v4-0001-system_user-implementation.patch

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index e1fe4fec57..fe99e65dd5 100644
--- a/doc/src/sgml/func.sgml
+++ b/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>name</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>
diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index bc93101ff7..c2a08e9414 100644
--- a/src/backend/access/transam/parallel.c
+++ b/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);
 
diff --git a/src/backend/executor/execExprInterp.c b/src/backend/executor/execExprInterp.c
index 9b9bbf00a9..c51578c0b9 100644
--- a/src/backend/executor/execExprInterp.c
+++ b/src/backend/executor/execExprInterp.c
@@ -2537,6 +2537,11 @@ ExecEvalSQLValueFunction(ExprState *state, ExprEvalStep *op)
 			*op->resvalue = session_user(fcinfo);
 			*op->resnull = fcinfo->isnull;
 			break;
+		case SVFOP_SYSTEM_USER:
+			InitFunctionCallInfoData(*fcinfo, NULL, 0, InvalidOid, NULL, NULL);
+			*op->resvalue = system_user(fcinfo);
+			*op->resnull = fcinfo->isnull;
+			break;
 		case SVFOP_CURRENT_CATALOG:
 			InitFunctionCallInfoData(*fcinfo, NULL, 0, InvalidOid, NULL, NULL);
 			*op->resvalue = current_database(fcinfo);
diff --git a/src/backend/nodes/nodeFuncs.c b/src/backend/nodes/nodeFuncs.c
index 3bac350bf5..453ba84494 100644
--- a/src/backend/nodes/nodeFuncs.c
+++ b/src/backend/nodes/nodeFuncs.c
@@ -1142,7 +1142,8 @@ exprSetCollation(Node *expr, Oid collation)
 			((MinMaxExpr *) expr)->minmaxcollid = collation;
 			break;
 		case T_SQLValueFunction:
-			Assert((((SQLValueFunction *) expr)->type == NAMEOID) ?
+			Assert((((SQLValueFunction *) expr)->type == NAMEOID ||
+					((SQLValueFunction *) expr)->type == TEXTOID) ?
 				   (collation == C_COLLATION_OID) :
 				   (collation == InvalidOid));
 			break;
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 82f03fc9c9..480e6d8c4b 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -744,7 +744,7 @@ 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 +15239,10 @@ func_expr_common_subexpr:
 				{
 					$$ = makeSQLValueFunction(SVFOP_SESSION_USER, -1, @1);
 				}
+			| SYSTEM_USER
+				{
+					$$ = makeSQLValueFunction(SVFOP_SYSTEM_USER, -1, @1);
+				}
 			| USER
 				{
 					$$ = makeSQLValueFunction(SVFOP_USER, -1, @1);
@@ -17120,6 +17124,7 @@ reserved_keyword:
 			| SESSION_USER
 			| SOME
 			| SYMMETRIC
+			| SYSTEM_USER
 			| TABLE
 			| THEN
 			| TO
@@ -17500,6 +17505,7 @@ bare_label_keyword:
 			| SYMMETRIC
 			| SYSID
 			| SYSTEM_P
+			| SYSTEM_USER
 			| TABLE
 			| TABLES
 			| TABLESAMPLE
diff --git a/src/backend/parser/parse_collate.c b/src/backend/parser/parse_collate.c
index 7582faabb3..95d21dab67 100644
--- a/src/backend/parser/parse_collate.c
+++ b/src/backend/parser/parse_collate.c
@@ -707,6 +707,19 @@ assign_collations_walker(Node *node, assign_collations_context *context)
 				 * Now figure out what collation to assign to this node.
 				 */
 				typcollation = get_typcollation(exprType(node));
+
+				/*
+				 * If this is a SQLValueFunction with a TEXT type, then
+				 * assign its collation to the standard C collation.
+				 */
+				if (IsA(node, SQLValueFunction))
+				{
+					if (((SQLValueFunction *) node)->type == TEXTOID)
+					{
+						typcollation = C_COLLATION_OID;
+					}
+				}
+
 				if (OidIsValid(typcollation))
 				{
 					/* Node's result is collatable; what about its input? */
diff --git a/src/backend/parser/parse_expr.c b/src/backend/parser/parse_expr.c
index 7aaf1c673f..a9c5c48102 100644
--- a/src/backend/parser/parse_expr.c
+++ b/src/backend/parser/parse_expr.c
@@ -2239,6 +2239,9 @@ transformSQLValueFunction(ParseState *pstate, SQLValueFunction *svf)
 		case SVFOP_CURRENT_SCHEMA:
 			svf->type = NAMEOID;
 			break;
+		case SVFOP_SYSTEM_USER:
+			svf->type = TEXTOID;
+			break;
 	}
 
