Overhaul the documentation of shared library SONAMEs

Provide much more background information about shared library
versioning and how the different symlinks are used.  Clearly
require the SONAME and corresponding binary package name to be
changed if the library ABI changes.  Add more cautions to the
practice of bundling multiple shared libraries together in one
package.  Clarify that the shared library section does not
apply to private libraries and loadable modules.

diff --git a/policy.sgml b/policy.sgml
index 7736ddb8e0e9f1ec76933d6afae1089d930d6b1f..15b34a9cf950c75bf99bd20fa80a5391fdd49d71 100644 (file)
--- a/policy.sgml
+++ b/policy.sgml
@@ -5106,55 +5106,138 @@ Replaces: mail-transport-agent
       </p>
 
       <p>
-       Packages involving shared libraries should be split up into
-       several binary packages. This section mostly deals with how
-       this separation is to be accomplished; rules for files within
-       the shared library packages are in <ref id="libraries"> instead.
+       This section deals only with public shared libraries: shared
+       libraries that are placed in directories searched by the dynamic
+       linker by default or which are intended to be linked against
+       normally and possibly used by other, independent packages.  Shared
+       libraries that are internal to a particular package or that are
+       only loaded as dynamic modules are not covered by this section and
+       are not subject to its requirements.
       </p>
 
-      <sect id="sharedlibs-runtime">
-       <heading>Run-time shared libraries</heading>
+      <p>
+       A shared library must be uniquely identified by an <tt>SONAME</tt>
+       attribute stored in its dynamic section.  When a binary is linked
+       against a shared library, the <tt>SONAME</tt> of the shared
+       library is recorded in the binary's <tt>NEEDED</tt> section so
+       that the dynamic linker knows that library must be loaded at
+       runtime.  The full name of the shared library (which usually
+       contains additional version information not needed in
+       the <tt>SONAME</tt>) is therefore not referenced directly.
+       Instead, the shared library is loaded by its <tt>SONAME</tt>,
+       which exists on the file system as a symlink pointing to the full
+       name of the shared library.<footnote>
+         Some unusual libraries have an <tt>SONAME</tt> which matches the
+         full library name, but normally there is a minor revision that
+         changes even though the ABI has not changed in a
+         backward-incompatible way.  The <tt>SONAME</tt> only changes
+         when binaries linked with the earlier version of the shared
+         library may no longer work.  See <ref id="sharedlibs-runtime">
+         for more information.
+       </footnote>
+       This symlink is updated and its location cached
+       by <prgn>ldconfig</prgn>, but must also be created by the
+       package.  <ref id="sharedlibs-runtime"> describes how to do this.
+      </p>
 
       <p>
-       The run-time shared library needs to be placed in a package
-        whose name changes whenever the shared object version
-        changes.<footnote>
-            <p>
-              Since it is common place to install several versions of a
-              package that just provides shared libraries, it is a
-              good idea that the library package should not
-              contain any extraneous non-versioned files, unless they
-              happen to be in versioned directories.</p>
-          </footnote>
-          The most common mechanism is to place it in a package
-        called
-        <package><var>libraryname</var><var>soversion</var></package>,
-        where <file><var>soversion</var></file> is the version number
-        in the soname of the shared library<footnote>
-             The soname is the shared object name: it's the thing
-             that has to match exactly between building an executable
-             and running it for the dynamic linker to be able run the
-             program.  For example, if the soname of the library is
-             <file>libfoo.so.6</file>, the library package would be
-             called <file>libfoo6</file>.
-         </footnote>.
-       Alternatively, if it would be confusing to directly append
-       <var>soversion</var> to <var>libraryname</var> (e.g. because
-       <var>libraryname</var> itself ends in a number), you may use
-       <package><var>libraryname</var>-<var>soversion</var></package> and
-       <package><var>libraryname</var>-<var>soversion</var>-dev</package>
-       instead.
+       When linking a binary or another shared library against a shared
+       library, the <tt>SONAME</tt> for that shared library is not yet
+       known.  Instead, the shared library is found by library name
+       with <tt>.so</tt> appended.  This file exists on the file system
+       as a symlink pointing to the shared library.<footnote>
+         Some shared libraries have no version information in
+         their <tt>SONAME</tt> and therefore need no symlink since the
+         file found when linking is the same as the library name.  This
+         is highly unusual, however, and means the shared library cannot
+         be versioned.  It is used only for very special situations where
+         the shared library provides a tiny set of symbols and must not
+         be versioned for some reason.
+       </footnote>
+      </p>
+
+      <p>
+       Shared libraries are normally split into several binary packages.
+       The <tt>SONAME</tt> symlink is installed by the runtime shared
+       library package, and the bare <tt>.so</tt> symlink is installed in
+       the development package since it's only used when linking binaries
+       or shared libraries.  However, there are some exceptions for
+       unusual shared libraries or for shared libraries that are also
+       loaded as dynamic modules by other programs.
       </p>
 
