Record async "task backtraces"

base/pending_task.cc

Index: base/pending_task.cc
diff --git a/base/pending_task.cc b/base/pending_task.cc
index 3d7891414c8dc7df431f0c4d8a6bf8dcb26eb9a5..e08b781d0fee4f8b060e193fb5c2074520a965f6 100644
--- a/base/pending_task.cc
+++ b/base/pending_task.cc
@@ -9,6 +9,7 @@
 namespace base {
 
 PendingTask::PendingTask(const tracked_objects::Location& posted_from,
+                         const PendingTask* parent_task,
                          const base::Closure& task)
     : base::TrackingInfo(posted_from, TimeTicks()),
       task(task),
@@ -16,9 +17,15 @@ PendingTask::PendingTask(const tracked_objects::Location& posted_from,
       sequence_num(0),
       nestable(true),
       is_high_res(false) {
+  task_backtrace[0] = posted_from.program_counter();
+  if (parent_task) {
+    memcpy(&task_backtrace[1], &parent_task->task_backtrace[0],
+           sizeof(task_backtrace[0]) * ARRAYSIZE_UNSAFE(task_backtrace) - 1);
+  }
 }
 
 PendingTask::PendingTask(const tracked_objects::Location& posted_from,
+                         const PendingTask* parent_task,
                          const base::Closure& task,
                          TimeTicks delayed_run_time,
                          bool nestable)
@@ -28,6 +35,11 @@ PendingTask::PendingTask(const tracked_objects::Location& posted_from,
       sequence_num(0),
       nestable(nestable),
       is_high_res(false) {
+  task_backtrace[0] = posted_from.program_counter();
+  if (parent_task) {
+    memcpy(&task_backtrace[1], &parent_task->task_backtrace[0],
+           sizeof(task_backtrace[0]) * ARRAYSIZE_UNSAFE(task_backtrace) - 1);
+  }
 }
 
 PendingTask::~PendingTask() {