Clarify the definition and restrictions on Essential

Add a brief rationale for Essential to the section on Essential, moving
it from an earlier footnote.  Say that maintainers should take care in
adding any functionality to essential packages because it's very
difficult to remove such functionality later, moving some additional
text from the earlier footnote.

Closes: #479080

diff --git a/policy.sgml b/policy.sgml
index c9bd84f23d9057d7825af62d195a88cf7866ff46..d0dc2dc82d5318aa49ab35e0ac53a27d5dad3711 100644 (file)
--- a/policy.sgml
+++ b/policy.sgml
@@ -985,29 +985,23 @@
          (see below), and should not do so unless they depend on a
          particular version of that package.<footnote>
             <p>
-              Essential is defined as the minimal set of functionality
-              that must be available and usable on the system even
-              when packages are in an unconfigured (but unpacked)
-              state.  This is needed to avoid unresolvable dependency
-              loops on upgrade.  If packages add unnecessary
-              dependencies on packages in this set, the chances that
-              there <strong>will</strong> be an unresolvable
-              dependency loop caused by forcing these Essential
-              packages to be configured first before they need to be
-              is greatly increased.  It also increases the chances
-              that frontends will be unable to
-              <strong>calculate</strong> an upgrade path, even if one
-              exists.
+             Essential is needed in part to avoid unresolvable dependency
+             loops on upgrade.  If packages add unnecessary dependencies
+             on packages in this set, the chances that there
+             <strong>will</strong> be an unresolvable dependency loop
+             caused by forcing these Essential packages to be configured
+             first before they need to be is greatly increased.  It also
+             increases the chances that frontends will be unable to
+             <strong>calculate</strong> an upgrade path, even if one
+             exists.
             </p>
             <p>
-              Also, it's pretty unlikely that functionality from
-              Essential shall ever be removed (which is one reason why
-              care must be taken before adding to the Essential
-              packages set), but <em>packages</em> have been removed
-              from the Essential set when the functionality moved to a
-              different package. So depending on these packages
-              <em>just in case</em> they stop being essential does way
-              more harm than good.
+             Also, functionality is rarely ever removed from the
+             Essential set, but <em>packages</em> have been removed from
+             the Essential set when the functionality moved to a
+             different package. So depending on these packages <em>just
+             in case</em> they stop being essential does way more harm
+             than good.
             </p>
           </footnote>
        </p>
@@ -1094,10 +1088,13 @@
        <heading>Essential packages</heading>
 
        <p>
-         Some packages are tagged <tt>essential</tt> for a system
-         using the <tt>Essential</tt> control file field.
-         The format of the <tt>Essential</tt> control field is
-         described in <ref id="f-Essential">.
+         Essential is defined as the minimal set of functionality that
+         must be available and usable on the system at all times, even
+         when packages are in an unconfigured (but unpacked) state.
+         Packages are tagged <tt>essential</tt> for a system using the
+         <tt>Essential</tt> control file field.  The format of the
+         <tt>Essential</tt> control field is described in <ref
+         id="f-Essential">.
        </p>
 
        <p>
@@ -1121,6 +1118,19 @@
             appropriate.
        </p>
 
+       <p>
+         Maintainers should take great care in adding any programs,
+         interfaces, or functionality to <tt>essential</tt> packages.
+         Packages may assume that functionality provided by
+         <tt>essential</tt> packages is always available without
+         declaring explicit dependencies, which means that removing
+         functionality from the Essential set is very difficult and is
+         almost never done.  Any capability added to an
+         <tt>essential</tt> package therefore creates an obligation to
+         support that capability as part of the Essential set in
+         perpetuity.
+       </p>
+
        <p>
          You must not tag any packages <tt>essential</tt> before
          this has been discussed on the <tt>debian-devel</tt>