Introduce SYSTEM_USER

SYSTEM_USER is a reserved keyword of the SQL specification that,
roughly described, is aimed at reporting some information about the
system user who has connected to the database server.  It may include
implementation-specific information about the means by the user
connected, like an authentication method.

This commit implements SYSTEM_USER as of auth_method:identity, where
"auth_method" is a keyword about the authentication method used to log
into the server (like peer, md5, scram-sha-256, gss, etc.) and
"identity" is the authentication identity as introduced by 9afffcb (peer
sets authn to the OS user name, gss to the user principal, etc.).  This
format has been suggested by Tom Lane.

Note that thanks to d951052, SYSTEM_USER is available to parallel
workers.

Bump catalog version.

Author: Bertrand Drouvot
Reviewed-by: Jacob Champion, Joe Conway, Álvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/7e692b8c-0b11-45db-1cad-3afc5b57409f@amazon.com

doc/src/sgml/func.sgml

@@ -22623,6 +22623,25 @@ SELECT * FROM pg_ls_dir('.') WITH ORDINALITY AS t(ls,n);
        </para></entry>
       </row>
 
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>system_user</primary>
+        </indexterm>
+        <function>system_user</function>
+        <returnvalue>text</returnvalue>
+       </para>
+       <para>
+        Returns the authentication method and the identity (if any) that the
+        user presented during the authentication cycle before they were
+        assigned a database role. It is represented as
+        <literal>auth_method:identity</literal> or
+        <literal>NULL</literal> if the user has not been authenticated (for
+        example if <link linkend="auth-trust">Trust authentication</link> has
+        been used).
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>

src/backend/access/transam/parallel.c

@@ -1496,6 +1496,14 @@ ParallelWorkerMain(Datum main_arg)
 										 false);
 	RestoreClientConnectionInfo(clientconninfospace);
 
+	/*
+	 * Initialize SystemUser now that MyClientConnectionInfo is restored.
+	 * Also ensure that auth_method is actually valid, aka authn_id is not NULL.
+	 */
+	if (MyClientConnectionInfo.authn_id)
+		InitializeSystemUser(MyClientConnectionInfo.authn_id,
+							 hba_authname(MyClientConnectionInfo.auth_method));
+
 	/* Attach to the leader's serializable transaction, if SERIALIZABLE. */
 	AttachSerializableXact(fps->serializable_xact_handle);
 

src/backend/parser/gram.y

@@ -743,7 +743,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 	SERIALIZABLE SERVER SESSION SESSION_USER SET SETS SETOF SHARE SHOW
 	SIMILAR SIMPLE SKIP SMALLINT SNAPSHOT SOME SQL_P STABLE STANDALONE_P
 	START STATEMENT STATISTICS STDIN STDOUT STORAGE STORED STRICT_P STRIP_P
-	SUBSCRIPTION SUBSTRING SUPPORT SYMMETRIC SYSID SYSTEM_P
+	SUBSCRIPTION SUBSTRING SUPPORT SYMMETRIC SYSID SYSTEM_P SYSTEM_USER
 
 	TABLE TABLES TABLESAMPLE TABLESPACE TEMP TEMPLATE TEMPORARY TEXT_P THEN
 	TIES TIME TIMESTAMP TO TRAILING TRANSACTION TRANSFORM
@@ -15239,6 +15239,13 @@ func_expr_common_subexpr:
 				{
 					$$ = makeSQLValueFunction(SVFOP_SESSION_USER, -1, @1);
 				}
+			| SYSTEM_USER
+				{
+					$$ = (Node *) makeFuncCall(SystemFuncName("system_user"),
+											   NIL,
+											   COERCE_SQL_SYNTAX,
+											   @1);
+				}
 			| USER
 				{
 					$$ = makeSQLValueFunction(SVFOP_USER, -1, @1);
@@ -17120,6 +17127,7 @@ reserved_keyword:
 			| SESSION_USER
 			| SOME
 			| SYMMETRIC
+			| SYSTEM_USER
 			| TABLE
 			| THEN
 			| TO
@@ -17500,6 +17508,7 @@ bare_label_keyword:
 			| SYMMETRIC
 			| SYSID
 			| SYSTEM_P
+			| SYSTEM_USER
 			| TABLE
 			| TABLES
 			| TABLESAMPLE

src/backend/utils/adt/ruleutils.c

@@ -10317,6 +10317,10 @@ get_func_sql_syntax(FuncExpr *expr, deparse_context *context)
 			appendStringInfoChar(buf, ')');
 			return true;
 
