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<body class="article">
<div id="header">
<h1>i3 testsuite</h1>
<span id="author">Michael Stapelberg</span><br />
<span id="email"><code>&lt;<a href="mailto:[email protected]">[email protected]</a>&gt;</code></span><br />
<span id="revdate">September 2012</span>
<div id="toc">
  <div id="toctitle">Table of Contents</div>
  <noscript><p><b>JavaScript must be enabled in your browser to display the table of contents.</b></p></noscript>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>This document explains how the i3 testsuite works, how to use it and extend it.
It is targeted at developers who not necessarily have been doing testing before
or have not been testing in Perl before. In general, the testsuite is not of
interest for end users.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_introduction">1. Introduction</h2>
<div class="sectionbody">
<div class="paragraph"><p>The i3 testsuite is a collection of files which contain testcases for various
i3 features. Some of them test if a certain workflow works correctly (moving
windows, focus behaviour, …). Others are regression tests and contain code
which previously made i3 crash or lead to unexpected behaviour. They then check
if i3 still runs (meaning it did not crash) and if it handled everything
correctly.</p></div>
<div class="paragraph"><p>The goal of having these tests is to automatically find problems and to
automatically get a feel for whether a change in the source code breaks any
existing feature. After every modification of the i3 sourcecode, the developer
should run the full testsuite. If one of the tests fails, the corresponding
problem should be fixed (or, in some cases, the testcase has to be modified).
For every bugreport, a testcase should be written to test the correct
behaviour. Initially, it will fail, but after fixing the bug, it will pass.
This ensures (or increases the chance) that bugs which have been fixed once
will never be found again.</p></div>
<div class="paragraph"><p>Also, when implementing a new feature, a testcase might be a good way to be
able to easily test if the feature is working correctly. Many developers will
test manually if everything works. Having a testcase not only helps you with
that, but it will also be useful for every future change.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_relevant_documentation">2. Relevant documentation</h2>
<div class="sectionbody">
<div class="paragraph"><p>Apart from this document, you should also have a look at:</p></div>
<div class="olist arabic"><ol class="arabic">
<li>
<p>
The "Modern Perl" book, which can be found at
   <a href="http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf">http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf</a>
</p>
</li>
<li>
<p>
The latest Perl documentation of the "i3test" (general testcase setup) and
   "i3test::Test" (additional test instructions) modules:
   <a href="https://build.i3wm.org/docs/lib-i3test.html">https://build.i3wm.org/docs/lib-i3test.html</a> respectively
   <a href="https://build.i3wm.org/docs/lib-i3test-test.html">https://build.i3wm.org/docs/lib-i3test-test.html</a>
</p>
</li>
<li>
<p>
The latest documentation on i3’s IPC interface:
   <a href="https://build.i3wm.org/docs/ipc.html">https://build.i3wm.org/docs/ipc.html</a>
</p>
</li>
</ol></div>
</div>
</div>
<div class="sect1">
<h2 id="_implementation">3. Implementation</h2>
<div class="sectionbody">
<div class="paragraph"><p>For several reasons, the i3 testsuite has been implemented in Perl:</p></div>
<div class="olist arabic"><ol class="arabic">
<li>
<p>
Perl has a long tradition of testing. Every popular/bigger Perl module which
   you can find on CPAN will not only come with documentation, but also with
   tests. Therefore, the available infrastructure for tests is comprehensive.
   See for example the excellent <a href="http://search.cpan.org/perldoc?Test::More">http://search.cpan.org/perldoc?Test::More</a>
   and the referenced <a href="http://search.cpan.org/perldoc?Test::Tutorial">http://search.cpan.org/perldoc?Test::Tutorial</a>.
</p>
</li>
<li>
<p>
Perl is widely available and has a well-working package infrastructure.
</p>
</li>
<li>
<p>
The author is familiar with Perl :).
</p>
</li>
<li>
<p>
It is a good idea to use a different language for the tests than the
   implementation itself.
</p>
</li>
</ol></div>
<div class="paragraph"><p>Please do not start programming language flamewars at this point.</p></div>
<div class="sect2">
<h3 id="_installing_the_dependencies">3.1. Installing the dependencies</h3>
<div class="paragraph"><p>As usual with Perl programs, the testsuite ships with a <code>Makefile.PL</code>.
This file specifies which Perl modules the testsuite depends on and can be used
to install all of them.</p></div>
<div class="paragraph"><p>Perl modules are distributed via CPAN, and there is the official, standard CPAN
client, simply called <code>cpan</code>. It comes with every Perl installation and can be
used to install the testsuite. Many users prefer to use the more modern
<code>cpanminus</code> instead, though (because it asks no questions and just works):</p></div>
<div class="paragraph"><p>The tests additionally require <code>Xephyr(1)</code> to run a nested X server. Install
<code>xserver-xephyr</code> on Debian or <code>xorg-server-xephyr</code> on Arch Linux.</p></div>
<div class="listingblock">
<div class="title">Installing testsuite dependencies using cpanminus (preferred)</div>
<div class="content">
<pre><code>$ cd ~/i3/testcases
$ sudo apt-get install cpanminus
$ sudo cpanm .
$ cd ~/i3/AnyEvent-I3
$ sudo cpanm Module::Install
$ sudo cpanm .</code></pre>
</div></div>
<div class="paragraph"><p>If you don’t want to use cpanminus for some reason, the same works with cpan:</p></div>
<div class="listingblock">
<div class="title">Installing testsuite dependencies using cpan</div>
<div class="content">
<pre><code>$ cd ~/i3/testcases
$ sudo cpan .
$ cd ~/i3/AnyEvent-I3
$ sudo cpan Module::Install
$ sudo cpan .</code></pre>
</div></div>
<div class="paragraph"><p>In case you don’t have root permissions, you can also install into your home
directory, see <a href="https://michael.stapelberg.de/cpan/">https://michael.stapelberg.de/cpan/</a></p></div>
</div>
<div class="sect2">
<h3 id="_mechanisms">3.2. Mechanisms</h3>
<div class="sect3">
<h4 id="_script_complete_run">3.2.1. Script: complete-run</h4>
<div class="paragraph"><p>The testcases are run by a script called <code>complete-run.pl</code>. It runs all
testcases by default, but you can be more specific and let it only run one or
more testcases. Also, it takes care of starting up a separate instance of i3
with an appropriate configuration file and creates a folder for each run
containing the appropriate i3 logfile for each testcase. The latest folder can
always be found under the symlink <code>latest/</code>. Unless told differently, it will
run the tests on a separate X server instance (using Xephyr).</p></div>
<div class="paragraph"><p>Xephyr will open a window where you can inspect the running test. By default,
tests are run under Xvfb.</p></div>
<div class="listingblock">
<div class="title">Example invocation of <code>complete-run.pl</code></div>
<div class="content">
<pre><code>$ cd ~/i3

