Bug 1712574 - Avoid trying to register the legacy listener twice. r=jdescottes

This code calling `watchResourcesForTarget` should only be called when
we already called `TargetCommand.watchTargets()`.
When we call `ResourceCommand.watchResources` for the first time, `TargetCommand.watchTargets`
will process already existing targets and call `ResourceCommand.onTargetAvailable` (which calls `watchResourcesForTarget`).
But for any subsequent call, we should use `TargetCommand.getAllTargets` and call `watchResourcesForTarget` manually.

Differential Revision: https://phabricator.services.mozilla.com/D116027

devtools/shared/commands/resource/resource-command.js

@@ -261,6 +261,9 @@ class ResourceCommand {
    */
   async _watchAllTargets() {
     if (!this._watchTargetsPromise) {
+      // If this is the very first listener registered, of all kind of resource types:
+      // * we want to start observing targets via TargetCommand
+      // * _onTargetAvailable will be called for each already existing targets and the next one to come
       this._watchTargetsPromise = this.targetCommand.watchTargets(
         this.targetCommand.ALL_TYPES,
         this._onTargetAvailable,
@@ -286,6 +289,37 @@ class ResourceCommand {
     );
   }
 
+  /**
+   * For a given resource type, start the legacy listeners for all already existing targets.
+   * Do that only if we have to. If this resourceType requires legacy listeners.
+   */
+  async _startLegacyListenersForExistingTargets(resourceType) {
+    // If we were already listening to targets, we want to start the legacy listeners
+    // for all already existing targets.
+    const shouldRunLegacyListeners =
+      !this.hasResourceCommandSupport(resourceType) ||
+      this._shouldRunLegacyListenerEvenWithWatcherSupport(resourceType);
+    if (shouldRunLegacyListeners) {
+      const promises = [];
+      const targets = this.targetCommand.getAllTargets(
+        this.targetCommand.ALL_TYPES
+      );
+      for (const targetFront of targets) {
+        // We disable warning in case we already registered the legacy listener for this target
+        // as this code may race with the call from onTargetAvailable if we end up having multiple
+        // calls to _startListening in parallel.
+        promises.push(
+          this._watchResourcesForTarget({
+            targetFront,
+            resourceType,
+            disableWarning: true,
+          })
+        );
+      }
+      await Promise.all(promises);
+    }
+  }
+
   /**
    * Method called by the TargetCommand for each already existing or target which has just been created.
    *
@@ -346,7 +380,7 @@ class ResourceCommand {
         // ...request existing resource and new one to come from this one target
         // *but* only do that for backward compat, where we don't have the watcher API
         // (See bug 1626647)
-        await this._watchResourcesForTarget(targetFront, resourceType);
+        await this._watchResourcesForTarget({ targetFront, resourceType });
       }
     }
 
@@ -778,27 +812,6 @@ class ResourceCommand {
 
     this._processingExistingResources.add(resourceType);
 
-    const shouldRunLegacyListeners =
-      !this.hasResourceCommandSupport(resourceType) ||
-      this._shouldRunLegacyListenerEvenWithWatcherSupport(resourceType);
-    if (shouldRunLegacyListeners) {
-      // If this is the very first listener registered, of all kind of resource types:
-      // 1) TargetCommand may not be initialized yet, so that targetCommand.getAllTargets will return an empty array
-      // 2) The following call to watchAllTargets will process all existing targets when it will call onTargetAvailable
-      //
-      // So this code is meant for all but the very first registered listener of all kinds.
-      // TargetCommand will already be watching for targets and the following call to watchAllTargets will be a no-op.
-      // So that we have to manually process all existing targets here.
-      const promises = [];
-      const targets = this.targetCommand.getAllTargets(
-        this.targetCommand.ALL_TYPES
-      );
-      for (const target of targets) {
-        promises.push(this._watchResourcesForTarget(target, resourceType));
-      }
-      await Promise.all(promises);
-    }
-
     // Ensuring enabling listening to targets.
     // This will be a no-op expect for the very first call to `_startListening`,
     // where it is going to call `onTargetAvailable` for all already existing targets,
@@ -806,7 +819,13 @@ class ResourceCommand {
     //
     // Do this *before* calling WatcherActor.watchResources in order to register "resource-available"
     // listeners on targets before these events start being emitted.
-    await this._watchAllTargets();
+    await this._watchAllTargets(resourceType);
+
+    // When we are calling _startListening for the first time, _watchAllTargets
+    // will register legacylistener when it will call onTargetAvailable for all existing targets.
+    // But for any next calls to _startListening, _watchAllTargets will be a no-op,
+    // and nothing will start legacy listener for each already registered targets.
+    await this._startLegacyListenersForExistingTargets(resourceType);
 
     // If the server supports the Watcher API and the Watcher supports
     // this resource type, use this API
@@ -850,7 +869,11 @@ class ResourceCommand {
    * Call backward compatibility code from `LegacyListeners` in order to listen for a given
    * type of resource from a given target.
    */
-  async _watchResourcesForTarget(targetFront, resourceType) {
+  async _watchResourcesForTarget({
+    targetFront,
+    resourceType,
+    disableWarning = false,
+  }) {
     if (this._hasResourceCommandSupportForTarget(resourceType, targetFront)) {
       // This resource / target pair should already be handled by the watcher,
       // no need to start legacy listeners.
@@ -872,9 +895,11 @@ class ResourceCommand {
     const legacyListeners =
       this._existingLegacyListeners.get(targetFront) || [];
     if (legacyListeners.includes(resourceType)) {
+      if (!disableWarning) {
         console.warn(
           `Already started legacy listener for ${resourceType} on ${targetFront.actorID}`
         );
+      }
       return;
     }
     this._existingLegacyListeners.set(