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docs: significant expansion of custom hook script information

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<h1>Hooks for specific system management</h1>
<p>Libvirt includes synchronous hooks, starting from version 0.8.0, as a
way to tie in specific tailored system actions at a specific time.
If these scripts are present on the host where the hypervisor
is running, then they are called when the libvirt daemon is doingi
some significant action.</p>
<p>The scripts are expected to execute quickly, return a zero exit
status if all conditions are set for the daemon to continue the
action (non zero will be considered a failure which may
be ignored but in general will stop the ongoing operation).
The script also should not call back into libvirt as the daemon
is waiting for the script exit and deadlock is likely to occur.</p>
<p>The scripts are stored in the directory <code>/etc/libvirt/hooks/</code>
when using a standard installation path
(<code>$SYSCONF_DIR/libvirt/hooks/</code> in general).</p>
<p>Each script is given the following command line arguments:</p>
<ul>
<li> the first argument is the name of the object involved in the
operation, or '-' if there is none.
<li> the second argument is the name of the operation.
<li> the third argument is a suboperation indication like 'start' or
'end', or '-' if there is none.
<li> the last argument is an extra argument string or '-' if there
is none.
</ul>
<p>There are currently scripts for 3 domains of operation:
<ul id="toc"></ul>
<h2><a name="intro">Custom event scripts</a></h2>
<p>Beginning with libvirt 0.8.0, specific events on a host system will
trigger custom scripts.</p>
<p>These custom <b>hook</b> scripts are executed when any of the following
actions occur:</p>
<ul>
<li><p><code>/etc/libvirt/hooks/daemon</code> script if
present is called at 3 points in time:</p>
<p>at daemon startup, typically started with the following
arguments:</p>
<pre>/etc/libvirt/hooks/daemon - start - start</pre>
<p>at daemon shutdown when it is about to exit, with the following
arguments:</p>
<pre>/etc/libvirt/hooks/daemon - shutdown - shutdown</pre>
<p>When the daemon is asked to reload its driver state when
receiving the SIGHUP signal, arguments are:</p>
<pre>/etc/libvirt/hooks/daemon - reload begin SIGHUP</pre>
</li>
<li><p><code>/etc/libvirt/hooks/qemu</code> script and <br/>
<code>/etc/libvirt/hooks/lxc</code> associate hooks for domain
operation on the respective QEmu/KVM and LXC drivers.</p>
<p> The domain related hooks also receive the full XML description
for the concerned domain on their stdin, which allows them to get
all the information from the domain, including UUID or storage
if that is needed for the script operation.</p>
<p> Currently only domain startup and domain end operations
involve the hook, the first one just before the domain gets
created.
For example if starting a QEmu domain named <code>test</code>
the following script will get called:</p>
<pre>/etc/libvirt/hooks/qemu test start begin -</pre>
<p> note that a non-zero return value from the script will abort the
domain startup operation, and if an error string is passed on
stderr by the hook script, it will be provided back to the user
at the libvirt API level.</p>
<p> For domain shutdown, the script will be called just after the
domain has finished execution, and the script will get:</p>
<pre>/etc/libvirt/hooks/qemu test stopped end -</pre>
<p> It is expected that other operations will be associated to hooks
but at the time of 0.8.0 only those 2 are associated to the
domains life cycle</p>
</li>
<li>The libvirt daemon starts, stops, or reloads its
configuration<br/><br/></li>
<li>A QEMU guest is started or stopped<br/><br/></li>
<li>An LXC guest is started or stopped<br/><br/></li>
</ul>
<p></p>
<h2><a name="location">Script location</a></h2>
<p>The libvirt hook scripts are located in the directory
<code>$SYSCONF_DIR/libvirt/hooks/</code>.</p>
<ul>
<li>In Linux distributions such as Fedora and RHEL, this is
<code>/etc/libvirt/hooks/</code>. Other Linux distributions may do
this differently.</li>
<li>If your installation of libvirt has instead been compiled from
source, it is likely to be
<code>/usr/local/etc/libvirt/hooks/</code>.</li>
</ul>
<p>To use hook scripts, you will need to create this <code>hooks</code>
directory manually, place the desired hook scripts inside, then make
them executable.</p>
<br/>
<h2><a name="names">Script names</a></h2>
<p>At present, there are three hook scripts that can be called:</p>
<ul>
<li><code>/etc/libvirt/hooks/daemon</code><br/><br/>
Executed when the libvirt daemon is started, stopped, or reloads
its configuration<br/><br/></li>
<li><code>/etc/libvirt/hooks/qemu</code><br/><br/>
Executed when a QEMU guest is started, stopped, or migrated<br/><br/></li>
<li><code>/etc/libvirt/hooks/lxc</code><br /><br/>
Executed when an LXC guest is started or stopped</li>
</ul>
<br/>
<h2><a name="structure">Script structure</a></h2>
<p>The hook scripts are executed using standard Linux process creation
functions. Therefore, they must begin with the declaration of the
command interpreter to use.</p>
<p>For example:</p>
<pre>#!/bin/bash</pre>
<p>or:</p>
<pre>#!/usr/bin/python</pre>
<p>Other command interpreters are equally valid, as is any executable
binary, so you are welcome to use your favourite languages.</p>
<br/>
<h2><a name="arguments">Script arguments</a></h2>
<p>The hook scripts are called with specific command line arguments,
depending upon the script, and the operation being performed.</p>
<p>The guest hook scripts, qemu and lxc, are also given the <b>full</b>
