Record async "task backtraces"

base/debug/task_annotator.cc

Index: base/debug/task_annotator.cc
diff --git a/base/debug/task_annotator.cc b/base/debug/task_annotator.cc
index 437d69a7f8be025b17dabe29adb12c7dedc63816..a6c3e83595626756302540c86702a6aab5a97895 100644
--- a/base/debug/task_annotator.cc
+++ b/base/debug/task_annotator.cc
@@ -41,13 +41,15 @@ void TaskAnnotator::RunTask(const char* queue_function,
       TRACE_ID_MANGLE(GetTaskTraceID(*pending_task)), TRACE_EVENT_FLAG_FLOW_IN,
       "queue_duration", queue_duration.InMilliseconds());
 
-  // Before running the task, store the program counter where it was posted
-  // and deliberately alias it to ensure it is on the stack if the task
-  // crashes. Be careful not to assume that the variable itself will have the
-  // expected value when displayed by the optimizer in an optimized build.
-  // Look at a memory dump of the stack.
-  const void* program_counter = pending_task->posted_from.program_counter();
-  debug::Alias(&program_counter);
+  // Before running the task, store the task backtrace with the chain of
+  // PostTasks that resulted in this call and deliberately alias it to ensure
+  // it is on the stack if the task crashes. Be careful not to assume that the
+  // variable itself will have the expected value when displayed by the
+  // optimizer in an optimized build. Look at a memory dump of the stack.
+  const void* task_backtrace[4];

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

can you static assert that this is the same size as the pending task's array?
(or 1 larger if we dedupe the 0th element)

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

good idea. done. Actually did better and just tied the size of the array in the
stack to the array in the struct.

+  memcpy(&task_backtrace[0], &pending_task->task_backtrace[0],
+         sizeof(task_backtrace));
+  debug::Alias(&task_backtrace);
 
   std::move(pending_task->task).Run();