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# How to Extend the Layout Test Framework
The Layout Test Framework that Blink uses is a regression testing tool that is
multi-platform and it has a large amount of tools that help test varying types
of regression, such as pixel diffs, text diffs, etc. The framework is mainly
used by Blink, however it was made to be extensible so that other projects can
use it test different parts of chrome (such as Print Preview). This is a guide
to help people who want to actually the framework to test whatever they want.
## Background
Before you can start actually extending the framework, you should be familiar
with how to use it. See the
[layout tests documentation](testing/
## How to Extend the Framework
There are two parts to actually extending framework to test a piece of software.
The first part is extending certain files in:
The code in `webkitpy/layout_tests` is the layout test framework itself
The second part is creating a driver (program) to actually communicate the
layout test framework. This part is significantly more tricky and dependent on
what exactly exactly is being tested.
### Part 1
This part isnt too difficult. There are basically two classes that need to be
extended (ideally, just inherited from). These classes are:
* `Driver`. Located in `layout_tests/port/`. Each instance of this is
the class that will actually an instance of the program that produces the
test data (program in Part 2).
* `Port`. Located in `layout_tests/port/`. This class is responsible
creating drivers with the correct settings, giving access to certain OS
functionality to access expected files, etc.
#### Extending Driver
As said, Driver launches the program from Part 2. Said program will communicate
with the driver class to receive instructions and send back data. All of the
work for driver gets done in `Driver.run_test`. Everything else is a helper or
initialization function.
`run_test()` steps:
1. On the very first call of this function, it will actually run the test
program. On every subsequent call to this function, at the beginning it will
verify that the process doesnt need to be restarted, and if it does, it
will create a new instance of the test program.
1. It will then create a command to send the program
* This command generally consists of an html file path for the test
program to navigate to.
* After creating it, the command is sent
1. After the command has been sent, it will then wait for data from the
* It will actually wait for 2 blocks of data.
* The first part being text or audio data. This part is required (the
program will always send something, even an empty string)
* The second block is optional and is image data and an image hash
(md5) this block of data is used for pixel tests
1. After it has received all the data, it will proceed to check if the program
has timed out or crashed, and if so fail this instance of the test (it can
be retried later if need be).
Luckily, `run_test()` most likely doesnt need to be overridden unless extra
blocks of data need to be sent to/read from the test program. However, you do
need to know how it works because it will influence what functions you need to
override. Here are the ones youre probably going to need to override
This function creates a set of command line arguments to run the test program,
so the function will almost certainly need to be overridden.
It creates the command line to run the program. `Driver` uses `subprocess.popen`
to create the process, which takes the name of the test program and any options
it might need.
The first item in the list of arguments should be the path to test program using
this function:
This is an absolute path to the test program. This is the bare minimum you need
to get the driver to launch the test program, however if you have options you
need to append, just append them to the list.
If your program has any special startup needs, then this will be the place to
put it.
Thats mostly it. The Driver class has almost all the functionality you could
want, so there isnt much to override here. If extra data needs to be read or
sent, extra data members should be added to `ContentBlock`.
#### Extending Port
This class is responsible for providing functionality such as where to look for
tests, where to store test results, what driver to run, what timeout to use,
what kind of files can be run, etc. It provides a lot of functionality, however
it isnt really sufficient because it doesnt account of platform specific
problems, therefore port itself shouldnt be extend. Instead LinuxPort, WinPort,
and MacPort (and maybe the android port class) should be extended as they
provide platform specific overrides/extensions that implement most of the
important functionality. While there are many functions in Port, overriding one
function will affect most of the other ones to get the desired behavior. For
example, if `layout_tests_dir()` is overridden, not only will the code look for
tests in that directory, but it will find the correct TestExpectations file, the
platform specific expected files, etc.
Here are some of the functions that most likely need to be overridden.
* `driver_class`
* This should be overridden to allow the testing program to actually run.
By default the code will run content_shell, which might or might not be
what you want.
* It should be overridden to return the driver extension class created
earlier. This function doesnt return an instance on the driver, just
the class itself.
* `driver_name`
* This should return the name of the program test p. By default it returns
content_shell’, but you want to have it return the program you want to
run, such as `chrome` or `browser_tests`.
* `layout_tests_dir`
* This tells the port where to look for all the and everything associated
with them such as resources files.
* By default it returns the absolute path to the layout tests directory.
* If you are planning on running something in the chromium src/ directory,
there are helper functions to allow you to return a path relative to the
base of the chromium src directory.
