Debugging i3: How To
====================
Michael Stapelberg <michael@i3wm.org>
January 2014

This document describes how to debug i3 to send us useful bug
reports, even if you have no knowledge of C programming.

Thank you for being interested in debugging i3. It really means
something to us to get your bug fixed. If you have any questions about the
process and/or need further help, do not hesitate to contact us!

== Verify you are using i3 ≥ 4.10

Only the latest major version of i3 is supported. To verify which version
you are running, use:

---------------
$ i3 --moreversion 2>&- || i3 --version
Binary i3 version:  4.7 (2013-12-22, branch "tags/4.7")
Running i3 version: 4.7-84-gac74a63 (2014-01-01, branch "next") (pid 1995)
---------------

Your version can look like this:

4.7 (release version)::
You are using a release version. In many cases, bugs are already
fixed in the development version of i3. Even if the bug is not a known fixed
one, we will still ask you to reproduce your error with the most recent
development version of i3. Therefore, please upgrade to a development version
if you can.

4.7-85-g9c15b95 (development version)::
Your version is 85 commits newer than 4.7, and the git revision of your
version is +9c15b95+. Go to https://github.com/i3/i3/commits/next and see if
the most recent commit starts with the same revision. If so, you are using the
latest version.

Development versions of i3 have logging enabled by default and are compiled
with debug symbols.

== Enabling logging

If you are using a development version (see previous section), you don’t need
to do anything -- skip to section 3.

If you are using a release version with a custom +~/.xsession+ (or xinitrc)
file, execute i3 with a line like this:

----------------------------------
# Use 25 MiB of RAM for debug logs
exec i3 --shmlog-size=26214400
----------------------------------

If you are *NOT* using an +~/.xsession+ file but you just chose "i3" from the
list of sessions in your desktop manager (gdm, lxdm, …), edit
+/usr/share/xsessions/i3.desktop+ and replace the +Exec=i3+ line with:

------------------------------
Exec=i3 --shmlog-size=26214400
------------------------------

If you cannot restart i3 for some reason, you can enable debug logging on the
fly:

---------------------------------------
i3-msg 'debuglog on; shmlog on; reload'
---------------------------------------

== Reproducing the problem

Before submitting an issue, please make sure to close down on the problem as
much as you can yourself. Here are some steps you should consider:

* Find a deterministic, reliable way to reproduce the problem and provide it
  with your bug report.
* Try using the default i3 config to reproduce the problem. If the issue does
  not appear with the default config, gradually adapt it to track down what 
  change(s) to the config introduce the problem.
* Reproduce the problem with a minimal setup, i.e., only use as few applications,
  windows and steps as necessary.
* In addition, try to stick to applications that are common and, even more
  importantly, free / open source.
* Before obtaining the log file, restart i3 in-place, execute the steps to
  reproduce the problem and then save the logs. This keeps the log file as
  small as possible and necessary.

Please be aware that we cannot support compatibility issues with closed-source
software, as digging into compatibility problems without having access to the
source code is too time-consuming. Additionally, experience has shown that
often, the software in question is responsible for the issue. Please raise an
issue with the software in question, not i3.

== Obtaining the debug logfile

[CAUTION]
================================================================================
Logs may contain sensitive information, so please inspect the log before
submitting it. Logs may be viewed by anyone, once posted. If you choose to
redact the log, make an effort not to discard information which may be relevant
to the issue you are reporting.

The best way to avoid submitting such information is to only run the necessary
steps to reproduce the behavior when saving the log file. This will also make
analyzing the log file easier.
================================================================================

No matter whether i3 misbehaved in some way without crashing or whether it just
crashed, the logfile provides all information necessary to debug the problem.

To upload a compressed version of the logfile (for a bugreport), use:
-------------------------------------------------------------------------------
DISPLAY=:0 i3-dump-log | bzip2 -c | curl --data-binary @- https://logs.i3wm.org
-------------------------------------------------------------------------------

This command does not depend on i3 (it also works while i3 displays
the crash dialog), but it requires a working X11 connection.

After running it, you will get a URL to the logfile. Please include that URL in
your bug report.

== On crashes: Obtaining a backtrace

When i3 crashes, it will display a dialog stating “i3 just crashed”, offering
you to save a backtrace to a text file.

To actually get useful backtraces, you should make sure that your version of i3
is compiled with debug symbols:

------------------------------------------------------------------------------
$ file `which i3`
/usr/bin/i3: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically
linked (uses shared libs), for GNU/Linux 2.6.18, not stripped
------------------------------------------------------------------------------

Notice the +not stripped+, which is the important part. If you have a version
which is stripped, please check whether your distribution provides debug
symbols (package +i3-wm-dbg+ on Debian for example) or if you can turn off
stripping. If nothing helps, please build i3 from source.

Once you have made sure that your i3 is compiled with debug symbols and the C
debugger +gdb+ is installed on your machine, you can let i3 generate a
backtrace in the crash dialog.

After pressing "b" in the crash dialog, you will get a file called
+/tmp/i3-backtrace.%d.%d.txt+ where the first +%d+ is replaced by i3’s process
id (PID) and the second one is incremented each time you generate a backtrace,
starting at 0.

In Linux, if the backtrace just says +No stack.+, that's because gdb does not
have necessary permissions to attach to a running process. You can fix that by
running from a terminal (you can open a new tty, e.g. with ctrl-alt-F2):

---------------------------------------------------------------------
echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
---------------------------------------------------------------------

Afterwards, try re-generating the stack trace. Note that this setting re-sets
after reboot, see more info at
https://www.kernel.org/doc/Documentation/security/Yama.txt.

== Debugging i3bar

To debug i3bar problems, use the +--verbose+ commandline parameter,
or add +verbose yes+ to all +bar {}+ blocks in your i3
config, reload your config and then restart all i3bar instances like this:

---------------------------------------------------------------------
$ i3 reload
$ killall i3bar
$ for c in $(i3-msg -t get_bar_config | python -c \
      'import json,sys;print("\n".join(json.load(sys.stdin)))'); do \
    (i3bar --bar_id=$c >i3bar.$c.log 2>&1) & \
  done;
---------------------------------------------------------------------

There will now be +i3bar.*.log+ files in your current directory that you can provide
in your bug report.
