Lazily initialize background tracing agents

Bug: 971859
Change-Id: I9213044a1555ddebffb7562ebd0a9448cfba9a4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1670318Reviewed-by: oysteine <oysteine@chromium.org>
Commit-Queue: Darin Fisher <darin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671429}

content/browser/tracing/background_tracing_manager_impl.cc

@@ -99,20 +99,21 @@ BackgroundTracingManagerImpl* BackgroundTracingManagerImpl::GetInstance() {
 void BackgroundTracingManagerImpl::ActivateForProcess(
     int child_process_id,
     mojom::ChildControl* child_control) {
+  // NOTE: Called from any thread.
+
   mojo::PendingRemote<tracing::mojom::BackgroundTracingAgentProvider>
       pending_provider;
   child_control->GetBackgroundTracingAgentProvider(
       pending_provider.InitWithNewPipeAndPassReceiver());
 
-  if (!BrowserThread::CurrentlyOn(BrowserThread::UI)) {
+  auto constructor =
+      base::BindOnce(&BackgroundTracingAgentClientImpl::Create,
+                     child_process_id, std::move(pending_provider));
+
   base::PostTaskWithTraits(
       FROM_HERE, {BrowserThread::UI},
-        base::BindOnce(&BackgroundTracingAgentClientImpl::Create,
-                       child_process_id, std::move(pending_provider)));
-    return;
-  }
-  BackgroundTracingAgentClientImpl::Create(child_process_id,
-                                           std::move(pending_provider));
+      base::BindOnce(&BackgroundTracingManagerImpl::AddPendingAgentConstructor,
+                     std::move(constructor)));
 }
 
 BackgroundTracingManagerImpl::BackgroundTracingManagerImpl()
@@ -289,6 +290,8 @@ void BackgroundTracingManagerImpl::AddAgentObserver(AgentObserver* observer) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   agent_observers_.insert(observer);
 
+  MaybeConstructPendingAgents();
+
   for (auto* agent : agents_) {
     observer->OnAgentAdded(agent);
   }
@@ -481,4 +484,27 @@ void BackgroundTracingManagerImpl::OnScenarioAborted() {
   }
 }
 
+// static
+void BackgroundTracingManagerImpl::AddPendingAgentConstructor(
+    base::OnceClosure constructor) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  // Stash away the parameters here, and delay agent initialization until we
+  // have an interested AgentObserver.
+
+  GetInstance()->pending_agent_constructors_.push_back(std::move(constructor));
+  GetInstance()->MaybeConstructPendingAgents();
+}
+
+void BackgroundTracingManagerImpl::MaybeConstructPendingAgents() {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  if (agent_observers_.empty())
+    return;
+
+  for (auto& constructor : pending_agent_constructors_)
+    std::move(constructor).Run();
+  pending_agent_constructors_.clear();
+}
+
 }  // namespace content

content/browser/tracing/background_tracing_manager_impl.h

@@ -9,6 +9,7 @@
 #include <memory>
 #include <set>
 #include <string>
+#include <vector>
 
 #include "base/macros.h"
 #include "content/browser/tracing/background_tracing_config_impl.h"
@@ -87,6 +88,7 @@ class BackgroundTracingManagerImpl : public BackgroundTracingManager {
 
   CONTENT_EXPORT static BackgroundTracingManagerImpl* GetInstance();
 
+  // Callable from any thread.
   static void ActivateForProcess(int child_process_id,
                                  mojom::ChildControl* child_control);
 
@@ -149,6 +151,8 @@ class BackgroundTracingManagerImpl : public BackgroundTracingManager {
       perfetto::protos::pbzero::ChromeMetadataPacket* metadata);
   bool IsTriggerHandleValid(TriggerHandle handle) const;
   void OnScenarioAborted();
+  static void AddPendingAgentConstructor(base::OnceClosure constructor);
+  void MaybeConstructPendingAgents();
 
   std::unique_ptr<BackgroundTracingActiveScenario> active_scenario_;
 
@@ -162,6 +166,8 @@ class BackgroundTracingManagerImpl : public BackgroundTracingManager {
   std::set<tracing::mojom::BackgroundTracingAgent*> agents_;
   std::set<AgentObserver*> agent_observers_;
 
+  std::vector<base::OnceClosure> pending_agent_constructors_;
+
   IdleCallback idle_callback_;
   base::RepeatingClosure tracing_enabled_callback_for_testing_;