memory-infra: Avoid dereferencing polling_state_ when it could be null

base/trace_event/memory_dump_scheduler.cc

 // Copyright 2017 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_scheduler.h"
 
 #include "base/process/process_metrics.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/trace_event/memory_dump_manager.h"
@@ skipping 153 matching lines @@
     level_of_detail = MemoryDumpLevelOfDetail::LIGHT;
   if (periodic_state_->heavy_dumps_rate > 0 &&
       periodic_state_->dump_count % periodic_state_->heavy_dumps_rate == 0)
     level_of_detail = MemoryDumpLevelOfDetail::DETAILED;
   ++periodic_state_->dump_count;
 
   mdm_->RequestGlobalDump(MemoryDumpType::PERIODIC_INTERVAL, level_of_detail);
 }
 
 void MemoryDumpScheduler::PollMemoryOnPollingThread() {
-  if (polling_state_->current_state != PollingTriggerState::ENABLED)
+  if (!polling_state_)

[Primiano Tucci (use gerrit), 2017/03/29 16:21:22]

I think you need both checks, i.e.
if (!polling_state_ || polling_state_->current_ctate...)

[ssid, 2017/03/29 16:41:45]

No the fix is correct as it is. As soon as disabled is set to null. I just
checked the won't condition.

[ssid, 2017/03/29 16:46:38]

Maybe a dcheck(enabled)

     return;
 
   uint64_t polled_memory = 0;
   bool res = mdm_->PollFastMemoryTotal(&polled_memory);
   DCHECK(res);
   if (polling_state_->level_of_detail == MemoryDumpLevelOfDetail::DETAILED) {
     TRACE_COUNTER1(MemoryDumpManager::kTraceCategory, "PolledMemoryMB",
                    polled_memory / 1024 / 1024);
   }
 
@@ skipping 132 matching lines @@
         last_memory_totals_kb[last_memory_totals_kb_index] * 1024;
   }
   num_polls_from_last_dump = 0;
   for (uint32_t i = 0; i < kMaxNumMemorySamples; ++i)
     last_memory_totals_kb[i] = 0;
   last_memory_totals_kb_index = 0;
 }
 
 }  // namespace trace_event
 }  // namespace base