       <p>
-       If you have several shared libraries built from the same
-       source tree you may lump them all together into a single
-       shared library package, provided that you change all of
-       their sonames at once (so that you don't get filename
-       clashes if you try to install different versions of the
-       combined shared libraries package).
+       This section is primarily concerned with how the separation of
+       shared libraries into multiple packages should be done and how
+       dependencies on and between shared library binary packages are
+       managed in Debian.  <ref id="libraries"> should be read in
+       conjunction with this section and contains additional rules for
+       the files contained in the shared library packages.
       </p>
 
+      <sect id="sharedlibs-runtime">
+       <heading>Run-time shared libraries</heading>
+
+       <p>
+         The run-time shared library must be placed in a package
+         whose name changes whenever the <tt>SONAME</tt> of the shared
+         library changes.  This allows several versions of the shared
+         library to be installed at the same time, allowing installation
+         of the new version of the shared library without immediately
+         breaking binaries that depend on the old version.  Normally, the
+         run-time shared library and its <tt>SONAME</tt> symlink should
+         be placed in a package named
+         <package><var>libraryname</var><var>soversion</var></package>,
+         where <var>soversion</var> is the version number in
+         the <tt>SONAME</tt> of the shared library.
+         See <ref id="shlibs"> for detailed information on how to
+         determine this version.  Alternatively, if it would be confusing
+         to directly append <var>soversion</var>
+         to <var>libraryname</var> (if, for example, <var>libraryname</var>
+         itself ends in a number), you should use
+         <package><var>libraryname</var>-<var>soversion</var></package>
+         instead.
+       </p>
+
+       <p>
+         If you have several shared libraries built from the same source
+         tree, you may lump them all together into a single shared
+         library package provided that all of their <tt>SONAME</tt>s will
+         always change together.  Be aware that this is not normally the
+         case, and if the <tt>SONAME</tt>s do not change together,
+         upgrading such a merged shared library package will be
+         unnecessarily difficult because of file conflicts with the old
+         version of the package.  When in doubt, always split shared
+         library packages so that each binary package installs a single
+         shared library.
+       </p>
+
+       <p>
+         Every time the shared library ABI changes in a way that may
+         break binaries linked against older versions of the shared
+         library, the <tt>SONAME</tt> of the library and the
+         corresponding name for the binary package containing the runtime
+         shared library should change.  Normally, this means
+         the <tt>SONAME</tt> should change any time an interface is
+         removed from the shared library or the signature of an interface
+         (the number of parameters or the types of parameters that it
+         takes, for example) is changed.  This practice is vital to
+         allowing clean upgrades from older versions of the package and
+         clean transitions between the old ABI and new ABI without having
+         to upgrade every affected package simultaneously.
+       </p>
+
+       <p>
+         The <tt>SONAME</tt> and binary package name need not, and indeed
+         normally should not, change if new interfaces are added but none
+         are removed or changed, since this will not break binaries
+         linked against the old shared library.  Correct versioning of
+         dependencies on the newer shared library by binaries that use
+         the new interfaces is handled via
+         the <qref id="sharedlibs-shlibdeps"><tt>shilbs</tt>
+         system</qref>.
+       </p>
+
       <p>
        The package should install the shared libraries under
        their normal names.  For example, the <package>libgdbm3</package>
@@ -5174,10 +5257,11 @@ Replaces: mail-transport-agent
       </p>
 
       <p>
-       The run-time library package should include the symbolic link that
-       <prgn>ldconfig</prgn> would create for the shared libraries.
-       For example, the <package>libgdbm3</package> package should include
-       a symbolic link from <file>/usr/lib/libgdbm.so.3</file> to
+       The run-time library package should include the symbolic link for
+       the <tt>SONAME</tt> that <prgn>ldconfig</prgn> would create for
+       the shared libraries.  For example,
+       the <package>libgdbm3</package> package should include a symbolic
+       link from <file>/usr/lib/libgdbm.so.3</file> to
        <file>libgdbm.so.3.0.0</file>.  This is needed so that the dynamic
        linker (for example <prgn>ld.so</prgn> or
        <prgn>ld-linux.so.*</prgn>) can find the library between the