Revise the permission checking on user mapping DDL commands.

CREATE/ALTER/DROP USER MAPPING are now allowed either by the server owner or
by a user with USAGE privileges for his own user name.  This is more or less
what the SQL standard wants anyway (plus "implementation-defined")

Hide information_schema.user_mapping_options.option_value, unless the current
user is the one associated with the user mapping, or is the server owner and
the mapping is for PUBLIC, or is a superuser.  This is to protect passwords.

Also, fix a bug in information_schema._pg_foreign_servers, which hid servers
using wrappers where the current user did not have privileges on the wrapper.
The correct behavior is to hide servers where the current user has no
privileges on the server.

doc/src/sgml/information_schema.sgml

@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/information_schema.sgml,v 1.36 2008/12/19 16:25:16 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/information_schema.sgml,v 1.37 2009/01/20 09:10:20 petere Exp $ -->
 
 <chapter id="information-schema">
  <title>The Information Schema</title>
@@ -5081,7 +5081,12 @@ ORDER BY c.ordinal_position;
      <row>
       <entry><literal>option_value</literal></entry>
       <entry><type>character_data</type></entry>
-      <entry>Value of the option</entry>
+      <entry>Value of the option.  This column will show as null
+      unless the current user is the user being mapped, or the mapping
+      is for <literal>PUBLIC</literal> and the current user is the
+      server owner, or the current user is a superuser.  The intent is
+      to protect password information stored as user mapping
+      option.</entry>
      </row>
     </tbody>
    </tgroup>

doc/src/sgml/ref/alter_user_mapping.sgml

@@ -1,5 +1,5 @@
 <!--
-$PostgreSQL: pgsql/doc/src/sgml/ref/alter_user_mapping.sgml,v 1.1 2008/12/19 16:25:16 petere Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/alter_user_mapping.sgml,v 1.2 2009/01/20 09:10:20 petere Exp $
 PostgreSQL documentation
 -->
 
@@ -31,10 +31,15 @@ ALTER USER MAPPING FOR { <replaceable class="parameter">username</replaceable> |
 
   <para>
    <command>ALTER USER MAPPING</command> changes the definition of a
-   user mapping.  Only the owner of the server can change the user
-   mappings of that server.
+   user mapping.
   </para>
 
+  <para>
+   The owner of a foreign server can alter user mappings for that
+   server for any user.  Also, a user can alter a user mapping for
+   his own user name if <literal>USAGE</> privilege on the server has
+   been granted to the user.
+  </para>
  </refsect1>
 
  <refsect1>

doc/src/sgml/ref/create_user_mapping.sgml

@@ -1,5 +1,5 @@
 <!--
-$PostgreSQL: pgsql/doc/src/sgml/ref/create_user_mapping.sgml,v 1.2 2009/01/17 04:24:41 neilc Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/create_user_mapping.sgml,v 1.3 2009/01/20 09:10:20 petere Exp $
 PostgreSQL documentation
 -->
 
@@ -31,10 +31,15 @@ CREATE USER MAPPING FOR { <replaceable class="parameter">username</replaceable>
 
   <para>
    <command>CREATE USER MAPPING</command> defines a mapping of a user
-   to a foreign server.  You must be the owner of the server to define
-   user mappings for it.
+   to a foreign server.
   </para>
 
+  <para>
+   The owner of a foreign server can create user mappings for that
+   server for any user.  Also, a user can create a user mapping for
+   his own user name if <literal>USAGE</> privilege on the server has
+   been granted to the user.
+  </para>
  </refsect1>
 
  <refsect1>

doc/src/sgml/ref/drop_user_mapping.sgml

@@ -1,5 +1,5 @@
 <!--
-$PostgreSQL: pgsql/doc/src/sgml/ref/drop_user_mapping.sgml,v 1.1 2008/12/19 16:25:16 petere Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/drop_user_mapping.sgml,v 1.2 2009/01/20 09:10:20 petere Exp $
 PostgreSQL documentation
 -->
 
@@ -29,8 +29,14 @@ DROP USER MAPPING [ IF EXISTS ] FOR { <replaceable class="parameter">username</r
 
   <para>
    <command>DROP USER MAPPING</command> removes an existing user
-   mapping from foreign server.  To execute this command, the current
-   user must be the owner of the server containing the mapping.
+   mapping from foreign server.
+  </para>
+
+  <para>
+   The owner of a foreign server can drop user mappings for that server
+   for any user.  Also, a user can drop a user mapping for his own
+   user name if <literal>USAGE</> privilege on the server has been
+   granted to the user.
   </para>
  </refsect1>