+		case F_SYSTEM_USER:
+			appendStringInfoString(buf, "SYSTEM_USER");
+			return true;
+
 		case F_XMLEXISTS:
 			/* XMLEXISTS ... extra parens because args are c_expr */
 			appendStringInfoString(buf, "XMLEXISTS((");

src/backend/utils/init/miscinit.c

@@ -477,6 +477,7 @@ static Oid	AuthenticatedUserId = InvalidOid;
 static Oid	SessionUserId = InvalidOid;
 static Oid	OuterUserId = InvalidOid;
 static Oid	CurrentUserId = InvalidOid;
+static const char *SystemUser = NULL;
 
 /* We also have to remember the superuser state of some of these levels */
 static bool AuthenticatedUserIsSuperuser = false;
@@ -548,6 +549,16 @@ SetSessionUserId(Oid userid, bool is_superuser)
 	CurrentUserId = userid;
 }
 
+/*
+ * Return the system user representing the authenticated identity.
+ * It is defined in InitializeSystemUser() as auth_method:authn_id.
+ */
+const char *
+GetSystemUser(void)
+{
+	return SystemUser;
+}
+
 /*
  * GetAuthenticatedUserId - get the authenticated user ID
  */
@@ -818,6 +829,45 @@ InitializeSessionUserIdStandalone(void)
 	SetSessionUserId(BOOTSTRAP_SUPERUSERID, true);
 }
 
+/*
+ * Initialize the system user.
+ *
+ * This is built as auth_method:authn_id.
+ */
+void
+InitializeSystemUser(const char *authn_id, const char *auth_method)
+{
+	char	   *system_user;
+
+	/* call only once */
+	Assert(SystemUser == NULL);
+
+	/*
+	 * InitializeSystemUser should be called only when authn_id is not NULL,
+	 * meaning that auth_method is valid.
+	 */
+	Assert(authn_id != NULL);
+
+	system_user = psprintf("%s:%s", auth_method, authn_id);
+
+	/* Store SystemUser in long-lived storage */
+	SystemUser = MemoryContextStrdup(TopMemoryContext, system_user);
+	pfree(system_user);
+}
+
+/*
+ * SQL-function SYSTEM_USER
+ */
+Datum
+system_user(PG_FUNCTION_ARGS)
+{
+	const char *sysuser = GetSystemUser();
+
+	if (sysuser)
+		PG_RETURN_DATUM(CStringGetTextDatum(sysuser));
+	else
+		PG_RETURN_NULL();
+}
 
 /*
  * Change session auth ID while running

src/backend/utils/init/postinit.c

@@ -904,6 +904,10 @@ InitPostgres(const char *in_dbname, Oid dboid,
 		Assert(MyProcPort != NULL);
 		PerformAuthentication(MyProcPort);
 		InitializeSessionUserId(username, useroid);
+		/* ensure that auth_method is actually valid, aka authn_id is not NULL */
+		if (MyClientConnectionInfo.authn_id)
+			InitializeSystemUser(MyClientConnectionInfo.authn_id,
+								 hba_authname(MyClientConnectionInfo.auth_method));
 		am_superuser = superuser();
 	}
 

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202209261
+#define CATALOG_VERSION_NO	202209291
 
 #endif

src/include/catalog/pg_proc.dat

@@ -1508,6 +1508,9 @@
 { oid => '746', descr => 'session user name',
   proname => 'session_user', provolatile => 's', prorettype => 'name',
   proargtypes => '', prosrc => 'session_user' },
+{ oid => '9977', descr => 'system user name',
+  proname => 'system_user', provolatile => 's', prorettype => 'text',
+  proargtypes => '', prosrc => 'system_user' },
 
