[tracing] Don't pop context from an empty stack.

base/trace_event/heap_profiler_allocation_context_tracker.cc

 // Copyright 2015 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/trace_event/heap_profiler_allocation_context_tracker.h"
 
 #include <algorithm>
 #include <iterator>
 
 #include "base/atomicops.h"
@@ skipping 95 matching lines @@
 
 void AllocationContextTracker::PushCurrentTaskContext(const char* context) {
   DCHECK(context);
   if (task_contexts_.size() < kMaxTaskDepth)
     task_contexts_.push_back(context);
   else
     NOTREACHED();
 }
 
 void AllocationContextTracker::PopCurrentTaskContext(const char* context) {
+  // Guard for stack underflow. If tracing was started with a TRACE_EVENT in
+  // scope, the context was never pushed, so it is possible that pop is called
+  // on an empty stack.
+  if (task_contexts_.empty())
+    return;
+
   DCHECK_EQ(context, task_contexts_.back())

[Primiano Tucci (use gerrit), 2016/04/29 12:02:36]

ok following this logic (that is correcT) we should also remove this DCHECK then
(and the |context| argument alltogether)

[ssid, 2016/04/29 12:10:44]

This check is still valid because the contexts cannot be misordered. Either the
context can go missing which means the stack will be empty or the context should
match the top.

This is unlike trace event because once the enabled flag is set it's never
unset. while tracing can be disabled and events can miss ordering in pseudo
stack.

       << "Encountered an unmatched context end";
   task_contexts_.pop_back();
 }
 
 // static
 AllocationContext AllocationContextTracker::GetContextSnapshot() {
   AllocationContext ctx;
 
   if (ignore_scope_depth_) {
     ctx.backtrace.frames[0] = StackFrame::FromTraceEventName(kTracingOverhead);
@@ skipping 24 matching lines @@
 
   // TODO(ssid): Fix crbug.com/594803 to add file name as 3rd dimension
   // (component name) in the heap profiler and not piggy back on the type name.
   ctx.type_name = task_contexts_.empty() ? nullptr : task_contexts_.back();
 
   return ctx;
 }
 
 }  // namespace trace_event
 }  // namespace base