Remove db_user_namespace.

This feature was intended to be a temporary measure to support
per-database user names.  A better one hasn't materialized in the
~21 years since it was added, and nobody claims to be using it, so
let's just remove it.

Reviewed-by: Michael Paquier, Magnus Hagander
Discussion: https://postgr.es/m/20230630200509.GA2830328%40nathanxps13
Discussion: https://postgr.es/m/20230630215608.GD2941194%40nathanxps13

doc/src/sgml/client-auth.sgml

@@ -1253,11 +1253,6 @@ omicron         bryanh                  guest1
        attacks.
       </para>
 
-      <para>
-       The <literal>md5</literal> method cannot be used with
-       the <xref linkend="guc-db-user-namespace"/> feature.
-      </para>
-
       <para>
        To ease transition from the <literal>md5</literal> method to the newer
        SCRAM method, if <literal>md5</literal> is specified as a method

doc/src/sgml/config.sgml

@@ -1188,58 +1188,6 @@ include_dir 'conf.d'
        </para>
       </listitem>
      </varlistentry>
-
-     <varlistentry id="guc-db-user-namespace" xreflabel="db_user_namespace">
-      <term><varname>db_user_namespace</varname> (<type>boolean</type>)
-      <indexterm>
-       <primary><varname>db_user_namespace</varname> configuration parameter</primary>
-      </indexterm>
-      </term>
-      <listitem>
-       <para>
-        This parameter enables per-database user names.  It is off by default.
-        This parameter can only be set in the <filename>postgresql.conf</filename>
-        file or on the server command line.
-       </para>
-
-       <para>
-        If this is on, you should create users as <replaceable>username@dbname</replaceable>.
-        When <replaceable>username</replaceable> is passed by a connecting client,
-        <literal>@</literal> and the database name are appended to the user
-        name and that database-specific user name is looked up by the
-        server. Note that when you create users with names containing
-        <literal>@</literal> within the SQL environment, you will need to
-        quote the user name.
-       </para>
-
-       <para>
-        With this parameter enabled, you can still create ordinary global
-        users.  Simply append <literal>@</literal> when specifying the user
-        name in the client, e.g., <literal>joe@</literal>.  The <literal>@</literal>
-        will be stripped off before the user name is looked up by the
-        server.
-       </para>
-
-       <para>
-        <varname>db_user_namespace</varname> causes the client's and
-        server's user name representation to differ.
-        Authentication checks are always done with the server's user name
-        so authentication methods must be configured for the
-        server's user name, not the client's.  Because
-        <literal>md5</literal> uses the user name as salt on both the
-        client and server, <literal>md5</literal> cannot be used with
-        <varname>db_user_namespace</varname>.
-       </para>
-
-       <note>
-        <para>
-         This feature is intended as a temporary measure until a
-         complete solution is found.  At that time, this option will
-         be removed.
-        </para>
-       </note>
-      </listitem>
-     </varlistentry>
      </variablelist>
      </sect2>
 

src/backend/libpq/auth.c

@@ -873,11 +873,6 @@ CheckMD5Auth(Port *port, char *shadow_pass, const char **logdetail)
 	char	   *passwd;
 	int			result;
 
-	if (Db_user_namespace)
-		ereport(FATAL,
-				(errcode(ERRCODE_INVALID_AUTHORIZATION_SPECIFICATION),
-				 errmsg("MD5 authentication is not supported when \"db_user_namespace\" is enabled")));
-
 	/* include the salt to use for computing the response */
 	if (!pg_strong_random(md5Salt, 4))
 	{

src/backend/libpq/hba.c

@@ -1741,19 +1741,7 @@ parse_hba_line(TokenizedAuthLine *tok_line, int elevel)
 	else if (strcmp(token->string, "reject") == 0)
 		parsedline->auth_method = uaReject;
 	else if (strcmp(token->string, "md5") == 0)
-	{
-		if (Db_user_namespace)
-		{
-			ereport(elevel,
-					(errcode(ERRCODE_CONFIG_FILE_ERROR),
-					 errmsg("MD5 authentication is not supported when \"db_user_namespace\" is enabled"),
-					 errcontext("line %d of configuration file \"%s\"",
-								line_num, file_name)));
-			*err_msg = "MD5 authentication is not supported when \"db_user_namespace\" is enabled";
-			return NULL;
-		}
 		parsedline->auth_method = uaMD5;
-	}
 	else if (strcmp(token->string, "scram-sha-256") == 0)
 		parsedline->auth_method = uaSCRAM;
 	else if (strcmp(token->string, "pam") == 0)

src/backend/postmaster/postmaster.c

@@ -236,7 +236,6 @@ int			AuthenticationTimeout = 60;
 
 bool		log_hostname;		/* for ps display and logging */
 bool		Log_connections = false;
-bool		Db_user_namespace = false;
 
 bool		enable_bonjour = false;
 char	   *bonjour_name;
@@ -2272,24 +2271,6 @@ retry1:
 	if (port->database_name == NULL || port->database_name[0] == '\0')
 		port->database_name = pstrdup(port->user_name);
 
-	if (Db_user_namespace)
-	{
-		/*
-		 * If user@, it is a global user, remove '@'. We only want to do this
-		 * if there is an '@' at the end and no earlier in the user string or
-		 * they may fake as a local user of another database attaching to this
-		 * database.
-		 */
-		if (strchr(port->user_name, '@') ==
-			port->user_name + strlen(port->user_name) - 1)
-			*strchr(port->user_name, '@') = '\0';
-		else
-		{
-			/* Append '@' and dbname */
-			port->user_name = psprintf("%s@%s", port->user_name, port->database_name);
-		}
-	}
-
 	if (am_walsender)
 		MyBackendType = B_WAL_SENDER;
 	else

src/backend/utils/misc/guc_tables.c

@@ -1545,15 +1545,6 @@ struct config_bool ConfigureNamesBool[] =
 		false,
 		NULL, NULL, NULL
 	},
-	{
-		{"db_user_namespace", PGC_SIGHUP, CONN_AUTH_AUTH,
-			gettext_noop("Enables per-database user names."),
-			NULL
-		},
-		&Db_user_namespace,
-		false,
-		NULL, NULL, NULL
-	},
 	{
 		{"default_transaction_read_only", PGC_USERSET, CLIENT_CONN_STATEMENT,
 			gettext_noop("Sets the default read-only status of new transactions."),

src/backend/utils/misc/postgresql.conf.sample

@@ -96,7 +96,6 @@
 #authentication_timeout = 1min		# 1s-600s
 #password_encryption = scram-sha-256	# scram-sha-256 or md5
 #scram_iterations = 4096
-#db_user_namespace = off
 
 # GSSAPI using Kerberos
 #krb_server_keyfile = 'FILE:${sysconfdir}/krb5.keytab'

src/include/libpq/pqcomm.h

@@ -103,8 +103,6 @@ typedef ProtocolVersion MsgType;
 
 typedef uint32 PacketLen;
 
-extern PGDLLIMPORT bool Db_user_namespace;
-
 /*
  * In protocol 3.0 and later, the startup packet length is not fixed, but
  * we set an arbitrary limit on it anyway.  This is just to prevent simple