admin: Add an `unlock` command, and libostree API

I'm trying to improve the developer experience on OSTree-managed
systems, and I had an epiphany the other day - there's no reason we
have to be absolutely against mutating the current rootfs live.  The
key should be making it easy to rollback/reset to a known good state.

I see this command as useful for two related but distinct workflows:

 - `ostree admin unlock` will assume you're doing "development".  The
   semantics hare are that we mount an overlayfs on `/usr`, but the
   overlay data is in `/var/tmp`, and is thus discarded on reboot.
 - `ostree admin unlock --hotfix` first clones your current deployment,
   then creates an overlayfs over `/usr` persistent
   to this deployment.  Persistent in that now the initramfs switchroot
   tool knows how to mount it as well.  In this model, if you want
   to discard the hotfix, at the moment you roll back/reboot into
   the clone.

Note originally, I tried using `rofiles-fuse` over `/usr` for this,
but then everything immediately explodes because the default (at least
CentOS 7) SELinux policy denies tons of things (including `sshd_t`
access to `fusefs_t`).  Sigh.

So the switch to `overlayfs` came after experimentation.  It still
seems to have some issues...specifically `unix_chkpwd` is broken,
possibly because it's setuid?  Basically I can't ssh in anymore.

But I *can* `rpm -Uvh strace.rpm` which is handy.

NOTE: I haven't tested the hotfix path fully yet, specifically
the initramfs bits.

1 files changed, 88 insertions, 0 deletions

diff --git a/man/ostree-admin-unlock.xml b/man/ostree-admin-unlock.xml
new file mode 100644
index 00000000..ca02bbde
--- /dev/null
+++ b/man/ostree-admin-unlock.xml
@@ -0,0 +1,88 @@
+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+    "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<!--
+Copyright 2016 Colin Walters <walters@verbum.org>
+
+This library 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 of the License, or (at your option) any later version.
+
+This library 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 this library; if not, write to the
+Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+Boston, MA 02111-1307, USA.
+-->
+
+<refentry id="ostree">
+
+    <refentryinfo>
+        <title>ostree admin unlock</title>
+        <productname>OSTree</productname>
+
+        <authorgroup>
+            <author>
+                <contrib>Developer</contrib>
+                <firstname>Colin</firstname>
+                <surname>Walters</surname>
+                <email>walters@verbum.org</email>
+            </author>
+        </authorgroup>
+    </refentryinfo>
+
+    <refmeta>
+        <refentrytitle>ostree admin unlock</refentrytitle>
+        <manvolnum>1</manvolnum>
+    </refmeta>
+
+    <refnamediv>
+        <refname>ostree-admin-unlock</refname>
+        <refpurpose>Prepare the current deployment for hotfix or development</refpurpose>
+    </refnamediv>
+
+    <refsynopsisdiv>
+            <cmdsynopsis>
+                <command>ostree admin unlock</command> <arg choice="opt" rep="repeat">OPTIONS</arg>
+            </cmdsynopsis>
+    </refsynopsisdiv>
+
+    <refsect1>
+        <title>Description</title>
+
+        <para>
+	  Remove the read-only bind mount on <literal>/usr</literal>
+	  and replace it with a writable overlay filesystem.  This
+	  default invocation of "unlock" is intended for
+	  development/testing purposes.  All changes in the overlay
+	  are lost on reboot.  However, this command also supports
+	  "hotfixes", see below.
+        </para>
+    </refsect1>
+
+    <refsect1>
+        <title>Options</title>
+
+        <variablelist>
+            <varlistentry>
+                <term><option>--hotfix</option></term>
+
+                <listitem><para>If this option is provided, the
+                current deployment will be cloned as a rollback
+                target.  This option is intended for things like
+                emergency security updates to userspace components
+                such as <literal>sshd</literal>.  The semantics here
+		differ from the default "development" unlock mode
+		in that reboots will retain any changes (which is what
+		you likely want for security hotfixes).
+                </para></listitem>
+            </varlistentry>
+        </variablelist>
+    </refsect1>
+</refentry>