The rest of the functions can definitely be overridden for your projects
specific needs, however these are the bare minimum needed to get it running.
There are also functions you can override to make certain actions that arent on
by default always take place. For example, the layout test framework always
checks for system dependencies unless you pass in a switch. If you want them
disabled for your project, just override `check_sys_deps` to always return OK.
This way you dont need to pass in so many switches.
As said earlier, you should override LinuxPort, MacPort, and/or WinPort. You
should create a class that implements the platform independent overrides (such
as `driver_class`) and then create a separate class for each platform specific
port of your program that inherits from the class with the independent overrides
and the platform port you want. For example, you might want to have a different
timeout for your project, but on Windows the timeout needs to be vastly
different than the others. In this case you can just create a default override
that every class uses except your Windows port. In that port you can just
override the function again to provide the specific timeout you need. This way
you dont need to maintain the same function on each platform if they all do the
same thing.
For `Driver` and `Port` thats basically it unless you need to make many odd
modifications. Lots of functionality is already there so you shouldnt really
need to do much.
### Part 2
This is the part where you create the program that your driver class launches.
This part is very application dependent, so it will not be a guide on how
implement certain features, just what should be implemented and the order in
which events should occur and some guidelines about what to do/not do. For a
good example of how to implement your test program, look at MockDRT in
`mock_drt.pyin` the same directory as `` and ``. It goes through
all the steps described below and is very clear and concise. It is written in
python, but your driver can be anything that can be run by `subprocess.popen`
and has stdout, stdin, stderr.
#### Goals
Your goal for this part of the project is to create a program (or extend a
program) to interface with the layout test framework. The layout test framework
will communicate with this program to tell it what to do and it will accept data
from this program to perform the regression testing or create new base line
#### Structure
This is how your code should be laid out.
1. Initialization
* The creation of any directories or the launching of any programs should
be done here and should be done once.
* After the program is initialized, “#READY\n should be sent to progress
the `run_test()` in the driver.
1. Infinite Loop (!)
* After initialization, your program needs to actually wait for input,
then process that input to carry out the test. In the context of layout
testing, the `content_shell` needs to wait for an html file to navigate
to, render it, then convert that rendering to a PNG. It does this
constantly, until a signal/message is sent to indicate that no more
tests should be processed
* Details:
* The first thing you need is your test file path and any other
additional information about the test that is required (this is sent
during the write() step in `run_tests()` is ``. This
information will be passed through stdin and is just one large
string, with each part of the command being split with apostrophes
(ex: “/pathfoo is path to the test file, then foo is some setting
that your program might need).
* After that, your program should act on this input, how it does this
is dependent on your program, however in `content_shell`, this would
be the part where it navigates to the test file, then renders it.
After the program acts on the input, it needs to send some text to
the driver code to indicate that it has acted on the input. This
text will indicate something that you want to test. For example, if
you want to make sure you program always prints foo you should
send it to the driver. If the program every prints bar (or
anything else), that would indicate a failure and the test will
* Then you need to send any image data in the same manner as you did
for step 2.
* Cleanup everything related to processing the input from step i, then
go back to step 1.
* This is where the infinite loop part comes in, your program
should constantly accept input from the driver until the driver
indicates that there are no more tests to run. The driver does this
by closing stdin, which will cause std::cin to go into a bad state.
However, you can also modify the driver to send a special string
such as QUIT to exit the while loop.
Thats basically what the skeleton of your program should be.
### Details
This is information about how to do some specific things, such as sending data
to the layout test framework.
* Content Blocks
* The layout test framework accepts output from your program in blocks of
data through stdout. Therefore, printing to stdout is really sending
data to the layout test framework.
* Structure of block
* Header: Data\n
* Header indicates what type of data will be sent through. A list
of valid headers is listed in ``.
* Data is the data that you actually want to send. For pixel
tests, you want to send the actual PNG data here.
* The newline is needed to indicate the end of a header.
* End of a content block
* To indicate the end of a a content block and cause the driver to
progress, you need to write “#EOF\n to stdout (mandatory) and
to stderr for certain types of content, such as image data.
* Multiple headers per block
* Some blocks require different sets of data. For PNGs, not only
is the PNG needed, but so is a hash of the bitmap used to create
the PNG.
* In this case this is how your output should look.
* Content-type: image/png\n
* ActualHash: hashData\n
* Content-Length: lengthOfPng\n
* pngdata
* This part doesnt need a header specifying that you are
sending png data, just send it
* “#EOF\n on both stdout and stderr
* To see the structure of the data required, look at the
`read_block` functions in