$ autoreconf -fi

$ mkdir -p build &amp;&amp; cd build

$ ../configure

$ make -j8
# output omitted because it is very long

$ cd testcases

$ ./complete-run.pl
# output omitted because it is very long
All tests successful.
Files=78, Tests=734, 27 wallclock secs ( 0.38 usr  0.48 sys + 17.65 cusr  3.21 csys = 21.72 CPU)
Result: PASS

$ ./complete-run.pl t/04-floating.t
[:3] i3 startup: took 0.07s, status = 1
[:3] Running t/04-floating.t with logfile testsuite-2011-09-24-16-06-04-4.0.2-226-g1eb011a/i3-log-for-04-floating.t
[:3] t/04-floating.t finished
[:3] killing i3
output for t/04-floating.t:
ok 1 - use X11::XCB::Window;
ok 2 - The object isa X11::XCB::Window
ok 3 - Window is mapped
ok 4 - i3 raised the width to 75
ok 5 - i3 raised the height to 50
ok 6 - i3 did not map it to (0x0)
ok 7 - The object isa X11::XCB::Window
ok 8 - i3 let the width at 80
ok 9 - i3 let the height at 90
ok 10 - i3 mapped it to x=1
ok 11 - i3 mapped it to y=18
ok 12 - The object isa X11::XCB::Window
ok 13 - i3 let the width at 80
ok 14 - i3 let the height at 90
1..14

