Clarify documentation on PITR and warm standby on the fact that the standby

restore_command should report failure on non-existent .backup and .history
files. Tidy up some related text along the way.

Patch by Markus Bertheau, with some editing by Simon Riggs and myself.

doc/src/sgml/backup.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.115 2008/03/07 01:46:41 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.116 2008/03/28 15:00:28 heikki Exp $ -->
 
 <chapter id="backup">
  <title>Backup and Restore</title>
@@ -577,11 +577,10 @@ cp -i pg_xlog/00000001000000A900000065 /mnt/server/archivedir/00000001000000A900
    <para>
     It is important that the archive command return zero exit status if and
     only if it succeeded.  Upon getting a zero result,
-    <productname>PostgreSQL</> will assume that the WAL segment file has been
-    successfully archived, and will remove or recycle it.
-    However, a nonzero status tells
-    <productname>PostgreSQL</> that the file was not archived; it will try
-    again periodically until it succeeds.
+    <productname>PostgreSQL</> will assume that the file has been
+    successfully archived, and will remove or recycle it.  However, a nonzero
+    status tells <productname>PostgreSQL</> that the file was not archived;
+    it will try again periodically until it succeeds.
    </para>
 
    <para>
@@ -1001,11 +1000,13 @@ restore_command = 'cp /mnt/server/archivedir/%f %p'
 
    <para>
     It is important that the command return nonzero exit status on failure.
-    The command <emphasis>will</> be asked for log files that are not present
+    The command <emphasis>will</> be asked for files that are not present
     in the archive; it must return nonzero when so asked.  This is not an
-    error condition.  Be aware also that the base name of the <literal>%p</>
-    path will be different from <literal>%f</>; do not expect them to be
-    interchangeable.
+    error condition.  Not all of the requested files will be WAL segment
+    files; you should also expect requests for files with a suffix of
+    <literal>.backup</> or <literal>.history</>. Also be aware that
+    the base name of the <literal>%p</> path will be different from
+    <literal>%f</>; do not expect them to be interchangeable.
    </para>
 
    <para>
@@ -1576,19 +1577,21 @@ archive_command = 'local_backup_script.sh'
 
    <para>
     The magic that makes the two loosely coupled servers work together is
-    simply a <varname>restore_command</> used on the standby that waits
-    for the next WAL file to become available from the primary. The
-    <varname>restore_command</> is specified in the
+    simply a <varname>restore_command</> used on the standby that,
+    when asked for the next WAL file, waits for it to become available from
+    the primary. The <varname>restore_command</> is specified in the
     <filename>recovery.conf</> file on the standby server. Normal recovery
     processing would request a file from the WAL archive, reporting failure
     if the file was unavailable.  For standby processing it is normal for
-    the next file to be unavailable, so we must be patient and wait for
-    it to appear. A waiting <varname>restore_command</> can be written as
-    a custom script that loops after polling for the existence of the next
-    WAL file. There must also be some way to trigger failover, which should
-    interrupt the <varname>restore_command</>, break the loop and return
-    a file-not-found error to the standby server. This ends recovery and
-    the standby will then come up as a normal server.
+    the next WAL file to be unavailable, so we must be patient and wait for
+    it to appear. For files ending in <literal>.backup</> or
+    <literal>.history</> there is no need to wait, and a non-zero return
+    code must be returned. A waiting <varname>restore_command</> can be
+    written as a custom script that loops after polling for the existence of
+    the next WAL file. There must also be some way to trigger failover, which
+    should interrupt the <varname>restore_command</>, break the loop and
+    return a file-not-found error to the standby server. This ends recovery
+    and the standby will then come up as a normal server.
    </para>
 
    <para>
@@ -1608,9 +1611,10 @@ if (!triggered)
 
    <para>
     A working example of a waiting <varname>restore_command</> is provided
-    as a <filename>contrib</> module named <application>pg_standby</>. This
-    example can be extended as needed to support specific configurations or
-    environments.
+    as a <filename>contrib</> module named <application>pg_standby</>. It
+    should be used as a reference on how to correctly implement the logic
+    described above. It can also be extended as needed to support specific
+    configurations or environments.
    </para>
 
    <para>