Add to glossary and clarify garbage collection

While trying to understand garbage collection it was not immediately
clear that only the runtime dependency closure of output paths
would be kept (instead of the build-time dependency closure).
This commit attempts to clarify this by expanding some of the
glossary definitions and extending the Garbage Collection
section.

1 files changed, 34 insertions, 15 deletions

diff --git a/doc/manual/glossary/glossary.xml b/doc/manual/glossary/glossary.xml
index e0636044cc25..a13630d2a4e9 100644
--- a/doc/manual/glossary/glossary.xml
+++ b/doc/manual/glossary/glossary.xml
@@ -85,29 +85,48 @@
 
 <glossentry xml:id="gloss-reference"><glossterm>reference</glossterm>
 
-  <glossdef><para>A store path <varname>P</varname> is said to have a
-  reference to a store path <varname>Q</varname> if the store object
-  at <varname>P</varname> contains the path <varname>Q</varname>
-  somewhere.  This implies than an execution involving
-  <varname>P</varname> potentially needs <varname>Q</varname> to be
-  present.  The <emphasis>references</emphasis> of a store path are
-  the set of store paths to which it has a reference.</para></glossdef>
+  <glossdef>
+    <para>A store path <varname>P</varname> is said to have a
+    reference to a store path <varname>Q</varname> if the store object
+    at <varname>P</varname> contains the path <varname>Q</varname>
+    somewhere. The <emphasis>references</emphasis> of a store path are
+    the set of store paths to which it has a reference.
+    </para>
+    <para>A derivation can reference other derivations and sources
+    (but not output paths), whereas an output path only references other
+    output paths.
+    </para>
+  </glossdef>
 
 </glossentry>
 
+<glossentry xml:id="gloss-reachable"><glossterm>reachable</glossterm>
+
+  <glossdef><para>A store path <varname>Q</varname> is reachable from
+  another store path <varname>P</varname> if <varname>Q</varname> is in the
+  <link linked="gloss-closure">closure</link> of the
+  <link linkend="gloss-reference">references</link> relation.
+  </para></glossdef>
+</glossentry>
 
 <glossentry xml:id="gloss-closure"><glossterm>closure</glossterm>
 
   <glossdef><para>The closure of a store path is the set of store
   paths that are directly or indirectly “reachable” from that store
   path; that is, it’s the closure of the path under the <link
-  linkend="gloss-reference">references</link> relation.  For instance,
-  if the store object at path <varname>P</varname> contains a
-  reference to path <varname>Q</varname>, then <varname>Q</varname> is
-  in the closure of <varname>P</varname>.  For correct deployment it
-  is necessary to deploy whole closures, since otherwise at runtime
-  files could be missing.  The command <command>nix-store
-  -qR</command> prints out closures of store paths.</para></glossdef>
+  linkend="gloss-reference">references</link> relation. For a package, the
+  closure of its derivation is equivalent to the build-time
+  dependencies, while the closure of its output path is equivalent to its
+  runtime dependencies. For correct deployment it is necessary to deploy whole
+  closures, since otherwise at runtime files could be missing. The command
+  <command>nix-store -qR</command> prints out closures of store paths.
+  </para>
+  <para>As an example, if the store object at path <varname>P</varname> contains
+  a reference to path <varname>Q</varname>, then <varname>Q</varname> is
+  in the closure of <varname>P</varname>. Further, if <varname>Q</varname>
+  references <varname>R</varname> then <varname>R</varname> is also in
+  the closure of <varname>P</varname>.
+  </para></glossdef>
 
 </glossentry>
 
@@ -147,7 +166,7 @@
   linkend="sec-profiles" />.</para>
 
   </glossdef>
-  
+
 </glossentry>