All tests successful.
Files=1, Tests=14,  0 wallclock secs ( 0.01 usr  0.00 sys +  0.19 cusr  0.03 csys =  0.23 CPU)
Result: PASS

$ less latest/i3-log-for-04-floating.t</code></pre>
</div></div>
<div class="paragraph"><p>If your attempt to run the tests with a bare call to ./complete-run.pl fails, try this:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ ./complete-run.pl --parallel=1 --keep-xserver-output</code></pre>
</div></div>
<div class="paragraph"><p>This will show the output of Xephyr, which is the X server implementation we
use for testing.</p></div>
<div class="sect4">
<h5 id="_make_command_code_make_check_code">make command: <code>make check</code></h5>
<div class="paragraph"><p>Make check runs the i3 testsuite.
You can still use ./testcases/complete-run.pl to get the interactive progress output.</p></div>
<div class="listingblock">
<div class="title">Example invocation of <code>make check</code></div>
<div class="content">
<pre><code>$ cd ~/i3

$ autoreconf -fi

$ mkdir -p build &amp;&amp; cd build

$ ../configure

$ make -j8
# output omitted because it is very long

$ make check
# output omitted because it is very long
PASS: testcases/complete-run.pl
============================================================================
Testsuite summary for i3 4.13
============================================================================
# TOTAL: 1
# PASS:  1
# SKIP:  0
# XFAIL: 0
# FAIL:  0
# XPASS: 0
# ERROR: 0
============================================================================

$ less test-suite.log</code></pre>
</div></div>
</div>
</div>
<div class="sect3">
<h4 id="_coverage_testing">3.2.2. Coverage testing</h4>
<div class="paragraph"><p>Coverage testing is possible with <code>lcov</code>, the front-end for GCC&#8217;s coverage
testing tool <code>gcov</code>. The testcases can generate a nice html report that tells
you which functions and lines were covered during a run of the tests. You can
use this tool to judge how effective your tests are.</p></div>
<div class="paragraph"><p>To use test coverage tools, first compile with coverage enabled.</p></div>
<div class="listingblock">
<div class="content">
<pre><code>COVERAGE=1 make</code></pre>
</div></div>
<div class="paragraph"><p>Then run the tests with the <code>--coverage-testing</code> flag.</p></div>
<div class="listingblock">
<div class="content">
<pre><code>./complete-run.pl --coverage-testing</code></pre>
</div></div>
<div class="paragraph"><p>Then open <code>latest/i3-coverage/index.html</code> in your web browser.</p></div>
</div>
<div class="sect3">
<h4 id="_ipc_interface">3.2.3. IPC interface</h4>
<div class="paragraph"><p>The testsuite makes extensive use of the IPC (Inter-Process Communication)
interface which i3 provides. It is used for the startup process of i3, for
terminating it cleanly and (most importantly) for modifying and getting the
current state (layout tree).</p></div>
<div class="paragraph"><p>See [<a href="https://i3wm.org/docs/ipc.html">https://i3wm.org/docs/ipc.html</a>] for documentation on the IPC interface.</p></div>
</div>
<div class="sect3">
<h4 id="_x11_xcb">3.2.4. X11::XCB</h4>
<div class="paragraph"><p>In order to open new windows, change attributes, get events, etc., the
testsuite uses X11::XCB, a new (and quite specific to i3 at the moment) Perl
module which uses the XCB protocol description to generate Perl bindings to
X11. They work in a very similar way to libxcb (which i3 uses) and provide
relatively high-level interfaces (objects such as <code>X11::XCB::Window</code>) as well as
access to the low-level interface, which is very useful when testing a window
manager.</p></div>
</div>
</div>
<div class="sect2">
<h3 id="_filesystem_structure">3.3. Filesystem structure</h3>
<div class="paragraph"><p>In the git root of i3, the testcases live in the folder <code>testcases</code>. This
folder contains the <code>complete-run.pl</code> and a base configuration file which will
be used for the tests. The different testcases (their file extension is .t, not
.pl) themselves can be found in the conventionally named subfolder <code>t</code>:</p></div>
<div class="listingblock">
<div class="title">Filesystem structure</div>
<div class="content">
<pre><code>├── testcases
│   ├── complete-run.pl
│   ├── i3-test.config
│   ├── lib
│   │   ├── i3test.pm
│   │   ├── SocketActivation.pm
│   │   └── StartXDummy.pm
│   ├── t
│   │   ├── 00-load.t
│   │   ├── 01-tile.t
│   │   ├── 02-fullscreen.t
│   │   ├── ...
│   │   ├── omitted for brevity
│   │   ├── ...
│   │   └── 74-regress-focus-toggle.t</code></pre>
</div></div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_anatomy_of_a_testcase">4. Anatomy of a testcase</h2>
<div class="sectionbody">
<div class="paragraph"><p>Learning by example is definitely a good strategy when you are wondering how to
write a testcase. Let&#8217;s take <code>t/11-goto.t</code> as an easy example and go through it
step by step:</p></div>
<div class="listingblock">
<div class="title">t/11-goto.t: Boilerplate</div>
<div class="content">
<pre><code>#!perl
# vim:ts=4:sw=4:expandtab

