Add support for Phased-Update-Percentage

This adds support for Phased-Update-Percentage by pinning
upgrades that are not to be installed down to 1.

The output of policy has been changed to add the level of
phasing, and documentation has been improved to document
how phased updates work.

The patch detects if it is running in a chroot, and if so, always
includes phased updates, restoring classic apt behavior to avoid
behavioral changes on buildd chroots.

Various options are added to control this all:

* APT::Get::{Always,Never}-Include-Phased-Updates and their legacy
  update-manager equivalents to always or never include phased updates
* APT::Machine-ID can be set to a UUID string to have all machines in a
  fleet phase the same
* Dir::Etc::Machine-ID is weird in that it's default is sort of like
  ../machine-id, but not really, as ../machine-id would look up
  $PWD/../machine-id and not relative to Dir::Etc; but it allows you to
  override the path to machine-id (as opposed to the value)
* Dir::Bin::ischroot is the path to the ischroot(1) binary which is used
  to detect whether we are running in a chroot.

1 files changed, 18 insertions, 1 deletions

diff --git a/doc/apt_preferences.5.xml b/doc/apt_preferences.5.xml
index 4ef5282e7..23f2d2d6b 100644
--- a/doc/apt_preferences.5.xml
+++ b/doc/apt_preferences.5.xml
@@ -102,7 +102,8 @@ algorithm to set the priorities of the versions of a package.  Assign:
 <term>priority 1</term>
 <listitem><simpara>to the versions coming from archives which in their <filename>Release</filename>
 files are marked as "NotAutomatic: yes" but <emphasis>not</emphasis> as "ButAutomaticUpgrades: yes"
-like the Debian <literal>experimental</literal> archive.</simpara></listitem>
+like the Debian <literal>experimental</literal> archive.</simpara></listitem>,
+as well as versions that are not phased on this systems.
 </varlistentry>
 
 <varlistentry>
@@ -175,6 +176,22 @@ because at least <emphasis>one</emphasis> of the available versions has a higher
 priority than the installed version.</para>
 </refsect2>
 
+<refsect2><title>Phased Updates</title>
+<para>APT understands a field called <literal>Phased-Update-Percentage</literal>
+which can be used to control the rollout of a new version. It is an integer between
+0 and 100.</para>
+
+<para>A system's eligibility to a phased update is determined by seeding
+random number generator with the package source name, the version number,
+and /etc/machine-id, and then calculating an integer in the range [0, 100].
+If this integer is larger than the <literal>Phased-Update-Percentage</literal>,
+the version is pinned to 1, and thus held back. Otherwise, normal policy rules apply.
+</para>
+
+<para>In case you have multiple systems that you want to receive the same set of updates, you can set
+<code>APT::Machine-ID</code> to a UUID such that they all phase the same, or set <code>APT::Get::Never-Include-Phased-Updates</code>
+or <code>APT::Get::Always-Include-Phased-Updates</code> to true such that APT will never/always consider phased updates.</para>
+</refsect2>
 <refsect2><title>The Effect of APT Preferences</title>
 
 <para>The APT preferences file allows the system administrator to control the