contributing: clear structure

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+.. _debugging:
+
+#########
+Debugging
+#########
+
+There are two flags available to aid in debugging configuration scripts.
+Since configuration loading issues will manifest during boot, the flags are
+passed as kernel boot parameters.
+
+ISO image build
+===============
+
+When having trouble compiling your own ISO image or debugging Jenkins issues
+you can follow the steps at :ref:`iso_build_issues`.
+
+System Startup
+==============
+
+The system startup can be debugged (like loading in the configuration
+file from ``/config/config.boot``. This can be achieve by extending the
+Kernel command-line in the bootloader.
+
+Kernel
+------
+
+* ``vyos-debug`` - Adding the parameter to the linux boot line will produce
+  timing results for the execution of scripts during commit. If one is seeing
+  an unexpected delay during manual or boot commit, this may be useful in
+  identifying bottlenecks. The internal flag is ``VYOS_DEBUG``, and is found
+  in vyatta-cfg_. Output is directed to ``/var/log/vyatta/cfg-stdout.log``.
+
+* ``vyos-config-debug`` - During development, coding errors can lead to a
+  commit failure on boot, possibly resulting in a failed initialization of the
+  CLI. In this circumstance, the kernel boot parameter ``vyos-config-debug``
+  will ensure access to the system as user ``vyos``, and will log a Python
+  stack trace to the file ``/tmp/boot-config-trace``.
+  File ``boot-config-trace`` will generate only if config loaded with a failure
+  status.
+
+Live System
+===========
+
+A number of flags can be set up to change the behaviour of VyOS at runtime.
+These flags can be toggled using either environment variables or creating
+files.
+
+For each feature, a file called ``vyos.feature.debug`` can be created to
+toggle the feature on. If a parameter is required it can be placed inside
+the file as its first line.
+
+The file can be placed in ``/tmp`` for one time debugging (as the file
+will be removed on reboot) or placed in '/config' to stay permanently.
+
+For example, ``/tmp/vyos.ifconfig.debug`` can be created to enable
+interface debugging.
+
+It is also possible to set up the debugging using environment variables.
+In that case, the name will be (in uppercase) VYOS_FEATURE_DEBUG.
+
+For example running, ``export VYOS_IFCONFIG_DEBUG=""`` on your vbash,
+will have the same effect as ``touch /tmp/vyos.ifconfig.debug``.
+
+* ``ifconfig`` - Once set, all commands used, and their responses received
+  from the OS, will be presented on the screen for inspection.
+
+* ``command`` - Once set, all commands used, and their responses received
+  from the OS, will be presented on the screen for inspection.
+
+* ``developer`` - Should a command fail, instead of printing a message to the
+  user explaining how to report issues, the python interpreter will start a
+  PBD post-mortem session to allow the developer to debug the issue. As the
+  debugger will wait from input from the developer, it has the capacity to
+  prevent a router to boot and therefore should only be permanently set up
+  on production if you are ready to see the OS fail to boot.
+
+* ``log`` - In some rare cases, it may be useful to see what the OS is doing,
+  including during boot. This option sends all commands used by VyOS to a
+  file. The default file is ``/tmp/full-log`` but it can be changed.
+
+.. note:: In order to retrieve the debug output on the command-line you need to
+  disable ``vyos-configd`` in addition. This can be run either one-time by
+  calling ``sudo systemctl stop vyos-configd`` or make this reboot-safe by
+  calling ``sudo systemctl disable vyos-configd``.
+
+FRR
+---
+
+Recent versions use the ``vyos.frr`` framework. The Python class is located
+inside our ``vyos-1x:python/vyos/frr.py``. It comes with an embedded debugging/
+(print style) debugger as vyos.ifconfig does.
+
+To enable debugging just run: ``$ touch /tmp/vyos.frr.debug``
+
+Debugging Python Code with PDB
+------------------------------
+
+Sometimes it might be useful to debug Python code interactively on the live
+system rather than a IDE. This can be achieved using pdb.