use i3test;
use File::Temp;

my $x = X11::XCB::Connection-&gt;new;</code></pre>
</div></div>
<div class="paragraph"><p>This is what we call boilerplate. It exists at the top of every test file (to
some extent). The first line is the shebang, which specifies that this file is
a Perl script. The second line contains VIM specific settings on how to
edit/format this file (use spaces instead of tabs, indent using 4 spaces).
Afterwards, the <code>i3test</code> module is used. This module contains i3 testsuite
specific functions which you are strongly encouraged to use. They make writing
testcases a lot easier and will make it easier for other people to read your
tests.</p></div>
<div class="paragraph"><p>The next line uses the <code>File::Temp</code> module. This is specific to this testcase,
because it needs to generate a temporary name during the test. Many testcases
use only the <code>i3test</code> module.</p></div>
<div class="paragraph"><p>The last line opens a connection to X11. You might or might not need this in
your testcase, depending on whether you are going to open windows (etc.) or
only use i3 commands.</p></div>
<div class="listingblock">
<div class="title">t/11-goto.t: Setup</div>
<div class="content">
<pre><code>my $tmp = fresh_workspace;

cmd 'split h';</code></pre>
</div></div>
<div class="paragraph"><p>The first line calls i3test&#8217;s <code>fresh_workspace</code> function which looks for a
currently unused workspace, switches to it, and returns its name. The variable
<code>$tmp</code> will end up having a value such as <code>"/tmp/87kBVcHbA9"</code>. Note that this
is not (necessarily) a valid path, it&#8217;s just a random workspace name.</p></div>
<div class="paragraph"><p>So, now that we are on a new workspace, we ensure that the workspace uses
horizontal orientation by issuing the <code>split h</code> command (see the i3 User&#8217;s
Guide for a list of commands). This is not strictly necessary, but good style.
In general, the <code>cmd</code> function executes the specified i3 command by using the
IPC interface and returns once i3 acknowledged the command.</p></div>
<div class="listingblock">
<div class="title">t/11-goto.t: Setup</div>
<div class="content">
<pre><code>#####################################################################
# Create two windows and make sure focus switching works
#####################################################################

