add a new tool for creating transient mount and automount units

This adds "systemd-mount" which is for transient mount and automount units what
"systemd-run" is for transient service, scope and timer units.

The tool allows establishing mounts and automounts during runtime. It is very
similar to the usual /bin/mount commands, but can pull in additional
dependenices on access (for example, it pulls in fsck automatically), an take
benefit of the automount logic.

This tool is particularly useful for mount removable file systems (such as USB
sticks), as the automount logic (together with automatic unmount-on-idle), as
well as automatic fsck on first access ensure that the removable file system
has a high chance to remain in a fully clean state even when it is unplugged
abruptly, and returns to a clean state on the next re-plug.

This is a follow-up for #2471, as it adds a simple client-side for the
transient automount logic added in that PR.

In later work it might make sense to invoke this tool automatically from udev
rules in order to implement a simpler and safer version of removable media
management á la udisks.

1 files changed, 295 insertions, 0 deletions

diff --git a/man/systemd-mount.xml b/man/systemd-mount.xml
new file mode 100644
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+<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<!--
+  This file is part of systemd.
+
+  Copyright 2016 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+-->
+
+<refentry id="systemd-mount"
+          xmlns:xi="http://www.w3.org/2001/XInclude">
+
+  <refentryinfo>
+    <title>systemd-mount</title>
+    <productname>systemd</productname>
+
+    <authorgroup>
+      <author>
+        <contrib>Developer</contrib>
+        <firstname>Lennart</firstname>
+        <surname>Poettering</surname>
+        <email>lennart@poettering.net</email>
+      </author>
+    </authorgroup>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>systemd-mount</refentrytitle>
+    <manvolnum>1</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>systemd-mount</refname>
+    <refpurpose>Establish a mount or auto-mount point transiently</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <cmdsynopsis>
+      <command>systemd-mount</command>
+      <arg choice="opt" rep="repeat"><replaceable>OPTIONS</replaceable></arg>
+      <arg choice="plain"><replaceable>WHAT</replaceable></arg>
+      <arg choice="opt"><replaceable>WHERE</replaceable></arg>
+    </cmdsynopsis>
+    <cmdsynopsis>
+      <command>systemd-mount</command>
+      <arg choice="opt" rep="repeat"><replaceable>OPTIONS</replaceable></arg>
+      <arg choice="plain"><option>--list</option></arg>
+    </cmdsynopsis>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para><command>systemd-mount</command> may be used to create and start a transient <filename>.mount</filename> or
+    <filename>.automount</filename> unit of the file system <replaceable>WHAT</replaceable> on the mount point
+    <replaceable>WHERE</replaceable>.</para>
+
+    <para>In many ways, <command>systemd-mount</command> is similar to the lower-level
+    <citerefentry><refentrytitle>mount</refentrytitle><manvolnum>8</manvolnum></citerefentry> command, however instead
+    of executing the mount operation directly and immediately, <command>systemd-mount</command> schedules it through
+    the service manager job queue, so that it may pull in further dependencies (such as parent mounts, or a file system
+    checker to execute a priori), and may make use of the auto-mounting logic.</para>
+
+    <para>The command takes either one or two arguments. If only one argument is specified it should refer to a block
+    device containing a file system (e.g. <literal>/dev/sdb1</literal>), which is then probed for a label and other
+    metadata, and is mounted to a directory whose name is generated from the label. In this mode the block device must
+    exist at the time of invocation of the command, so that it may be probed. If the device is found to be a removable
+    block device (e.g. a USB stick) an automount point instead of a regular mount point is created (i.e. the
+    <option>--automount=</option> option is implied, see below).</para>
+
+    <para>If two arguments are specified the first indicates the mount source (the <replaceable>WHAT</replaceable>) and
+    the second indicates the path to mount it on (the <replaceable>WHERE</replaceable>). In this mode no probing of the
+    source is attempted, and a backing device node doesn't have to exist yet. However, if this mode is combined with
+    <option>--discover</option>, device node probing for additional metadata is enabled, and – much like in the
+    single-argument case discussed above – the specified device has to exist at the time of invocation of the
+    command.</para>
+
+    <para>Use the <option>--list</option> command to show a terse table of all local, known block devices with file
+    systems that may be mounted with this command.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Options</title>
+
+    <para>The following options are understood:</para>
+
+    <variablelist>
+
+      <varlistentry>
+        <term><option>--no-block</option></term>
+
+        <listitem>
+          <para>Do not synchronously wait for the requested operation to finish. If this is not specified, the job will
+          be verified, enqueued and <command>systemd-mount</command> will wait until the mount or automount unit's
+          start-up is completed. By passing this argument, it is only verified and enqueued.</para>
+        </listitem>
+      </varlistentry>
+
+      <xi:include href="standard-options.xml" xpointer="no-pager"/>
+      <xi:include href="standard-options.xml" xpointer="no-ask-password"/>
+
+      <varlistentry>
+        <term><option>--quiet</option></term>
+        <term><option>-q</option></term>
+
+        <listitem><para>Suppresses additional informational output while running.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--discover</option></term>
+
+        <listitem><para>Enable probing of the mount source. This switch is implied if a single argument is specified on
+        the command line. If passed, additional metadata is read from the device to enhance the unit to create. For
+        example, a descriptive string for the transient units is generated from the file system label and device
+        model. Moreover if a removable block device (e.g. USB stick) is detected an automount unit instead of a regular
+        mount unit is created, with a short idle time-out, in order to ensure the file-system is placed in a clean
+        state quickly after each access.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--type=</option></term>
+        <term><option>-t</option></term>
+
+        <listitem><para>Specifies the file system type to mount (e.g. <literal>vfat</literal>, <literal>ext4</literal>,
