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 cca9ebfbff3d43bc1c840ce9493e372864e9e0d1..a7b9ff33ee560b2e189d6f6693b3d0625e934e4f 100644
--- a/base/pending_task.cc
+++ b/base/pending_task.cc
@@ -4,18 +4,14 @@
 
 #include "base/pending_task.h"
 
+#include "base/message_loop/message_loop.h"
 #include "base/tracked_objects.h"
 
 namespace base {
 
 PendingTask::PendingTask(const tracked_objects::Location& posted_from,
                          OnceClosure task)
-    : base::TrackingInfo(posted_from, TimeTicks()),
-      task(std::move(task)),
-      posted_from(posted_from),
-      sequence_num(0),
-      nestable(true),
-      is_high_res(false) {}
+    : PendingTask(posted_from, std::move(task), TimeTicks(), true) {}
 
 PendingTask::PendingTask(const tracked_objects::Location& posted_from,
                          OnceClosure task,
@@ -26,7 +22,19 @@ PendingTask::PendingTask(const tracked_objects::Location& posted_from,
       posted_from(posted_from),
       sequence_num(0),
       nestable(nestable),
-      is_high_res(false) {}
+      is_high_res(false) {
+  const PendingTask* parent_task =
+      MessageLoop::current() ? MessageLoop::current()->current_pending_task_
+                             : nullptr;
+  if (parent_task) {
+    task_backtrace[0] = parent_task->posted_from.program_counter();
+    std::copy(parent_task->task_backtrace.begin(),
+              parent_task->task_backtrace.begin() + (task_backtrace.size() - 1),

[danakj, 2017/02/09 16:39:51]

this is end() - 1 right?

[awong, 2017/02/09 19:49:02]

Yes...initially I thought it was illegal to subtract of end()...but apparently
it's okay for random access iterators.  (verified in [random.access.iterator]).

+              task_backtrace.begin() + 1);
+  } else {
+    std::fill(task_backtrace.begin(), task_backtrace.end(), nullptr);
+  }
+}
 
 PendingTask::PendingTask(PendingTask&& other) = default;