my $top = open_window($x);
my $mid = open_window($x);
my $bottom = open_window($x);</code></pre>
</div></div>
<div class="paragraph"><p>In every major section of a testcase, you should put a comment like the one
above. This makes it immediately clear how the file is structured.</p></div>
<div class="paragraph"><p>The <code>open_window</code> function opens a standard window, which will then be put into
tiling mode by i3. If you want a floating window, use the
<code>open_floating_window</code> function. These functions accept the same parameters as
<code>X11::XCB::Window&#8594;new</code>, see the i3test documentation at TODO.</p></div>
<div class="listingblock">
<div class="title">t/11-goto.t: Helper function</div>
<div class="content">
<pre><code>#
# Returns the input focus after sending the given command to i3 via IPC
# and syncing with i3
#
sub focus_after {
    my $msg = shift;

    cmd $msg;
    sync_with_i3 $x;
    return $x-&gt;input_focus;
}</code></pre>
</div></div>
<div class="paragraph"><p>This section defines a helper function which will be used over and over in this
testcase. If you have code which gets executed more than once or twice
(depending on the length of your test, use your best judgement), please put it
in a function. Tests should be short, concise and clear.</p></div>
<div class="paragraph"><p>The <code>focus_after</code> function executes a command and returns the X11 focus after
the command was executed. The <code>sync_with_i3</code> command makes sure that i3 could
push its state to X11. See <a href="#i3_sync">[i3_sync]</a> to learn how this works exactly.</p></div>
<div class="listingblock">
<div class="title">t/11-goto.t: Test assumptions</div>
<div class="content">
<pre><code>$focus = $x-&gt;input_focus;
is($focus, $bottom-&gt;id, "Latest window focused");

$focus = focus_after('focus left');
is($focus, $mid-&gt;id, "Middle window focused");</code></pre>
</div></div>
<div class="paragraph"><p>Now, we run the first two real tests. They use <code>Test::More</code>'s <code>is</code> function,
which compares two values and prints the differences if they are not the same.
After the arguments, we supply a short comment to indicate what we are testing
here. This makes it vastly more easy for the developer to spot which testcase
is the problem in case one fails.</p></div>
<div class="paragraph"><p>The first test checks that the most recently opened window is focused.
Afterwards, the command <code>focus left</code> is issued and it is verified that the
middle window now has focus.</p></div>
<div class="paragraph"><p>Note that this is not a comprehensive test of the <code>focus</code> command&#8201;&#8212;&#8201;we would
have to test wrapping, focus when using a more complex layout, focusing the
parent/child containers, etc. But that is not the point of this testcase.
Instead, we just want to know if <code>$x&#8594;input_focus</code> corresponds with what we are
expecting. If not, something is completely wrong with the test environment and
this trivial test will fail.</p></div>
<div class="listingblock">
<div class="title">t/11-goto.t: Test that the feature does not work (yet)</div>
<div class="content">
<pre><code>#####################################################################
# Now goto a mark which does not exist
#####################################################################

my $random_mark = mktemp('mark.XXXXXX');

$focus = focus_after(qq|[con_mark="$random_mark"] focus|);
is($focus, $mid-&gt;id, "focus unchanged");</code></pre>
</div></div>
<div class="paragraph"><p>Syntax hint: The qq keyword is the interpolating quote operator. It lets you
chose a quote character (in this case the <code>|</code> character, a pipe). This makes
having double quotes in our string easy.</p></div>
<div class="paragraph"><p>In this new major section, a random mark (mark is an identifier for a window,
see "VIM-like marks" in the i3 User’s Guide) will be generated. Afterwards, we
test that trying to focus that mark will not do anything. This is important: Do
not only test that using a feature has the expected outcome, but also test that
using it without properly initializing it does no harm. This command could for
example have changed focus anyways (a bug) or crash i3 (obviously a bug).</p></div>
<div class="listingblock">
<div class="title">t/11-goto.t: Test that the feature does work</div>
<div class="content">
<pre><code>cmd "mark $random_mark";

$focus = focus_after('focus left');
is($focus, $top-&gt;id, "Top window focused");

