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..0963fdec862eb3f57001b4a37989f7a5a8c07802 100644
--- a/base/debug/task_annotator.cc
+++ b/base/debug/task_annotator.cc
@@ -4,6 +4,8 @@
 
 #include "base/debug/task_annotator.h"
 
+#include <array>
+
 #include "base/debug/activity_tracker.h"
 #include "base/debug/alias.h"
 #include "base/pending_task.h"
@@ -41,13 +43,18 @@ 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.
+  std::array<const void*,
+             std::tuple_size<decltype(pending_task->task_backtrace)>::value + 1>

[danakj, 2017/02/09 16:39:50]

can you pull the std::tuple_size<decltype(pending_task->task_backtrace)>::value
part out into a constexpr variable, it would make this easier to read.

i wonder if we should make a base::array_size
http://stackoverflow.com/a/16866128

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

done.
Honestly seems overkill unless this sort of dependent array size becomes more
prevalent.

I am a conservative when it comes to adding concepts into low level libraries.

+      task_backtrace;
+  task_backtrace[0] = pending_task->posted_from.program_counter();
+  std::copy(pending_task->task_backtrace.begin(),
+            pending_task->task_backtrace.end(), task_backtrace.begin() + 1);
+  debug::Alias(&task_backtrace);
 
   std::move(pending_task->task).Run();