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[libvirt] [PATCHv2] docs: significant expansion of custom hook script information



---

This 2nd version incorporates feedback from Eric Blake and Osier, plus fixes
a typo, and a few other trivial wording tweaks.

It doesn't change the information regarding the return code of 256, as the
present text describes how the (to be fixed) return code mechanism in libvirt
is really operating.  We'll update this text when the return code problem
in libvirt is fixed. :)

 docs/hooks.html.in |  239 ++++++++++++++++++++++++++++++++++++++--------------
 1 files changed, 177 insertions(+), 62 deletions(-)

diff --git a/docs/hooks.html.in b/docs/hooks.html.in
index 2afdecf..411f2ec 100644
--- a/docs/hooks.html.in
+++ b/docs/hooks.html.in
@@ -2,69 +2,184 @@
 <html>
   <body>
     <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>
   </body>
 </html>
-- 
1.7.2


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