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Device detection in Tradefed

A new device connection triggers a series of asynchronous events that are not obvious yet worth understanding.

Physically connected

Tradefed uses the ddmlib library (a Java adb library) to provide the basic interaction with adb and devices. Part of this solution is the IDeviceChangeListener interface that allows reception of new device events, such as:

  • deviceConnected: When a new device is seen by adb
  • deviceDisconnected: When a device is not reporting to adb anymore
  • deviceChanged: When a major device state occurs (such as device offline or device online)

These events are enough at the adb level to decide whether or not a device is connected, online, or offline. But for the test harness, we need a stronger state than this to ensure a device is truly ready to start running tests; it should not be impacted by potential state flakiness that can come with a newly connected device.

This is the sequence of events in Tradefed:

  1. Device is recognized as deviceConnected and open to regular events from adb
  2. An internal Tradefed event is created that will:

    • Check if the device is known already; Tradefed keeps internal reference to some devices (especially the one current allocated and running tests) to avoid TF losing track of them randomly.
    • Check if the device is ONLINE or OFFLINE.
  3. If device is:

    • OFFLINE: Device will be switched to Tradefed CONNECTED_OFFLINE state, which doesn't allow the device to run tests yet. If the device is online later, it will go through the ONLINE cycle. If we receive a deviceDisconnect event, the device will simply be removed from the list.

    • ONLINE (as seen by adb): Device will be put in the state CONNECTED_ONLINE and will have its availability checked for test allocation: checking_availability.

  4. If availability check is successful, the device will be marked as available for test allocation; it will be able to run tests. If not, the device will be marked as unavailable for allocation and cannot receive any tests.

All of these states are reflected in the Tradefed console when listing the devices via: tf> list devices

It's important to note that when the device is currently allocated for a test, most of the above will not occur and Tradefed will determine the device state internally. So it's possible for a device to disappear from adb devices while still being listed by Tradefed. That can happen when a test is rebooting the device for example.

Virtual device connected via "adb connect"

When a remote virtual device is created, Tradefed connects to it using adb connect. This will usually trigger the device showing in adb devices as <some ip>:<port number> and will follow the same sequence as physically connected devices.

Device tracking when a deviceConnected event occurs

When deviceConnected occurs, ddmlib creates a new reference IDevice to track the newly connected device.

Tradefed uses that reference and wraps it in its own implementation of device interface ITestDevice to provide more advanced service. But the underlying IDevice is always the one coming from ddmlib.

This means if a new device is connected, a new ITestDevice is created and associated with the IDevice. When this happens during an invocation and the ITestDevice is being used, the underlying IDevice is still replaced so testing can proceed on the proper reference. This is done seamlessly each time a device is disconnected/reconnected (for example, during a reboot).