[sensors](Linux) Fix tsan data race in sensor reader

device/generic_sensor/platform_sensor_reader_linux.cc

Index: device/generic_sensor/platform_sensor_reader_linux.cc
diff --git a/device/generic_sensor/platform_sensor_reader_linux.cc b/device/generic_sensor/platform_sensor_reader_linux.cc
index 5c014a4b200b43b9f8697bcfa8c5ee1cfc457fb9..143b1987a455374e925c9ef8e8eef4f74b1d5373 100644
--- a/device/generic_sensor/platform_sensor_reader_linux.cc
+++ b/device/generic_sensor/platform_sensor_reader_linux.cc
@@ -74,6 +74,7 @@ PollingSensorReader::~PollingSensorReader() {
 
 bool PollingSensorReader::StartFetchingData(
     const PlatformSensorConfiguration& configuration) {
+  DCHECK(task_runner_->BelongsToCurrentThread());
   if (is_reading_active_)
     StopFetchingData();
 
@@ -83,6 +84,7 @@ bool PollingSensorReader::StartFetchingData(
 }
 
 void PollingSensorReader::StopFetchingData() {
+  DCHECK(task_runner_->BelongsToCurrentThread());
   is_reading_active_ = false;
   timer_->Stop();
 }
@@ -108,7 +110,9 @@ void PollingSensorReader::PollForData() {
     std::string new_read_value;
     if (!base::ReadFileToString(path, &new_read_value)) {
       NotifyReadError();
-      StopFetchingData();
+      task_runner_->PostTask(FROM_HERE,
+                             base::Bind(&PollingSensorReader::StopFetchingData,
+                                        weak_factory_.GetWeakPtr()));
       return;
     }
 
@@ -116,7 +120,9 @@ void PollingSensorReader::PollForData() {
     base::TrimWhitespaceASCII(new_read_value, base::TRIM_ALL, &new_read_value);
     if (!base::StringToDouble(new_read_value, &new_value)) {
       NotifyReadError();
-      StopFetchingData();
+      task_runner_->PostTask(FROM_HERE,
+                             base::Bind(&PollingSensorReader::StopFetchingData,
+                                        weak_factory_.GetWeakPtr()));

[gab, 2016/12/13 18:12:45]

WeakPtr is also not thread-safe (can't obtain it on one thread and check its
validity on another).

In general only a class's owning sequence should operate on its members (which
weak_factory_ is one of).

[maksims (do not use this acc), 2016/12/13 18:18:19]

Do you mean I need 2 to create two different weak factories for different
threads?

[gab, 2016/12/13 18:23:02]

classes in Chromium are typically bound to a single sequence
(SequencedTaskRunner -- or thread) in this case seems like some methods are
meant to run on one sequence and other methods on another sequence.

Typically this would be split in two classes, one for each sequence.

How do you ensure proper shutdown order today? i.e. there can be pending tasks
on two sequence to one class which can only be deleted on one sequence..?

       return;
     }
     readings.values[i++] = new_value;