Fix race condition when dump providers invoked within destroyed state

base/trace_event/memory_dump_manager.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/memory_dump_manager.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/atomic_sequence_num.h"
@@ skipping 389 matching lines @@
 // active at any time for a given PMD, regardless of status of the |lock_|.
 // |lock_| is used in these functions purely to ensure consistency w.r.t.
 // (un)registrations of |dump_providers_|.
 void MemoryDumpManager::SetupNextMemoryDump(
     std::unique_ptr<ProcessMemoryDumpAsyncState> pmd_async_state) {
   // Initalizes the ThreadLocalEventBuffer to guarantee that the TRACE_EVENTs
   // in the PostTask below don't end up registering their own dump providers
   // (for discounting trace memory overhead) while holding the |lock_|.
   TraceLog::GetInstance()->InitializeThreadLocalEventBufferIfSupported();
 
-  // If this was the last hop, create a trace event, add it to the trace and
-  // finalize process dump (invoke callback).
+  // |dump_thread_| might be destroyed before getting this point.
+  // It means that tracing was disabled a moment ago.

[Primiano Tucci (use gerrit), 2016/04/13 20:19:16]

s/a moment ago/right before starting this dump/

+  // Anyway either tracing is stopped or this was the last hop, create a trace
+  // event, add it to the trace and finalize process dump (invoke callback).

[Primiano Tucci (use gerrit), 2016/04/13 20:19:16]

s/(invoke callback)/, invoking the callback/

+  if (!pmd_async_state->dump_thread_task_runner.get()) {
+    pmd_async_state->dump_successful = false;
+    pmd_async_state->pending_dump_providers.clear();
+  }
   if (pmd_async_state->pending_dump_providers.empty())
     return FinalizeDumpAndAddToTrace(std::move(pmd_async_state));
 
   // Read MemoryDumpProviderInfo thread safety considerations in
   // memory_dump_manager.h when accessing |mdpinfo| fields.
   MemoryDumpProviderInfo* mdpinfo =
       pmd_async_state->pending_dump_providers.back().get();
 
   // If the dump provider did not specify a task runner affinity, dump on
-  // |dump_thread_|. Note that |dump_thread_| might have been destroyed
-  // meanwhile.
+  // |dump_thread_| which is already checked above for presence.
   SequencedTaskRunner* task_runner = mdpinfo->task_runner.get();
   if (!task_runner) {
     DCHECK(mdpinfo->options.dumps_on_single_thread_task_runner);
     task_runner = pmd_async_state->dump_thread_task_runner.get();

[bashi, 2016/04/13 23:39:01]

DCHECK(task_runner) ?

-    if (!task_runner) {
-      // If tracing was disabled before reaching CreateProcessDump() the
-      // dump_thread_ would have been already torn down. Nack current dump and
-      // continue.
-      pmd_async_state->dump_successful = false;
-      pmd_async_state->pending_dump_providers.pop_back();
-      return SetupNextMemoryDump(std::move(pmd_async_state));
-    }
   }
 
   if (mdpinfo->options.dumps_on_single_thread_task_runner &&
       task_runner->RunsTasksOnCurrentThread()) {
     // If |dumps_on_single_thread_task_runner| is true then no PostTask is
     // required if we are on the right thread.
     return InvokeOnMemoryDump(pmd_async_state.release());
   }
 
   bool did_post_task = task_runner->PostTask(
@@ skipping 308 matching lines @@
   if (iter == process_dumps.end()) {
     std::unique_ptr<ProcessMemoryDump> new_pmd(
         new ProcessMemoryDump(session_state));
     iter = process_dumps.insert(std::make_pair(pid, std::move(new_pmd))).first;
   }
   return iter->second.get();
 }
 
 }  // namespace trace_event
 }  // namespace base