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

@@ -1066,6 +1066,8 @@ 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
@@ -1088,6 +1090,59 @@ OutputPluginWrite(ctx, true);
       than one database is actively used.
      </para>
     </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>