Record async "task backtraces"

base/pending_task.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #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,
                          TimeTicks delayed_run_time,
                          bool nestable)
     : base::TrackingInfo(posted_from, delayed_run_time),
       task(std::move(task)),
       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;
+  task_backtrace[0] = posted_from.program_counter();
+  if (parent_task) {
+    memcpy(&task_backtrace[1], &parent_task->task_backtrace[0],

[danakj, 2017/02/08 18:06:22]

I can't help but notice that we store the program_location twice now, once in
posted_from and once in task_backtrace. If you made the task_backtrace only 3
large, and copied the parent_task's program_counter to 0 instead of this task's,
then we wouldn't have that redundancy. TaskAnnotator makes its own copy anyways
also so it's not requiring this info to be a contiguous block. wdyt?

[awong, 2017/02/08 23:20:38]

sure...

nice catch!

I actually left the backtrace at size 4 so now we have 5 frames. :)

+           sizeof(task_backtrace[0]) * (arraysize(task_backtrace) - 1));
+  } else {
+    memset(&task_backtrace[1], 0,
+           sizeof(task_backtrace[0]) * (arraysize(task_backtrace) - 1));
+  }
+}
 
 PendingTask::PendingTask(PendingTask&& other) = default;
 
 PendingTask::~PendingTask() {
 }
 
 PendingTask& PendingTask::operator=(PendingTask&& other) = default;
 
 bool PendingTask::operator<(const PendingTask& other) const {
   // Since the top of a priority queue is defined as the "greatest" element, we
   // need to invert the comparison here.  We want the smaller time to be at the
   // top of the heap.
 
   if (delayed_run_time < other.delayed_run_time)
     return false;
 
   if (delayed_run_time > other.delayed_run_time)
     return true;
 
   // If the times happen to match, then we use the sequence number to decide.
   // Compare the difference to support integer roll-over.
   return (sequence_num - other.sequence_num) > 0;
 }
 
 }  // namespace base