device_sensors: make sure to clear the shared memory buffer on each start.

content/browser/device_sensors/data_fetcher_shared_memory_base.cc

 // Copyright 2014 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 "content/browser/device_sensors/data_fetcher_shared_memory_base.h"
 
 #include <stddef.h>
 #include <string.h>
 
 #include "base/bind.h"
@@ skipping 115 matching lines @@
 
 bool DataFetcherSharedMemoryBase::StartFetchingDeviceData(
     ConsumerType consumer_type) {
   if (started_consumers_ & consumer_type)
     return true;
 
   void* buffer = GetSharedMemoryBuffer(consumer_type);
   if (!buffer)
     return false;
 
+  size_t buffer_size = GetConsumerSharedMemoryBufferSize(consumer_type);
+  // buffer size should be strictly positive because buffer is non-zero.
+  DCHECK(buffer_size > 0);
+  // make sure to clear any potentially stale values in the memory buffer.
+  memset(buffer, 0, buffer_size);
+
   if (GetType() != FETCHER_TYPE_DEFAULT) {
     if (!InitAndStartPollingThreadIfNecessary())
       return false;
     polling_thread_->task_runner()->PostTask(
         FROM_HERE, base::Bind(&PollingThread::AddConsumer,
                               base::Unretained(polling_thread_.get()),
                               consumer_type, buffer));
   } else {
     if (!Start(consumer_type, buffer))
       return false;
@@ skipping 102 matching lines @@
 
 base::MessageLoop* DataFetcherSharedMemoryBase::GetPollingMessageLoop() const {
   return polling_thread_ ? polling_thread_->message_loop() : nullptr;
 }
 
 bool DataFetcherSharedMemoryBase::IsPollingTimerRunningForTesting() const {
   return polling_thread_ ? polling_thread_->IsTimerRunning() : false;
 }
 
 }  // namespace content