Document a few caveats in synchronous logical replication.

In a synchronous logical setup, locking [user] catalog tables can cause
deadlock. This is because logical decoding of transactions can lock
catalog tables to access them so exclusively locking those in transactions
can lead to deadlock. To avoid this users must refrain from having
exclusive locks on catalog tables.

Author: Takamichi Osumi
Reviewed-by: Vignesh C, Amit Kapila
Backpatch-through: 9.6
Discussion: https://www.postgresql.org/message-id/20210222222847.tpnb6eg3yiykzpky%40alap3.anarazel.de

doc/src/sgml/logicaldecoding.sgml

@@ -1066,28 +1066,83 @@ OutputPluginWrite(ctx, true);
 
   <sect1 id="logicaldecoding-synchronous">
    <title>Synchronous Replication Support for Logical Decoding</title>
+   <sect2>
+    <title>Overview</title>
 
-   <para>
-    Logical decoding can be used to build
-    <link linkend="synchronous-replication">synchronous
-    replication</link> solutions with the same user interface as synchronous
-    replication for <link linkend="streaming-replication">streaming
-    replication</link>.  To do this, the streaming replication interface
-    (see <xref linkend="logicaldecoding-walsender"/>) must be used to stream out
-    data. Clients have to send <literal>Standby status update (F)</literal>
-    (see <xref linkend="protocol-replication"/>) messages, just like streaming
-    replication clients do.
-   </para>
-
-   <note>
     <para>
-     A synchronous replica receiving changes via logical decoding will work in
-     the scope of a single database. Since, in contrast to
-     that, <parameter>synchronous_standby_names</parameter> currently is
-     server wide, this means this technique will not work properly if more
-     than one database is actively used.
+     Logical decoding can be used to build
+     <link linkend="synchronous-replication">synchronous
+     replication</link> solutions with the same user interface as synchronous
+     replication for <link linkend="streaming-replication">streaming
+     replication</link>.  To do this, the streaming replication interface
+     (see <xref linkend="logicaldecoding-walsender"/>) must be used to stream out
+     data. Clients have to send <literal>Standby status update (F)</literal>
+     (see <xref linkend="protocol-replication"/>) messages, just like streaming
+     replication clients do.
+    </para>
+
+    <note>
+     <para>
+      A synchronous replica receiving changes via logical decoding will work in
+      the scope of a single database. Since, in contrast to
+      that, <parameter>synchronous_standby_names</parameter> currently is
+      server wide, this means this technique will not work properly if more
+      than one database is actively used.
      </para>
-   </note>
+    </note>
+   </sect2>
+
+   <sect2 id="logicaldecoding-synchronous-caveats">
+    <title>Caveats</title>
+
+    <para>
+     In synchronous replication setup, a deadlock can happen, if the transaction
+     has locked [user] catalog tables exclusively. This is because logical decoding of
+     transactions can lock catalog tables to access them. To avoid this users
+     must refrain from taking an exclusive lock on [user] catalog tables. This can
+     happen in the following ways:
+
+     <itemizedlist>
+      <listitem>
+       <para>
+        Issuing an explicit <command>LOCK</command> on <structname>pg_class</structname>
+        (or any other catalog table) in a transaction.
+       </para>
+      </listitem>
+
+      <listitem>
+       <para>
+        Perform <command>CLUSTER</command> on <structname>pg_class</structname> in
+        a transaction.
+       </para>
+      </listitem>
+
+      <listitem>
+       <para>
+        <command>PREPARE TRANSACTION</command> after <command>LOCK</command> command
+        on <structname>pg_class</structname> and allow logical decoding of two-phase
+        transactions.
+       </para>
+      </listitem>
+
+      <listitem>
+       <para>
+        <command>PREPARE TRANSACTION</command> after <command>CLUSTER</command>
+        command on <structname>pg_trigger</structname> and allow logical decoding of
+        two-phase transactions. This will lead to deadlock only when published table
+        have a trigger.
+       </para>
+      </listitem>
+
+      <listitem>
+       <para>
+        Executing <command>TRUNCATE</command> on [user] catalog table in a
+        transaction.
+       </para>
+      </listitem>
+     </itemizedlist>
+    </para>
+   </sect2>
   </sect1>
 
   <sect1 id="logicaldecoding-streaming">
@@ -1253,9 +1308,10 @@ stream_commit_cb(...);  &lt;-- commit of the streamed transaction
       <para>
        The logical replication solution that builds distributed two phase commit
        using this feature can deadlock if the prepared transaction has locked
-       [user] catalog tables exclusively. They need to inform users to not have
-       locks on catalog tables (via explicit <command>LOCK</command> command) in
-       such transactions.
+       [user] catalog tables exclusively. To avoid this users must refrain from
+       having locks on catalog tables (e.g. explicit <command>LOCK</command> command)
+       in such transactions.
+       (See <xref linkend="logicaldecoding-synchronous-caveats"/> for the details.)
       </para>
      </listitem>
     </itemizedlist>