+
+Let us assume you want to debug a Python script that is called by an op-mode
+command. After you found the script by looking up the op-mode-defitions you
+can edit the script in the live system using e.g. vi:
+``vi /usr/libexec/vyos/op_mode/show_xyz.py``
+
+Insert the following statement right before the section where you want to
+investigate a problem (e.g. a statement you see in a backtrace):
+``import pdb; pdb.set_trace()``
+Optionally you can surrounded this statement by an ``if`` which only triggers
+under the condition you are interested in.
+
+Once you run ``show xyz`` and your condition is triggered you should be dropped
+into the python debugger:
+
+
+.. code-block:: none
+
+   > /usr/libexec/vyos/op_mode/show_nat_translations.py(109)process()
+   -> rule_type = rule.get('type', '')
+   (Pdb)
+
+You can type ``help`` to get an overview of the available commands, and
+``help command`` to get more information on each command.
+
+Useful commands are:
+
+* examine variables using ``pp(var)``
+* contine execution using ``cont``
+* get a backtrace using ``bt``
+
+Config Migration Scripts
+------------------------
+
+When writing a new configuration migrator it may happen that you see an error
+when you try to invoke it manually on a development system. This error will
+look like:
+
+.. code-block:: none
+
+  vyos@vyos:~$ /opt/vyatta/etc/config-migrate/migrate/ssh/0-to-1 /tmp/config.boot
+  Traceback (most recent call last):
+    File "/opt/vyatta/etc/config-migrate/migrate/ssh/0-to-1", line 31, in <module>
+      config = ConfigTree(config_file)
+    File "/usr/lib/python3/dist-packages/vyos/configtree.py", line 134, in __init__
+      raise ValueError("Failed to parse config: {0}".format(msg))
+  ValueError: Failed to parse config: Syntax error on line 240, character 1: Invalid syntax.
+
+The reason is that the configuration migration backend is rewritten and uses
+a new form of "magic string" which is applied on demand when real config
+migration is run on boot. When runnint individual migrators for testing,
+you need to convert the "magic string" on your own by:
+
+.. code-block:: none
+
+  vyos@vyos:~$ /usr/libexec/vyos/run-config-migration.py --virtual --set-vintage vyos /tmp/config.boot
+
+Configuration Error on System Boot
+----------------------------------
+
+Beeing brave and running the latest rolling releases will sometimes trigger
+bugs due to corner cases we missed in our design. Those bugs should be filed
+via Phabricator_ but you can help us to narrow doen the issue. Login to your
+VyOS system and change into configuration mode by typing ``configure``. Now
+re-load your boot configuration by simply typing ``load`` followed by return.
+
+You shoudl now see a Python backtrace which will help us to handle the issue,
+please attach it to the Phabricator_ task.
+
+Boot Timing
+-----------
+
+During the migration and extensive rewrite of functionality from Perl into
+Python a significant increase in the overall system boottime was noticed. The
+system boot time can be analysed and a graph can be generated in the end which
+shows in detail who called whom during the system startup phase.
+
+This is done by utilizing the ``systemd-bootchart`` package which is now
+installed by default on the VyOS 1.3 (equuleus) branch. The configuration is
+also versioned so we get comparable results. ``systemd-bootchart`` is configured
+using this file: bootchart.conf_
+
+To enable boot time graphing change the Kernel commandline and add the folowing
+string: ``init=/usr/lib/systemd/systemd-bootchart``
+
+This can also be done permanently by changing ``/boot/grub/grub.cfg``.
+
+Priorities
+==========
+
+VyOS CLI is all about priorities. Every CLI node has a corresponding
+``node.def`` file and possibly an attached script that is executed when the
+node is present. Nodes can have a priority, and on system bootup - or any
+other ``commit`` to the config all scripts are executed from lowest to higest
+priority. This is good as this gives a deterministic behavior.
+
+To debug issues in priorities or to see what's going on in the background
+you can use the ``/opt/vyatta/sbin/priority.pl`` script which lists to you
+the execution order of the scripts.
+
+.. stop_vyoslinter
+
+.. _vyatta-cfg: https://github.com/vyos/vyatta-cfg
+.. _bootchart.conf: https://github.com/vyos/vyos-build/blob/current/data/live-build-config/includes.chroot/etc/systemd/bootchart.conf
+.. include:: /_include/common-references.txt
+
+.. start_vyoslinter