 { oid => '744',
   proname => 'array_eq', prorettype => 'bool',

src/include/miscadmin.h

@@ -357,6 +357,9 @@ extern void InitializeSessionUserIdStandalone(void);
 extern void SetSessionAuthorization(Oid userid, bool is_superuser);
 extern Oid	GetCurrentRoleId(void);
 extern void SetCurrentRoleId(Oid roleid, bool is_superuser);
+extern void InitializeSystemUser(const char *authn_id,
+								 const char *auth_method);
+extern const char *GetSystemUser(void);
 
 /* in utils/misc/superuser.c */
 extern bool superuser(void);	/* current user is superuser */

src/include/parser/kwlist.h

@@ -409,6 +409,7 @@ PG_KEYWORD("support", SUPPORT, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("symmetric", SYMMETRIC, RESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("sysid", SYSID, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("system", SYSTEM_P, UNRESERVED_KEYWORD, BARE_LABEL)
+PG_KEYWORD("system_user", SYSTEM_USER, RESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("table", TABLE, RESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("tables", TABLES, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("tablesample", TABLESAMPLE, TYPE_FUNC_NAME_KEYWORD, BARE_LABEL)

src/test/authentication/t/001_password.pl

@@ -72,6 +72,11 @@ $node->safe_psql('postgres',
 $node->safe_psql('postgres',
 	"SET password_encryption='md5'; CREATE ROLE md5_role LOGIN PASSWORD 'pass';"
 );
+# Set up a table for tests of SYSTEM_USER.
+$node->safe_psql(
+	'postgres',
+	"CREATE TABLE sysuser_data (n) AS SELECT NULL FROM generate_series(1, 10);
+	 GRANT ALL ON sysuser_data TO md5_role;");
 $ENV{"PGPASSWORD"} = 'pass';
 
 # For "trust" method, all users should be able to connect. These users are not
@@ -82,6 +87,24 @@ test_role($node, 'scram_role', 'trust', 0,
 test_role($node, 'md5_role', 'trust', 0,
 	log_unlike => [qr/connection authenticated:/]);
 
+# SYSTEM_USER is null when not authenticated.
+my $res = $node->safe_psql('postgres', "SELECT SYSTEM_USER IS NULL;");
+is($res, 't', "users with trust authentication use SYSTEM_USER = NULL");
+
+# Test SYSTEM_USER with parallel workers when not authenticated.
+$res = $node->safe_psql(
+	'postgres', qq(
+        SET min_parallel_table_scan_size TO 0;
+        SET parallel_setup_cost TO 0;
+        SET parallel_tuple_cost TO 0;
+        SET max_parallel_workers_per_gather TO 2;
+
+        SELECT bool_and(SYSTEM_USER IS NOT DISTINCT FROM n) FROM sysuser_data;),
+	connstr => "user=md5_role");
+is($res, 't',
+	"users with trust authentication use SYSTEM_USER = NULL in parallel workers"
+);
+
 # For plain "password" method, all users should also be able to connect.
 reset_pg_hba($node, 'password');
 test_role($node, 'scram_role', 'password', 0,
@@ -120,6 +143,25 @@ test_role($node, 'md5_role', 'md5', 0,
 	log_like =>
 	  [qr/connection authenticated: identity="md5_role" method=md5/]);
 
+# Test SYSTEM_USER <> NULL with parallel workers.
+$node->safe_psql(
+	'postgres',
+	"TRUNCATE sysuser_data;
+INSERT INTO sysuser_data SELECT 'md5:md5_role' FROM generate_series(1, 10);",
+	connstr => "user=md5_role");
+$res = $node->safe_psql(
+	'postgres', qq(
+        SET min_parallel_table_scan_size TO 0;
+        SET parallel_setup_cost TO 0;
+        SET parallel_tuple_cost TO 0;
+        SET max_parallel_workers_per_gather TO 2;
+
+        SELECT bool_and(SYSTEM_USER IS NOT DISTINCT FROM n) FROM sysuser_data;),
+	connstr => "user=md5_role");
+is($res, 't',
+	"users with md5 authentication use SYSTEM_USER = md5:role in parallel workers"
+);
+
 # Tests for channel binding without SSL.
 # Using the password authentication method; channel binding can't work
 reset_pg_hba($node, 'password');

src/test/kerberos/t/001_auth.pl

@@ -4,8 +4,8 @@
 # Sets up a KDC and then runs a variety of tests to make sure that the
 # GSSAPI/Kerberos authentication and encryption are working properly,
 # that the options in pg_hba.conf and pg_ident.conf are handled correctly,
-# and that the server-side pg_stat_gssapi view reports what we expect to
-# see for each test.
+# that the server-side pg_stat_gssapi view reports what we expect to
+# see for each test and that SYSTEM_USER returns what we expect to see.
 #
 # Since this requires setting up a full KDC, it doesn't make much sense
 # to have multiple test scripts (since they'd have to also create their
@@ -180,6 +180,13 @@ $node->start;
 
