commit 2fe453d27f41d22f6f7d3804eb603dbb6c8490cc
author Burcu Dogan <jbd@google.com> Tue Oct 06 09:23:29 2015 -0700
committer Burcu Dogan <jbd@google.com> Tue Oct 06 21:59:44 2015 +0000
tree 8053ff2a006eaa64a055cade2eba4a98e9490b5d
parent 8a578fa6caebfe37737209387c5fd07cb2038f59

exp/sensor: don't block the looper thread indefinitely

The sensor package had a deadlock case if it started to poll the
events but all sensors have been disabled on the underlying ALooper.

Consider the following program:

  app.Main(func(a app.App) {
    sensor.Enable(a, sensor.Gyroscope, time.Millisecond)
    go func() {
      time.Sleep(5 * time.Second)
      sensor.Disable(sensor.Gyroscope)
      time.Sleep(2 * time.Second)
      sensor.Enable(a, sensor.Gyroscope, time.Millisecond)
    }()
    for e := range a.Events() {
    case sensor.Event:
      //...
    }

The initial Enable will enable the gyroscope and start polling events
from the ALooper. After 5 seconds, the gyroscope will be disabled.

  ALooper_pollAll(-1, NULL, &events, NULL)

will block indefinately because there are no events will be
available until another sensor is enabled.

After 2 seconds, we will attempt to enable the gyroscope again. But,
the underlying thread will be blocked at pollAll and won't be able
to make the sensor enabling call on the same thread.

In order to overcome this deadlock case, this thread introduces a
hard timeout limit during polling. If timeout occurs, the looper
thread will be unblocked and select{} statement will be able to
handle {enable,disable,close}Signal cases.

Fixes golang/go#12501.

Change-Id: I35efa2e29057ca37f8ac0f38be8dc59c9b8262b3
Reviewed-on: https://go-review.googlesource.com/15438
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>