$focus = focus_after(qq|[con_mark="$random_mark"] focus|);
is($focus, $mid-&gt;id, "goto worked");</code></pre>
</div></div>
<div class="paragraph"><p>Remember: Focus was on the middle window (we verified that earlier in "Test
assumptions"). We now mark the middle window with our randomly generated mark.
Afterwards, we switch focus away from the middle window to be able to tell if
focusing it via its mark will work. If the test works, the goto command seems
to be working.</p></div>
<div class="listingblock">
<div class="title">t/11-goto.t: Test corner case</div>
<div class="content">
<pre><code># check that we can specify multiple criteria

$focus = focus_after('focus left');
is($focus, $top-&gt;id, "Top window focused");

$focus = focus_after(qq|[con_mark="$random_mark" con_mark="$random_mark"] focus|);
is($focus, $mid-&gt;id, "goto worked");</code></pre>
</div></div>
<div class="paragraph"><p>Now we test the same feature, but specifying the mark twice in the command.
This should have no effect, but let’s be sure: test it and see if things go
wrong.</p></div>
<div class="listingblock">
<div class="title">t/11-goto.t: Test second code path</div>
<div class="content">
<pre><code>#####################################################################
# Check whether the focus command will switch to a different
# workspace if necessary
#####################################################################

my $tmp2 = fresh_workspace;

is(focused_ws(), $tmp2, 'tmp2 now focused');

cmd qq|[con_mark="$random_mark"] focus|;

is(focused_ws(), $tmp, 'tmp now focused');</code></pre>
</div></div>
<div class="paragraph"><p>This part of the test checks that focusing windows by mark works across
workspaces. It uses i3test&#8217;s <code>focused_ws</code> function to get the current
workspace.</p></div>
<div class="listingblock">
<div class="title">t/11-goto.t: Test second code path</div>
<div class="content">
<pre><code>done_testing;</code></pre>
</div></div>
<div class="paragraph"><p>The end of every testcase has to contain the <code>done_testing</code> line. This tells
<code>complete-run.pl</code> that the test was finished successfully. If it does not
occur, the test might have crashed during execution&#8201;&#8212;&#8201;some of the reasons why
that could happen are bugs in the used modules, bugs in the testcase itself or
an i3 crash resulting in the testcase being unable to communicate with i3 via
IPC anymore.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="i3_sync">5. Appendix A: The i3 sync protocol</h2>
<div class="sectionbody">
<div class="paragraph"><p>Consider the following situation: You open two windows in your testcase, then
you use <code>focus left</code> and want to verify that the X11 focus has been updated
properly. Sounds simple, right? Let’s assume you use this straight-forward
implementation:</p></div>
<div class="listingblock">
<div class="title">Racey focus testcase</div>
<div class="content">
<pre><code>my $left = open_window($x);
my $right = open_window($x);
cmd 'focus left';
is($x-&gt;input_focus, $left-&gt;id, 'left window focused');</code></pre>
</div></div>
<div class="paragraph"><p>However, the test fails. Sometimes. Apparently, there is a race condition in
your test. If you think about it, this is because you are using two different
pieces of software: You tell i3 to update focus, i3 confirms that, and then you
ask X11 to give you the current focus. There is a certain time i3 needs to
update the X11 state. If the testcase gets CPU time before X11 processed i3&#8217;s
requests, the test will fail.</p></div>
<div class="imageblock">
<div class="content">
<img src="i3-sync.png" alt="Diagram of the race condition" />
</div>
<div class="title">Figure 1. Diagram of the race condition</div>
</div>
<div class="paragraph"><p>One way to "solve" this would be to add <code>sleep 0.5;</code> after the <code>cmd</code> call.
After 0.5 seconds it should be safe to assume that focus has been updated,
right?</p></div>
<div class="paragraph"><p>In practice, this usually works. However, it has several problems:</p></div>
<div class="olist arabic"><ol class="arabic">
<li>
<p>
This is obviously not a clean solution, but a workaround. Ugly.
</p>
</li>
<li>
<p>
On very slow machines, this might not work. Unlikely, but in different
   situations (a delay to wait for i3 to startup) the necessary time is much
   harder to guess, even for fast machines.
</p>
</li>
<li>
<p>
This <strong>wastes a lot of time</strong>. Usually, your computer is much faster than 0.5s
   to update the status. However, sometimes, it might take 0.4s, so we can’t
   make it <code>sleep 0.1</code>.
</p>
</li>
</ol></div>
<div class="paragraph"><p>To illustrate how grave the problem with wasting time actually is: Before
removing all sleeps from the testsuite, a typical run using 4 separate X
servers took around 50 seconds on my machine. After removing all the sleeps,
we achieved times of about 25 seconds. This is very significant and influences
the way you think about tests&#8201;&#8212;&#8201;the faster they are, the more likely you are
to check whether everything still works quite often (which you should).</p></div>
<div class="paragraph"><p>What I am trying to say is: Delays adds up quickly and make the test suite
less robust.</p></div>
<div class="paragraph"><p>The real solution for this problem is a mechanism which I call "the i3 sync
protocol". The idea is to send a request (which does not modify state) via X11
to i3 which will then be answered. Due to the request&#8217;s position in the event
queue (<strong>after</strong> all previous events), you can be sure that by the time you
receive the reply, all other events have been dealt with by i3 (and, more
importantly, X11).</p></div>
<div class="imageblock">
<div class="content">
<img src="i3-sync-working.png" alt="Diagram of the i3 sync solution" />
</div>
<div class="title">Figure 2. Diagram of the i3 sync solution</div>
</div>
<div class="sect2">
<h3 id="_implementation_details">5.1. Implementation details</h3>
<div class="paragraph"><p>The client which wants to sync with i3 initiates the protocol by sending a
ClientMessage to the X11 root window:</p></div>
<div class="listingblock">
<div class="title">Send ClientMessage</div>
<div class="content">
<pre><code># Generate a ClientMessage, see xcb_client_message_t
my $msg = pack "CCSLLLLLLL",
    CLIENT_MESSAGE, # response_type
    32,     # format
    0,      # sequence
    $root,  # destination window
    $x-&gt;atom(name =&gt; 'I3_SYNC')-&gt;id,

