x/mobile/exp/sensor: can't reenable sensors once disabled on Android #12501
Comments
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I would like to reuse the the existing app looper to enable, disable and read the sensor events. The app looper is not suffering from the indefinitely blocked case because there will be new app lifecycle events during the lifetime of an app. Reusing it sounds like a better strategy than trying to invent a preemption strategy for the sensor looper. @crawshaw, any thoughts? |
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Given it a second thought, trying to wake the blocked pollAll with ALooper_wake or destroying the underlying ALooper if no sensors enabled sound like a better option than coupling the sensor package with app's ALooper. |
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CL https://golang.org/cl/15438 mentions this issue. |
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I would prefer minimizing the number of OS-specific hooks the app package needs to export, so if this can be done inside the sensor package that would be best. |
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Rather than going for one of the complex proposals above, I decided to introduce a hard poll timeout which allowed the thread not to be blocked indefinitely and gave the poller a fair execution time. |
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>
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>
The proxying goroutine will block on readSignal forever if all sensors are disabled on Android. Once every sensor is disabled, there is no way for other signals to be consumed by the same proxying goroutine.
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