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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="introduction-cancellation"></a>Cancellation</h2></div></div></div>
<p>
If you have a multipart transaction that can be aborted in one phase but
not another then the AllowCancel signal can be sent.
This allows for example the yum download to be cancelled, but not the
install transaction.
By cancelling a job all subtransactions are killed.
</p>
<p>
By default actions cannot be cancelled unless enabled in the backend.
Use <code class="literal">AllowCancel(true)</code> to enable cancellation
and <code class="literal">AllowCancel(false)</code> to disable it.
This can be done for any job type.
</p>
<p>
For compiled backends that are threaded, the
<code class="literal">cancel()</code> method can be used to terminate
the thread.
</p>
<p>
For spawned backends, there are two staggered signals send to allow
locks to be released or for the backend to clean up after itself:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>Send the process <code class="literal">SIGQUIT</code>.</p></li>
<li class="listitem"><p>Wait 500ms</p></li>
<li class="listitem"><p>
If the process has not already quit, send the process
<code class="literal">SIGKILL</code> to terminate it.
</p></li>
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