XML description for the domain on their stdin. This includes items
such the UUID of the domain and its storage information, and is
intended to provide all the libvirt information the script needs.</p>
<p>The command line arguments take this approach:</p>
<ol>
<li>The first argument is the name of the <b>object</b> involved in the
operation, or '-' if there is none.<br/><br/>
For example, the name of a guest being started.<br/><br/></li>
<li>The second argument is the name of the <b>operation</b> being
performed.<br/><br/>
For example, "start" if a guest is being started.<br/><br/></li>
<li>The third argument is a <b>sub-operation</b> indication, or '-' if there
is none.<br/><br/></li>
<li>The last argument is an <b>extra argument</b> string, or '-' if there is
none.</li>
</ol>
<h4><a name="arguments_specifics">Specifics</a></h4>
<p>This translates to the following specifics for each hook script:</p>
<h5><a name="daemon">/etc/libvirt/hooks/daemon</a></h5>
<ul>
<li>When the libvirt daemon is started, this script is called as:<br/>
<pre>/etc/libvirt/hooks/daemon - start - start</pre></li>
<li>When the libvirt daemon is shut down, this script is called as:<br/>
<pre>/etc/libvirt/hooks/daemon - shutdown - shutdown</pre></li>
<li>When the libvirt daemon receives the SIGHUP signal, it reloads its
configuration and triggers the hook script as:<br/>
<pre>/etc/libvirt/hooks/daemon - reload begin SIGHUP</pre></li>
</ul>
<p>Please note that when the libvirt daemon is restarted, the <i>qemu</i>
hook script is called once with the "shutdown" operation, and then once
with the "start" operation. There is no specific operation to indicate
a "restart" is occurring.</p>
<h5><a name="qemu">/etc/libvirt/hooks/qemu</a></h5>
<ul>
<li>When a QEMU guest is started, the qemu hook script is called as:<br/>
<pre>/etc/libvirt/hooks/qemu guest_name start begin -</pre></li>
<li>When a QEMU guest is stopped, the qemu hook script is called
as:<br/>
<pre>/etc/libvirt/hooks/qemu guest_name stopped end -</pre></li>
</ul>
<h5><a name="lxc">/etc/libvirt/hooks/lxc</a></h5>
<ul>
<li>When an LXC guest is started, the lxc hook script is called as:<br/>
<pre>/etc/libvirt/hooks/lxc guest_name start begin -</pre></li>
<li>When a LXC guest is stopped, the lxc hook script is called
as:<br/>
<pre>/etc/libvirt/hooks/lxc guest_name stopped end -</pre></li>
</ul>
<br/>
<h2><a name="execution">Script execution</a></h2>
<ul>
<li>The "start" operation for the guest hook scripts, qemu and lxc,
executes <b>prior</b> to the guest being created. This allows the
guest start operation to be aborted if the script returns indicating
failure.<br/><br/></li>
<li>The "shutdown" operation for the guest hook scripts, qemu and lxc,
executes <b>after</b> the guest has stopped. If the hook script
indicates failure in its return, the shut down of the guest cannot
be aborted because it has already been performed.<br/><br/></li>
<li>Hook scripts execute in a synchronous fashion. Libvirt waits
for them to return before continuing the given operation.<br/><br/>
This is most noticeable with the guest start operation, as a lengthy
operation in the hook script can mean an extended wait for the guest
to be available to end users.<br/><br/></li>
<li>For a hook script to be utilised, it must have its execute bit set
(ie. chmod o+rx <i>qemu</i>), and must be present when the libvirt
daemon is started.<br/><br/></li>
<li>If a hook script is added to a host after the libvirt daemon is
already running, it won't be used until the libvirt daemon
next starts.</li>
</ul>
<br/>
<h2><a name="qemu_migration">QEMU guest migration</a></h2>
<p>Migration of a QEMU guest involves running hook scripts on both the
source and destination hosts:</p>
<ol>
<li>At the beginning of the migration, the <i>qemu</i> hook script on
the <b>destination</b> host is executed with the "start"
operation.<br/><br/></li>
<li>If this hook script returns indicating success (error code 0), the
migration continues. Any other return code indicates failure, and
the migration is aborted.<br/><br/></li>
<li>The QEMU guest is then migrated to the destination host.<br/>
<br/></li>
<li>Unless an error occurs during the migration process, the <i>qemu</i>
hook script on the <b>source</b> host is then executed with the "stopped"
operation, to indicate it is no longer running on this
host.<br/><br/>
Regardless of the return code from this hook script, the migration
is not aborted as it has already been performed.</li>
</ol>
<br/>
<h2><a name="recursive">Calling libvirt functions from within a hook script</a></h2>
<p><b>DO NOT DO THIS!</b></p>
<p>A hook script must not call back into libvirt, as the libvirt daemon
is already waiting for the script to exit.</p>
<p>A deadlock is likely to occur.</p>
<br/>
<h2><a name="return_codes">Return codes and logging</a></h2>
<p>If a hook script returns with an exit code of 0, the libvirt daemon
regards this as successful and performs no logging of it.</p>
<p>However, if a hook script returns with a non zero exit code, the libvirt
daemon regards this as a failure, logs it with return code 256, and
additionally logs anything on stderr the hook script returns.</p>
<p>For example, a hook script might use this code to indicate failure,
and send a text string to stderr:</p>
<pre>echo "Could not find required XYZZY" &gt;&amp;2
exit 1</pre>
<p>The resulting entry in the libvirt log will appear as:</p>
<pre>20:02:40.297: error : virHookCall:416 : Hook script execution failed: Hook script /etc/libvirt/hooks/qemu qemu failed with error code 256:Could not find required XYZZY</pre>
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