Hooks, Signals and External Measurement

Hooks, Signals and External Measurement


This is work in progress! Expect changes in details until at least early 2013.

It is sometimes the case that an interesting test cannot be run solely on the device being tested: additional data from somewhere else is required. For example, a test of the sound subsystem may want to generate audio, play it, capture it on another system and then compare the generated and captured audio. A lava-test-shell test can be written to send signals to indicate when a test case starts and finishes which can be handled by a handler specified by the test definition.


A signal is a message from the system being tested (“device”) to the system the dispatcher is running on (“host”). The messaging is synchronous and uni-directional: lava-test-shell on the device will wait for the signal to be processesed and there is no way for the device to receieve data from the host.

Generally speaking, we expect a test author will only be interested in handling the “start test case” and “end test case” signals that are sent by lava-test-case --shell.


A handler is a Python class that subclasses:

class lava_dispatcher.signals.SignalHandler(testdef_obj)[source]

This class defines three methods that you almost certainly want to override:

  1. start_testcase(self, test_case_id):

    Called when a testcase starts on the device. The return value of this method is passed to both end_testcase and processes_test_run.

    The expected case is something like: starting a process that captures some data from or about the device and returning a dictionary that indicates the pid of that process and where its output is going.

  2. end_testcase(self, test_case_id, case_data):

    Called when a testcase ends on the device. case_data is whatever the corresponding start_testcase call returned.

    The expected case here is that you will terminate the process that was started by start_testcase.

  3. process_test_result(self, test_result, case_data):

    Here you are expected to add the data that was recorded during the test run to the results. You need to know about the bundle format to do this.

These methods are invoked with catch-all exception handlers around them so you don’t have to be super careful in their implementation: it should not be possible to crash the whole dispatcher with a typo in one of them.

There are other methods you might want to override in some situations – see the source for more.

Here is a very simple complete handler:

import datetime
import time

from json_schema_validator.extensions import timedelta_extension

from lava_dispatcher.signals import SignalHandler

class AddDuration(SignalHandler):

    def start_testcase(self, test_case_id):
        return {
            'starttime': time.time()

    def end_testcase(self, test_case_id, data):
        data['endtime'] = time.time()

    def postprocess_test_result(self, test_result, data):
        delta = datetime.timedelta(seconds=data['endtime'] - data['starttime'])
        test_result['duration'] = timedelta_extension.to_json(delta)

Specifying a handler

A handlers are named the test definition, for example:

  handler-name: add-duration

The name is the name of an entry point from the lava.signal_handlers “group”. The entry point must be provided by a package installed into the instance that the dispatcher is running from.

Providing handlers as shell scripts

Using the ‘shell-hooks’ handler that is distributed with the dispatcher it is possible to write handlers as scripts in the same VCS repository as the test definition itself.

The simplest usage looks like this:

  handler-name: shell-hooks
      start_testcase: start-testcase.sh
      end_testcase: end-testcase.sh
      postprocess_test_result: postprocess-test-result.sh

The scripts named in handlers are invoked with a test-case specific directory as the current working directory so they can store and access data in local paths. The scripts named by start_testcase and end_testcase are invoked with no arguments but postprocess_test_result is invoked with a single argument: a directory which contains the on-disk representation of the test result as produced on the device (this on-disk representation is not yet fully documented). If a hook produces output, it will be attached to the test result.

As many interesting hooks need to have information about the device being tested, there is a facility for putting values from the device config into the environment of the hooks. For example, the following test definition sets the environment variable $DEVICE_TYPE to the value of the device_type key:

  handler-name: shell-hooks
      DEVICE_TYPE: device_type

For a slightly silly example of a shell hook, let’s try to mark any test that takes more than 10 seconds (as viewed from the host) as failed, even if they report success on the device, and also attach some meta data about the device to each test result.

The start hook (start-hook.sh in the repository) just records the current unix timestamp in a file (we can just use the cwd as a scratch storage area):

date +%s > start-time

The end hook (end-hook.sh) just records the end time:

date +%s > end-time

The postprocess hook (post-process-result-hook.sh) reads the times recorded by the above hooks, overwrites the result if necessary and creates an attachment containing the device type:

start_time=`cat start-time`
end_time=`cat end-time`
if [ $((end_time - start_time)) -gt 10 ]; then
    echo fail > $1/result
echo $DEVICE_TYPE > $1/attachments/device-type.txt

A test definition that glues this all together would be:

  format: Lava-Test Test Definition 1.0
  name: shell-hook-example

    - lava-test-case pass-test --shell sleep 5
    - lava-test-case fail-test --shell sleep 15

  handler-name: shell-hooks
      DEVICE_TYPE: device_type
      start_testcase: start-hook.sh
      end_testcase: end-hook.sh
      postprocess_test_result: post-process-result-hook.sh

A repository with all the above piece is on Launchpad in the branch lp:~linaro-validation/+junk/shell-hook-example so an action for your job file might look like:

    "command": "lava_test_shell",
    "parameters": {
        "testdef_repos": [{"bzr-repo": "lp:~linaro-validation/+junk/shell-hook-example"}],
        "timeout": 1800