    $_sync_window-&gt;id,    # data[0]: our own window id
    $myrnd, # data[1]: a random value to identify the request
    0,
    0,
    0;

# Send it to the root window -- since i3 uses the SubstructureRedirect
# event mask, it will get the ClientMessage.
$x-&gt;send_event(0, $root, EVENT_MASK_SUBSTRUCTURE_REDIRECT, $msg);</code></pre>
</div></div>
<div class="paragraph"><p>i3 will then reply with the same ClientMessage, sent to the window specified in
<code>data[0]</code>. In the reply, <code>data[0]</code> and <code>data[1]</code> are exactly the same as in the
request. You should use a random value in <code>data[1]</code> and check that you received
the same one when getting the reply.</p></div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_appendix_b_socket_activation">6. Appendix B: Socket activation</h2>
<div class="sectionbody">
<div class="paragraph"><p>Socket activation is a mechanism which was made popular by systemd, an init
replacement. It basically describes creating a listening socket before starting
a program.  systemd will invoke the program only when an actual connection to
the socket is made, hence the term socket activation.</p></div>
<div class="paragraph"><p>The interesting part of this (in the i3 context) is that you can very precisely
detect when the program is ready (finished its initialization).</p></div>
<div class="sect2">
<h3 id="_preparing_the_listening_socket">6.1. Preparing the listening socket</h3>
<div class="paragraph"><p><code>complete-run.pl</code> will create a listening UNIX socket which it will then pass
to i3. This socket will be used by i3 as an additional IPC socket, just like
the one it will create on its own. Passing the socket happens implicitly
because children will inherit the parent’s sockets when fork()ing and sockets
will continue to exist after an exec() call (unless CLOEXEC is set of course).</p></div>
<div class="paragraph"><p>The only explicit things <code>complete-run.pl</code> has to do is setting the <code>LISTEN_FDS</code>
environment variable to the number of sockets which exist (1 in our case) and
setting the <code>LISTEN_PID</code> environment variable to the current process ID. Both
variables are necessary so that the program (i3) knows how many sockets it
should use and if the environment variable is actually intended for it. i3 will
then start looking for sockets at file descriptor 3 (since 0, 1 and 2 are used
for stdin, stdout and stderr, respectively).</p></div>
<div class="paragraph"><p>The actual Perl code which sets up the socket, fork()s, makes sure the socket
has file descriptor 3 and sets up the environment variables follows (shortened
a bit):</p></div>
<div class="listingblock">
<div class="title">Setup socket and environment</div>
<div class="content">
<pre><code>my $socket = IO::Socket::UNIX-&gt;new(
    Listen =&gt; 1,
    Local =&gt; $args{unix_socket_path},
);

