provide a public interface for acquiring changelogs

Provided is a specialized acquire item which given a version can figure
out the correct URI to try by itself and if not provides an error
message alongside with static methods to get just the URI it would try
to download if it should just be displayed or similar such.

The URI is constructed as follows:
Release files can provide an URI template in the "Changelogs" field,
otherwise we lookup a configuration item based on the "Label" or
"Origin" of the Release file to get a (hopefully known) default value
for now. This template should contain the string CHANGEPATH which is
replaced with the information about the version we want the changelog
for (e.g.  main/a/apt/apt_1.1). This middleway was choosen as this path
part was consistent over the three known implementations (+1 defunct),
while the rest of the URI varies widely between them.

The benefit of this construct is that it is now easy to get changelogs
for Debian packages on Ubuntu and vice versa – even at the moment where
the Changelogs field is present nowhere. Strictly better than what
apt-get had before as it would even fail to get changelogs from
security… Now it will notice that security identifies as Origin: Debian
and pick this setting (assuming again that no Changelogs field exists).
If on the other hand security would ship its changelogs in a different
location we could set it via the Label option overruling Origin.

Closes: 687147, 739854, 784027, 787190

3 files changed, 36 insertions, 18 deletions

diff --git a/doc/apt-get.8.xml b/doc/apt-get.8.xml
index da077afa7..5b6788ed4 100644
--- a/doc/apt-get.8.xml
+++ b/doc/apt-get.8.xml
@@ -230,16 +230,12 @@
      </varlistentry>
 
      <varlistentry><term><option>changelog</option></term>
-       <listitem><para><literal>changelog</literal> downloads a package changelog and displays
-           it through <command>sensible-pager</command>. The server name and base
-           directory is defined in the <literal>APT::Changelogs::Server</literal>
-	   variable (e.g. <ulink url="http://packages.debian.org/changelogs">packages.debian.org/changelogs</ulink> for
-	   Debian or <ulink url="http://changelogs.ubuntu.com/changelogs">changelogs.ubuntu.com/changelogs</ulink> for
-           Ubuntu).
-           By default it displays the changelog for the version that is
-           installed.  However, you can specify the same options as for
-           the <option>install</option> command.
-         </para>
+	<listitem><para><literal>changelog</literal> tries to download the
+	      changelog of a package and displays it through
+	      <command>sensible-pager</command>.  By default it
+	      displays the changelog for the version that is installed.
+	      However, you can specify the same options as for the
+	      <option>install</option> command.</para>
        </listitem>
      </varlistentry>
 
diff --git a/doc/apt.conf.5.xml b/doc/apt.conf.5.xml
index efe986ea8..7d5f7e9b3 100644
--- a/doc/apt.conf.5.xml
+++ b/doc/apt.conf.5.xml
@@ -618,6 +618,33 @@ DPkg::Pre-Install-Pkgs {"/usr/sbin/dpkg-preconfigure --apt";};
          </para></listitem>
      </varlistentry>
 
+     <varlistentry><term><option>Changelogs::URI</option> scope</term>
+	<listitem><para>
+	  Acquiring changelogs can only be done if an URI is known from where to get them.
+	  Preferable the Release file indicates this in a 'Changelogs' field. If this isn't
+	  available the Label/Origin field of the Release file is used to check if a
+	  <literal>Acquire::Changelogs::URI::Label::<replaceable>LABEL</replaceable></literal> or
+	  <literal>Acquire::Changelogs::URI::Origin::<replaceable>ORIGIN</replaceable></literal> option
+	  exists and if so this value is taken. The value in the Release file can be overridden
+	  with <literal>Acquire::Changelogs::URI::Override::Label::<replaceable>LABEL</replaceable></literal>
+	  or <literal>Acquire::Changelogs::URI::Override::Origin::<replaceable>ORIGIN</replaceable></literal>.
+
+	  The value should be a normal URI to a text file, expect that package specific data is
+	  replaced with the placeholder <literal>CHANGEPATH</literal>. The
+	  value for it is: 1. if the package is from a component (e.g. <literal>main</literal>)
+	  this is the first part otherwise it is omitted, 2. the first letter of source package name,
+	  expect if the source package name starts with '<literal>lib</literal>' in which case it will
+	  be the first four letters. 3. The complete source package name. 4. the complete name again and
+	  5. the source version.
+	  The first (if present), second, third and fourth part are separated by a slash ('<literal>/</literal>')
+	  and between the fourth and fifth part is an underscore ('<literal>_</literal>').
+
+	  The special value '<literal>no</literal>' is available for this option indicating that
+	  this source can't be used to acquire changelog files from. Another source will be tried
+	  if available in this case.
+	</para></listitem>
+    </varlistentry>
+
    </variablelist>
  </refsect1>
 
diff --git a/doc/examples/configure-index b/doc/examples/configure-index
index ef1ae056d..1339335fa 100644
--- a/doc/examples/configure-index
+++ b/doc/examples/configure-index
@@ -117,14 +117,6 @@ APT
   // does a ExecFork)
   Keep-Fds {};
 
-  Changelogs
-  {
-    // server the provides the changelogs, the code will assume
-    // the changlogs are in the pool/ under a srcpkg_ver directory 
-    // with the name "changelog"
-    Server "http://packages.debian.org/changelogs";
-  }:
-
   // control parameters for cron jobs by /etc/cron.daily/apt
   Periodic
   {
@@ -305,6 +297,9 @@ Acquire
      "none";
      "fr";
   };
+
+  // Location of the changelogs with the placeholder CHANGEPATH (e.g. "main/a/apt/apt_1.1")
+  Changelogs::URI::Origin::Debian "http://metadata.ftp-master.debian.org/changelogs/CHANGEPATH_changelog";
 };
 
 // Directory layout