+        …). If omitted (or set to <literal>auto</literal>) the file system is determined automatically.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--options=</option></term>
+        <term><option>-o</option></term>
+
+        <listitem><para>Additional mount options for the mount point.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--fsck=</option></term>
+
+        <listitem><para>Takes a boolean argument, defaults to on. Controls whether to run a file system check
+        immediately before the mount operation. In the automount case (see <option>--automount=</option> below) the
+        check will be run the moment the first access to the device is made, which might slightly delay the
+        access.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--description=</option></term>
+
+        <listitem><para>Provide a description for the mount or automount unit. See <varname>Description=</varname> in
+        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+        </para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--property=</option></term>
+        <term><option>-p</option></term>
+
+        <listitem><para>Sets a unit property for the mount unit that is created. This takes an assignment in the same
+        format as <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
+        <command>set-property</command> command.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--automount=</option></term>
+
+        <listitem><para>Takes a boolean argument. Controls whether to create an automount point or a regular mount
+        point. If true an automount point is created that is backed by the actual file system at the time of first
+        access. If false a plain mount point is created that is backed by the actual file system immediately. Automount
+        points have the benefit that the file system stays unmounted and hence in clean state until it is first
+        accessed. In automount mode the <option>--timeout-idle-sec=</option> switch (see below) may be used to ensure
+        the mount point is unmounted automatically after the last access and an idle period passed.</para>
+
+        <para>If this switch is not specified it defaults to false. If not specified and <option>--discover</option> is
+        used (or only a single argument passed, which implies <option>--discover</option>, see above), and the file
+        system block device is detected to be removable, it is set to true, in order to increase the chance that the
+        file system is in a fully clean state if the device is unplugged abruptly.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>-A</option></term>
+
+        <listitem><para>Equivalent to <option>--automount=yes</option>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--timeout-idle-sec=</option></term>
+
+        <listitem><para>Takes a time value that controls the idle timeout in automount mode. If set to
+        <literal>infinity</literal> (the default) no automatic unmounts are done. Otherwise the file system backing the
+        automount point is detached after the last access and the idle timeout passed. See
+        <citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry> for details on
+        the time syntax supported. This option has no effect if only a regular mount is established, and automounting
+        is not used.</para>
+
+        <para>Note that if <option>--discover</option> is used (or only a single argument passed, which implies
+        <option>--discover</option>, see above), and the file system block device is detected to be removable,
+        <option>--timeout-idle-sec=1s</option> is implied.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--automount-property=</option></term>
+
+        <listitem><para>Similar to <option>--property=</option>, but applies additional properties to the automount
+        unit created, instead of the mount unit.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--bind-device=</option></term>
+
+        <listitem><para>Takes a boolean argument, defaults to off. This option only has an effect in automount mode,
+        and controls whether the automount unit shall be bound to the backing device's lifetime. If enabled, the
+        automount point will be removed automatically when the backing device vanishes. If disabled the automount point
+        stays around, and subsequent accesses will block until backing device is replugged. This option has no effect
+        in case of non-device mounts, such as network or virtual file system mounts.</para>
+
+        <para>Note that if <option>--discover</option> is used (or only a single argument passed, which implies
+        <option>--discover</option>, see above), and the file system block device is detected to be removable, this
+        option is implied.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>--list</option></term>
+
+        <listitem><para>Instead of establishing a mount or automount point, print a terse list of block devices
+        containing file systems that may be mounted with <literal>systemd-mount</literal>, along with useful metadata
+        such as labels, etc.</para></listitem>
+      </varlistentry>
+
+      <xi:include href="user-system-options.xml" xpointer="user" />
+      <xi:include href="user-system-options.xml" xpointer="system" />
+      <xi:include href="user-system-options.xml" xpointer="host" />
+      <xi:include href="user-system-options.xml" xpointer="machine" />
+
+      <xi:include href="standard-options.xml" xpointer="help" />
+      <xi:include href="standard-options.xml" xpointer="version" />
+    </variablelist>
+
+  </refsect1>
+
+  <refsect1>
+    <title>Exit status</title>
+
+    <para>On success, 0 is returned, a non-zero failure
+    code otherwise.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>The udev Database</title>
+
+    <para>If <option>--discover</option> is used, <command>systemd-mount</command> honours a couple of additional udev
+    properties of block devices:</para>
+
+    <variablelist class='udev-directives'>
+      <varlistentry>
+        <term><varname>SYSTEMD_MOUNT_OPTIONS=</varname></term>
+
+        <listitem><para>The mount options to use, if <option>--options=</option> is not used.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>SYSTEMD_MOUNT_WHERE=</varname></term>
+
+        <listitem><para>The file system path to place the mount point at, instead of the automatically generated
+        one.</para></listitem>
+      </varlistentry>
+    </variablelist>
+  </refsect1>
+
+  <refsect1>
+    <title>See Also</title>
+    <para>
+      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>mount</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd-run</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+
+</refentry>