 $node->safe_psql('postgres', 'CREATE USER test1;');
 
+# Set up a table for SYSTEM_USER parallel worker testing.
+$node->safe_psql('postgres',
+	"CREATE TABLE ids (id) AS SELECT 'gss:test1\@$realm' FROM generate_series(1, 10);"
+);
+
+$node->safe_psql('postgres', 'GRANT SELECT ON ids TO public;');
+
 note "running tests";
 
 # Test connection success or failure, and if success, that query returns true.
@@ -311,6 +318,23 @@ test_query(
 	'gssencmode=require',
 	'sending 100K lines works');
 
+# Test that SYSTEM_USER works.
+test_query($node, 'test1', 'SELECT SYSTEM_USER;',
+	qr/^gss:test1\@$realm$/s, 'gssencmode=require', 'testing system_user');
+
+# Test that SYSTEM_USER works with parallel workers.
+test_query(
+	$node,
+	'test1', qq(
+	SET min_parallel_table_scan_size TO 0;
+	SET parallel_setup_cost TO 0;
+	SET parallel_tuple_cost TO 0;
+	SET max_parallel_workers_per_gather TO 2;
+	SELECT bool_and(SYSTEM_USER = id) FROM ids;),
+	qr/^t$/s,
+	'gssencmode=require',
+	'testing system_user with parallel workers');
+
 unlink($node->data_dir . '/pg_hba.conf');
 $node->append_conf('pg_hba.conf',
 	qq{hostgssenc all all $hostaddr/32 gss map=mymap});

src/test/regress/expected/create_view.out

@@ -1940,7 +1940,8 @@ select
   trim(trailing ' foo ') as rt,
   trim(E'\\000'::bytea from E'\\000Tom\\000'::bytea) as btb,
   trim(leading E'\\000'::bytea from E'\\000Tom\\000'::bytea) as ltb,
-  trim(trailing E'\\000'::bytea from E'\\000Tom\\000'::bytea) as rtb;
+  trim(trailing E'\\000'::bytea from E'\\000Tom\\000'::bytea) as rtb,
+  SYSTEM_USER as su;
 select pg_get_viewdef('tt201v', true);
                                         pg_get_viewdef                                         
 -----------------------------------------------------------------------------------------------
@@ -1961,7 +1962,8 @@ select pg_get_viewdef('tt201v', true);
      TRIM(TRAILING FROM ' foo '::text) AS rt,                                                 +
      TRIM(BOTH '\x00'::bytea FROM '\x00546f6d00'::bytea) AS btb,                              +
      TRIM(LEADING '\x00'::bytea FROM '\x00546f6d00'::bytea) AS ltb,                           +
-     TRIM(TRAILING '\x00'::bytea FROM '\x00546f6d00'::bytea) AS rtb;
+     TRIM(TRAILING '\x00'::bytea FROM '\x00546f6d00'::bytea) AS rtb,                          +
+     SYSTEM_USER AS su;
 (1 row)
 
 -- corner cases with empty join conditions

src/test/regress/sql/create_view.sql

@@ -721,7 +721,8 @@ select
   trim(trailing ' foo ') as rt,
   trim(E'\\000'::bytea from E'\\000Tom\\000'::bytea) as btb,
   trim(leading E'\\000'::bytea from E'\\000Tom\\000'::bytea) as ltb,
-  trim(trailing E'\\000'::bytea from E'\\000Tom\\000'::bytea) as rtb;
+  trim(trailing E'\\000'::bytea from E'\\000Tom\\000'::bytea) as rtb,
+  SYSTEM_USER as su;
 select pg_get_viewdef('tt201v', true);
 
 -- corner cases with empty join conditions