commit 0b858c1f42b6dcb34904e50d0285449e6c03ae54
author Brian Stack <bstack@google.com> Fri Oct 19 10:53:25 2018 -0700
committer Brian Stack <bstack@google.com> Fri Oct 19 13:45:39 2018 -0700
tree ef8415456f2bcdf0d93e60ad2d5bfea9394132c2
parent c284ebf192d86d0c942d39596efc05880c9caf23

Do not send stale event to on-change sensor

When a client requests to activate an on-change sensor, the
SensorService sends the previous sensor event to the new client if
the sensor is already active. This is necessary to ensure that the new
client receives a sensor event since it is unknown when the sensor may
generate a new sensor event.

If two clients simultaneously activate the same on-change sensor that
is currently not active, then the second client would receive a sensor
event from the previous activation which may be from a significant
time ago, leading to an inconsistent state between the clients.

This patch checks that the last sensor event it would send to the
second client is from the current sensor activation in order to
prevent sending any stale sensor events.

Bug: 116283108
Test: 1) Artificially added delay to generate on-change sensor event.
      2) Requested on-change sensor from client 1
      3) Requested on-change sensor from client 2
      4) Verified that client 2 did not receive a stale sensor event
      5) Verified that client 1 and client 2 both received the same
         up-to-date sensor event
Test: Verified that above test case fails at step 4 without this patch

Change-Id: I49587e3da7177882a1e8e67347bbb64acfe38200

services/sensorservice/RecentEventLogger.cpp

diff --git a/services/sensorservice/RecentEventLogger.cpp b/services/sensorservice/RecentEventLogger.cpp
index cec2ae5..207b097 100644
--- a/services/sensorservice/RecentEventLogger.cpp
+++ b/services/sensorservice/RecentEventLogger.cpp
@@ -32,19 +32,26 @@
 
 RecentEventLogger::RecentEventLogger(int sensorType) :
         mSensorType(sensorType), mEventSize(eventSizeBySensorType(mSensorType)),
-        mRecentEvents(logSizeBySensorType(sensorType)), mMaskData(false) {
+        mRecentEvents(logSizeBySensorType(sensorType)), mMaskData(false),
+        mIsLastEventCurrent(false) {
     // blank
 }
 
 void RecentEventLogger::addEvent(const sensors_event_t& event) {
     std::lock_guard<std::mutex> lk(mLock);
     mRecentEvents.emplace(event);
+    mIsLastEventCurrent = true;
 }
 
 bool RecentEventLogger::isEmpty() const {
     return mRecentEvents.size() == 0;
 }
 
+void RecentEventLogger::setLastEventStale() {
+    std::lock_guard<std::mutex> lk(mLock);
+    mIsLastEventCurrent = false;
+}
+
 std::string RecentEventLogger::dump() const {
     std::lock_guard<std::mutex> lk(mLock);
 
@@ -85,10 +92,10 @@
     }
 }
 
-bool RecentEventLogger::populateLastEvent(sensors_event_t *event) const {
+bool RecentEventLogger::populateLastEventIfCurrent(sensors_event_t *event) const {
     std::lock_guard<std::mutex> lk(mLock);
 
-    if (mRecentEvents.size()) {
+    if (mIsLastEventCurrent && mRecentEvents.size()) {
         // Index 0 contains the latest event emplace()'ed
         *event = mRecentEvents[0].mEvent;
         return true;