Ensure Origin Thread Affinity for BlameContext

base/trace_event/blame_context.cc

Index: base/trace_event/blame_context.cc
diff --git a/base/trace_event/blame_context.cc b/base/trace_event/blame_context.cc
index 27d2d2eb7509c2b57f161aa5263c2787cc337a3c..234c2e7a4f0a00ce7ac0b6b716ce193f4f91c632 100644
--- a/base/trace_event/blame_context.cc
+++ b/base/trace_event/blame_context.cc
@@ -5,6 +5,7 @@
 #include "base/trace_event/blame_context.h"
 
 #include "base/strings/stringprintf.h"
+#include "base/thread_task_runner_handle.h"
 #include "base/trace_event/trace_event.h"
 #include "base/trace_event/trace_event_argument.h"
 
@@ -24,17 +25,18 @@ BlameContext::BlameContext(const char* category,
       id_(id),
       parent_scope_(parent_context ? parent_context->scope() : nullptr),
       parent_id_(parent_context ? parent_context->id() : 0),
-      category_group_enabled_(nullptr) {
+      category_group_enabled_(nullptr),
+      task_runner_(ThreadTaskRunnerHandle::Get()),
+      weak_factory_(this) {
   DCHECK(!parent_context || !std::strcmp(name_, parent_context->name()))
       << "Parent blame context must have the same name";
 }
 
 BlameContext::~BlameContext() {
-  DCHECK(WasInitialized());
+  UnregisterFromTraceLog();
   TRACE_EVENT_API_ADD_TRACE_EVENT(
       TRACE_EVENT_PHASE_DELETE_OBJECT, category_group_enabled_, type_, scope_,
       id_, 0, nullptr, nullptr, nullptr, nullptr, TRACE_EVENT_FLAG_HAS_ID);
-  trace_event::TraceLog::GetInstance()->RemoveEnabledStateObserver(this);
 }
 
 void BlameContext::Enter() {
@@ -54,6 +56,7 @@ void BlameContext::Leave() {
 }
 
 void BlameContext::TakeSnapshot() {
+  DCHECK(thread_checker_.CalledOnValidThread());
   DCHECK(WasInitialized());
   if (!*category_group_enabled_)
     return;
@@ -71,9 +74,18 @@ void BlameContext::TakeSnapshot() {
                                   arg_values, TRACE_EVENT_FLAG_HAS_ID);
 }
 
+// Called by TraceLog from a thread possibly different from the one where this
+// BlameContext instance was constructed. Since any access to TraceLog's list
+// of observers is lock-guarded, we can access members of BlameContext from a
+// different thread via a raw pointer safely without worrying about threading
+// issues or pointer validity.
 void BlameContext::OnTraceLogEnabled() {
   DCHECK(WasInitialized());
-  TakeSnapshot();
+  if (task_runner()->BelongsToCurrentThread())
+    TakeSnapshot();
+  else
+    task_runner()->PostTask(FROM_HERE,
+                            Bind(&BlameContext::TakeSnapshot, get_weak()));
 }
 
 void BlameContext::OnTraceLogDisabled() {}
@@ -94,13 +106,22 @@ void BlameContext::Initialize() {
   TRACE_EVENT_API_ADD_TRACE_EVENT(
       TRACE_EVENT_PHASE_CREATE_OBJECT, category_group_enabled_, type_, scope_,
       id_, 0, nullptr, nullptr, nullptr, nullptr, TRACE_EVENT_FLAG_HAS_ID);
-  trace_event::TraceLog::GetInstance()->AddEnabledStateObserver(this);
   TakeSnapshot();
+  trace_event::TraceLog::GetInstance()->AddEnabledStateObserver(this);
+  registered_ = true;
 }
 
 bool BlameContext::WasInitialized() const {
   return category_group_enabled_ != nullptr;
 }
 
+void BlameContext::UnregisterFromTraceLog() {
+  DCHECK(WasInitialized());
+  if (!registered_)
+    return;
+  trace_event::TraceLog::GetInstance()->RemoveEnabledStateObserver(this);
+  registered_ = false;
+}
+
 }  // namespace trace_event
 }  // namespace base