Rework the CompressionTypes system by adding an Order subgroup to

simplify customisation of the order and improve the documentation
about this setting group.

2 files changed, 32 insertions, 9 deletions

diff --git a/doc/apt.conf.5.xml b/doc/apt.conf.5.xml
index e0ce0db40..92ac5ec04 100644
--- a/doc/apt.conf.5.xml
+++ b/doc/apt.conf.5.xml
@@ -90,7 +90,7 @@ DPkg::Pre-Install-Pkgs {"/usr/sbin/dpkg-preconfigure --apt";};
    <literal>#include</literal> will include the given file, unless the filename
    ends in a slash, then the whole directory is included.  
    <literal>#clear</literal> is used to erase a part of the configuration tree. The
-   specified element and all its descendents are erased.</para>
+   specified element and all its descendants are erased.</para>
 
    <para>All of the APT tools take a -o option which allows an arbitrary configuration 
    directive to be specified on the command line. The syntax is a full option
@@ -312,16 +312,30 @@ DPkg::Pre-Install-Pkgs {"/usr/sbin/dpkg-preconfigure --apt";};
      <varlistentry><term>CompressionTypes</term>
      <listitem><para>List of compression types which are understood by the acquire methods.
      Files like <filename>Packages</filename> can be available in various compression formats.
-     This list defines in which order the acquire methods will try to download these files.
-     Per default <command>bzip2</command> compressed files will be prefered over
-     <command>lzma</command>, <command>gzip</command> and uncompressed files. The syntax for
-     the configuration fileentry is
+     Per default the acquire methods can decompress <command>bzip2</command>, <command>lzma</command>
+     and <command>gzip</command> compressed files, with this setting more formats can be added
+     on the fly or the used method can be changed. The syntax for this is:
      <synopsis>Acquire::CompressionTypes::<replaceable>FileExtension</replaceable> "<replaceable>Methodname</replaceable>";</synopsis>
-     e.g. <synopsis>Acquire::CompressionTypes::bz2 "bzip2";</synopsis>
-     Note that at runtime the <literal>Dir::Bin::<replaceable>Methodname</replaceable></literal> will
+     </para><para>Also the <literal>Order</literal> subgroup can be used to define in which order
+     the acquire system will try to download the compressed files. The acquire system will try the first
+     and proceed with the next compression type in this list on error, so to prefer one over the other type
+     simple add the preferred type at first - not already added default types will be added at run time
+     to the end of the list, so e.g. <synopsis>Acquire::CompressionTypes::Order:: "gz";</synopsis> can
+     be used to prefer <command>gzip</command> compressed files over <command>bzip2</command> and <command>lzma</command>.
+     If <command>lzma</command> should be preferred over <command>gzip</command> and <command>bzip2</command> the
+     configure setting should look like this <synopsis>Acquire::CompressionTypes::Order { "lzma"; "gz"; };</synopsis>
+     It is not needed to add <literal>bz2</literal> explicit to the list as it will be added automatic.</para>
+     <para>Note that at run time the <literal>Dir::Bin::<replaceable>Methodname</replaceable></literal> will
      be checked: If this setting exists the method will only be used if this file exists, e.g. for
-     the bzip2 method above (the inbuilt) setting is <literallayout>Dir::Bin::bzip2 "/bin/bzip2";</literallayout>
-     </para></listitem>
+     the bzip2 method (the inbuilt) setting is <literallayout>Dir::Bin::bzip2 "/bin/bzip2";</literallayout>
+     Note also that list entries specified on the commandline will be added at the end of the list
+     specified in the configuration files, but before the default entries. To prefer a type in this case
+     over the ones specified in in the configuration files you can set the option direct - not in list style.
+     This will not override the defined list, it will only prefix the list with this type.</para>
+     <para>While it is possible to add an empty compression type to the order list, but APT in its current
+     version doesn't understand it correctly and will display many warnings about not downloaded files -
+     these warnings are most of the time false negatives. Future versions will maybe include a way to
+     really prefer uncompressed files to support the usage of local mirrors.</para></listitem>
      </varlistentry>
    </variablelist>
   </para>
diff --git a/doc/examples/configure-index b/doc/examples/configure-index
index 5dc7b5246..8025fa4b4 100644
--- a/doc/examples/configure-index
+++ b/doc/examples/configure-index
@@ -246,6 +246,15 @@ Acquire
   {
    Options {"--ignore-time-conflict";}	// not very useful on a normal system
   };
+
+  CompressionTypes
+  {
+    bz2 "bzip2";
+    lzma "lzma";
+    gz "gzip";
+
+    Order { "gz"; "lzma"; "bz2"; };
+  };
 };
 
 // Directory layout