From 3cabe45a819f8a2a282d9d57e45f259c84e97c3f Mon Sep 17 00:00:00 2001
From: Fujii Masao <fujii@postgresql.org>
Date: Tue, 26 Jul 2022 16:02:43 +0900
Subject: [PATCH] doc: Add note about re-archiving of same WAL files in docs.

The server may attempt to re-archive a WAL file that was previously archived.
This commit adds the note about how an archive library should handle such
a re-archiving.

Author: Nathan Bossart
Reviewed-by: David Steele, Kyotaro Horiguchi
Discussion: https://postgr.es/m/CA+TgmoaeTe4oUT39A4nt=8LD6UJ5u0vcmGc5+Aksn-4oKRb8-w@mail.gmail.com
---
 doc/src/sgml/backup.sgml | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/doc/src/sgml/backup.sgml b/doc/src/sgml/backup.sgml
index 73a774d3d7f..e432bb015a6 100644
--- a/doc/src/sgml/backup.sgml
+++ b/doc/src/sgml/backup.sgml
@@ -681,14 +681,29 @@ test ! -f /mnt/server/archivedir/00000001000000A900000065 &amp;&amp; cp pg_wal/0
     any pre-existing archive file.  This is an important safety feature to
     preserve the integrity of your archive in case of administrator error
     (such as sending the output of two different servers to the same archive
-    directory).
+    directory).  It is advisable to test your proposed archive library to ensure
+    that it does not overwrite an existing file.
    </para>
 
    <para>
-    It is advisable to test your proposed archive library to ensure that it
-    indeed does not overwrite an existing file, <emphasis>and that it returns
-    <literal>false</literal> in this case</emphasis>.
-    The example command above for Unix ensures this by including a separate
+    In rare cases, <productname>PostgreSQL</productname> may attempt to
+    re-archive a WAL file that was previously archived.  For example, if the
+    system crashes before the server makes a durable record of archival success,
+    the server will attempt to archive the file again after restarting (provided
+    archiving is still enabled).  When an archive library encounters a
+    pre-existing file, it may return <literal>true</literal> if the WAL file has
+    identical contents to the pre-existing archive and the pre-existing archive
+    is fully persisted to storage.  Alternatively, the archive library may
+    return <literal>false</literal> anytime a pre-existing file is encountered,
+    but this will require manual action by an administrator to resolve.  If a
+    pre-existing file contains different contents than the WAL file being
+    archived, the archive library <emphasis>must</emphasis> return
+    <literal>false</literal>.
+   </para>
+
+   <para>
+    The example command above for Unix avoids overwriting a pre-existing archive
+    by including a separate
     <command>test</command> step.  On some Unix platforms, <command>cp</command> has
     switches such as <option>-i</option> that can be used to do the same thing
     less verbosely, but you should not rely on these without verifying that