Bug 1620248 - [devtools] Ensure destroying ContentProcess target when the target is destroyed before process shutdown. r=nchevobbe,jdescottes

For now, we were emitting the "tabDetached" event on message-manager-close (=process shutdown),
or when the DevToolsServerConnection closes (=client closes the connection).
But not when the target is being destroyed by the frontend when calling ResourceWatcher.unwatchTargets,
which is calling process-helper's destroyTargets.
This was introducing test failure (browser_target_list_watchTargets.js) with never resolving RDP requests.

Differential Revision: https://phabricator.services.mozilla.com/D91559

devtools/client/fronts/targets/target-mixin.js

@@ -595,6 +595,12 @@ function TargetMixin(parentClass) {
     _addListeners() {
       this.client.on("closed", this.destroy);
 
+      // `tabDetached` is sent by all target targets types: frame, process and workers.
+      // This is sent when the target is destroyed:
+      // * the target context destroys itself (the tab closes for ex, or the worker shuts down)
+      //   in this case, it may be the connector that send this event in the name of the target actor
+      // * the target actor is destroyed, but the target context stays up and running (for ex, when we call Watcher.unwatchTargets)
+      // * the DevToolsServerConnection closes (client closes the connection)
       this.on("tabDetached", this.destroy);
     }
 

devtools/server/actors/targets/content-process.js

@@ -198,6 +198,11 @@ const ContentProcessTargetActor = ActorClassWithSpec(contentProcessTargetSpec, {
     if (this.isDestroyed()) {
       return;
     }
+
+    // Notify the client that this target is being destroyed.
+    // So that we can destroy the target front and all its children.
+    this.emit("tabDetached");
+
     Actor.prototype.destroy.call(this);
 
     // Tell the live lists we aren't watching any more.