        // line.
+        // TODO: move this to a more general location so that all EventListener implementations can benefit from it.
+        ClassLoader currentCL = Thread.currentThread().getContextClassLoader();
+        try {
+            Thread.currentThread().setContextClassLoader(this.getClass().getClassLoader());
+            // Register our Attach VFS Driver and inject a Component Manager in it.
+            TConfig config = TConfig.current();
+            // Note: Make sure we add our own Archive Detector to the existing Detector so that all archive formats
+            // supported by TrueVFS are handled properly.
+            config.setArchiveDetector(new TArchiveDetector(config.getArchiveDetector(), "attach",
+                new AttachDriver(this.componentManagerProvider.get())));
+        } finally {
+            Thread.currentThread().setContextClassLoader(currentCL);
+        }
     }
 }