Doc: word-smith the discussion of secure schema usage patterns.

Rearrange the discussion of user-private schemas so that details
applying only to upgraded-from-pre-v15 databases are in a follow-on
paragraph, not in the main description of how to set up this pattern.
This seems a little clearer even today, and it'll get more so as
pre-v15 systems fade into the sunset.

Wording contributions from Robert Haas, Tom Lane, Noah Misch.

Discussion: https://postgr.es/m/CA+TgmoYUHsfp90inEMAP0yNr7Y_L6EphPH1YOon1JKtBztXHyQ@mail.gmail.com

Author: tglsfdc

doc/src/sgml/ddl.sgml

@@ -3211,40 +3211,43 @@ REVOKE CREATE ON SCHEMA public FROM PUBLIC;
     query that database would take protective action at the beginning of each
     session.  Specifically, they would begin each session by
     setting <varname>search_path</varname> to the empty string or otherwise
-    removing non-superuser-writable schemas
+    removing schemas that are writable by non-superusers
     from <varname>search_path</varname>.  There are a few usage patterns
     easily supported by the default configuration:
     <itemizedlist>
      <listitem>
-      <!-- "DROP SCHEMA public" is inferior to this REVOKE, because pg_dump
-           doesn't preserve that DROP.
-
-           A database owner can attack the database's users via "CREATE SCHEMA
+      <para>
+       Constrain ordinary users to user-private schemas.
+       To implement this pattern, first ensure that no schemas have
+       public <literal>CREATE</literal> privileges.  Then, for every user
+       needing to create non-temporary objects, create a schema with the
+       same name as that user, for example
+       <literal>CREATE SCHEMA alice AUTHORIZATION alice</literal>.
+       (Recall that the default search path starts
+       with <literal>$user</literal>, which resolves to the user
+       name. Therefore, if each user has a separate schema, they access
+       their own schemas by default.)  This pattern is a secure schema
+       usage pattern unless an untrusted user is the database owner or
+       holds the <literal>CREATEROLE</literal> privilege, in which case no
+       secure schema usage pattern exists.
+      </para>
+      <!-- A database owner can attack the database's users via "CREATE SCHEMA
            trojan; ALTER DATABASE $mydb SET search_path = trojan, public;".  A
            CREATEROLE user can issue "GRANT $dbowner TO $me" and then use the
            database owner attack. -->
+
       <para>
-       Constrain ordinary users to user-private schemas.  To implement this,
-       first issue <literal>REVOKE CREATE ON SCHEMA public FROM
-       PUBLIC</literal>.  Then, for every user needing to create non-temporary
-       objects, create a schema with the same name as that user.  Recall that
-       the default search path starts with <literal>$user</literal>, which
-       resolves to the user name.  Therefore, if each user has a separate
-       schema, they access their own schemas by default.  After adopting this
-       pattern in a database where untrusted users had already logged in,
-       consider auditing the public schema for objects named like objects in
-       schema <literal>pg_catalog</literal>.  This pattern is a secure schema
-       usage pattern unless an untrusted user is the database owner or holds
-       the <literal>CREATEROLE</literal> privilege, in which case no secure
-       schema usage pattern exists.
-      </para>
-      <para>
-       If the database originated in an upgrade
-       from <productname>PostgreSQL</productname> 14 or earlier,
-       the <literal>REVOKE</literal> is essential.  Otherwise, the default
-       configuration follows this pattern; ordinary users can create only
-       temporary objects until a privileged user furnishes a schema.
+       In <productname>PostgreSQL</productname> 15 and later, the default
+       configuration supports this usage pattern.  In prior versions, or
+       when using a database that has been upgraded from a prior version,
+       you will need to remove the public <literal>CREATE</literal>
+       privilege from the <literal>public</literal> schema (issue
+       <literal>REVOKE CREATE ON SCHEMA public FROM PUBLIC</literal>).
+       Then consider auditing the <literal>public</literal> schema for
+       objects named like objects in schema <literal>pg_catalog</literal>.
       </para>
+      <!-- "DROP SCHEMA public" is inferior to this REVOKE, because pg_dump
+           doesn't preserve that DROP. -->
      </listitem>
 
      <listitem>