my $pid = fork;
if ($pid == 0) {
    $ENV{LISTEN_PID} = $$;
    $ENV{LISTEN_FDS} = 1;

    # Only pass file descriptors 0 (stdin), 1 (stdout),
    # 2 (stderr) and 3 (socket) to the child.
    $^F = 3;

    # If the socket does not use file descriptor 3 by chance
    # already, we close fd 3 and dup2() the socket to 3.
    if (fileno($socket) != 3) {
        POSIX::close(3);
        POSIX::dup2(fileno($socket), 3);
    }

    exec "/usr/bin/i3";
}</code></pre>
</div></div>
</div>
<div class="sect2">
<h3 id="_waiting_for_a_reply">6.2. Waiting for a reply</h3>
<div class="paragraph"><p>In the parent process, we want to know when i3 is ready to answer our IPC
requests and handle our windows. Therefore, after forking, we immediately close
the listening socket (i3 will handle this side of the socket) and connect to it
(remember, we are talking about a named UNIX socket) as a client. This connect
call will immediately succeed because the kernel buffers it. Then, we send a
request (of type GET_TREE, but that is not really relevant). Writing data to
the socket will also succeed immediately because, again, the kernel buffers it
(only up to a certain amount of data of course).</p></div>
<div class="paragraph"><p>Afterwards, we just blockingly wait until we get an answer. In the child
process, i3 will setup the listening socket in its event loop. Immediately
after actually starting the event loop, it will notice a new client connecting
(the parent process) and handle its request. Since all initialization has been
completed successfully by the time the event loop is entered, we can now assume
that i3 is ready.</p></div>
</div>
<div class="sect2">
<h3 id="_timing_and_conclusion">6.3. Timing and conclusion</h3>
<div class="paragraph"><p>A beautiful feature of this mechanism is that it does not depend on timing. It
does not matter when the child process gets CPU time or when the parent process
gets CPU time. On heavily loaded machines (or machines with multiple CPUs,
cores or unreliable schedulers), this makes waiting for i3 much more robust.</p></div>
<div class="paragraph"><p>Before using socket activation, we typically used a <code>sleep(1)</code> and hoped that
i3 was initialized by that time. Of course, this breaks on some (slow)
computers and wastes a lot of time on faster computers. By using socket
activation, we decreased the total amount of time necessary to run all tests
(72 files at the time of writing) from &gt; 100 seconds to 16 seconds. This makes
it significantly more attractive to run the test suite more often (or at all)
during development.</p></div>
<div class="paragraph"><p>An alternative approach to using socket activation is polling for the existence
of the IPC socket and connecting to it. While this might be slightly easier to
implement, it wastes CPU time and is considerably uglier than this solution
:). After all, <code>lib/SocketActivation.pm</code> contains only 54 SLOC.</p></div>
</div>
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</div>
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Last updated
 2019-08-30 23:06:47 CEST
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