Avoiding flush on-change sensors at subscription

Initial sensor flush at subscription is a mechanism to avoid sensors
to get stale samples before subscription happens. However, there is a
slight chance that a most recent sample will be lost during the flush
process. This is OK for continuous sensors but problematic in
on-change sensor as on-change event does not come continuously and
a lost event can cause inconsistent state in client. Flush at
subscription of on-change sensor is disabled in this CL to avoid new
important on-change event to be discarded during the initial flush
process.

Bugs: b/24647069
      b/25241873
      b/24804819
     
Change-Id: Ibda099c6b9f5fb6e200f13cf13a850b0026e9e7c

1 files changed, 9 insertions, 4 deletions

diff --git a/services/sensorservice/SensorService.cpp b/services/sensorservice/SensorService.cpp
index 40b21a9..fd72b23 100644
--- a/services/sensorservice/SensorService.cpp
+++ b/services/sensorservice/SensorService.cpp
@@ -912,10 +912,15 @@ status_t SensorService::enable(const sp<SensorEventConnection>& connection,
     status_t err = sensor->batch(connection.get(), handle, 0, samplingPeriodNs,
                                  maxBatchReportLatencyNs);
 
-    // Call flush() before calling activate() on the sensor. Wait for a first flush complete
-    // event before sending events on this connection. Ignore one-shot sensors which don't
-    // support flush(). Also if this sensor isn't already active, don't call flush().
-    if (err == NO_ERROR && sensor->getSensor().getReportingMode() != AREPORTING_MODE_ONE_SHOT &&
+    // Call flush() before calling activate() on the sensor. Wait for a first
+    // flush complete event before sending events on this connection. Ignore
+    // one-shot sensors which don't support flush(). Ignore on-change sensors
+    // to maintain the on-change logic (any on-change events except the initial
+    // one should be trigger by a change in value). Also if this sensor isn't
+    // already active, don't call flush().
+    if (err == NO_ERROR &&
+            sensor->getSensor().getReportingMode() != AREPORTING_MODE_ONE_SHOT &&
+            sensor->getSensor().getReportingMode() != AREPORTING_MODE_ON_CHANGE &&
             rec->getNumConnections() > 1) {
         connection->setFirstFlushPending(handle, true);
         status_t err_flush = sensor->flush(connection.get(), handle);