 	return (Node *) svf;
diff --git a/src/backend/parser/parse_target.c b/src/backend/parser/parse_target.c
index bd8057bc3e..1cd08a65e8 100644
--- a/src/backend/parser/parse_target.c
+++ b/src/backend/parser/parse_target.c
@@ -1907,6 +1907,9 @@ FigureColnameInternal(Node *node, char **name)
 				case SVFOP_SESSION_USER:
 					*name = "session_user";
 					return 2;
+				case SVFOP_SYSTEM_USER:
+					*name = "system_user";
+					return 2;
 				case SVFOP_CURRENT_CATALOG:
 					*name = "current_catalog";
 					return 2;
diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 2b7b1b0c0f..c621b4328e 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -9210,6 +9210,9 @@ get_rule_expr(Node *node, deparse_context *context,
 					case SVFOP_SESSION_USER:
 						appendStringInfoString(buf, "SESSION_USER");
 						break;
+					case SVFOP_SYSTEM_USER:
+						appendStringInfoString(buf, "SYSTEM_USER");
+						break;
 					case SVFOP_CURRENT_CATALOG:
 						appendStringInfoString(buf, "CURRENT_CATALOG");
 						break;
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 683f616b1a..fa38a76c37 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/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.
+ */
+void
+InitializeSystemUser(const char *authn_id, const char *auth_method)
+{
+	/* call only once */
+	Assert(SystemUser == NULL);
+
+	/*
+	 * InitializeSystemUser should already be called once we are sure that
+	 * authn_id is not NULL (means auth_method is actually valid).
+	 * But keep the test here also for safety.
+	 */
+	if (authn_id)
+	{
+		/* Build system user as auth_method:authn_id */
+		char	   *system_user;
+
+		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
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 4a207a7391..31b7e1de5d 100644
--- a/src/backend/utils/init/postinit.c
+++ b/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();
 	}
 
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index a07e737a33..68bb032d3e 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/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',
diff --git a/src/include/miscadmin.h b/src/include/miscadmin.h
index ee48e392ed..e7ebea4ff4 100644
--- a/src/include/miscadmin.h
+++ b/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 */
diff --git a/src/include/nodes/primnodes.h b/src/include/nodes/primnodes.h
index 40661334bb..74fbc6a4af 100644
--- a/src/include/nodes/primnodes.h
+++ b/src/include/nodes/primnodes.h
@@ -1318,6 +1318,7 @@ typedef enum SQLValueFunctionOp
 	SVFOP_CURRENT_USER,
 	SVFOP_USER,
 	SVFOP_SESSION_USER,
+	SVFOP_SYSTEM_USER,
 	SVFOP_CURRENT_CATALOG,
 	SVFOP_CURRENT_SCHEMA
 } SQLValueFunctionOp;
diff --git a/src/include/parser/kwlist.h b/src/include/parser/kwlist.h
index 9a7cc0c6bd..ccc927851c 100644
--- a/src/include/parser/kwlist.h
+++ b/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)
diff --git a/src/test/authentication/t/001_password.pl b/src/test/authentication/t/001_password.pl
index 3e3079c824..b0b7aac4c0 100644
--- a/src/test/authentication/t/001_password.pl
+++ b/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 +87,25 @@ 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 = $node->safe_psql(
+	'postgres', "
+        SET min_parallel_table_scan_size TO 0;
+        SET parallel_setup_cost TO 0;
+        SET parallel_tuple_cost TO 0;
+        SET max_parallel_workers_per_gather TO 2;
+
+        SELECT bool_and(SYSTEM_USER IS NOT DISTINCT FROM n) FROM sysuser_data;
+    ",
+	connstr => "user=md5_role");
+is($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 +144,26 @@ 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 = $node->safe_psql(
+	'postgres', "
+        SET min_parallel_table_scan_size TO 0;
+        SET parallel_setup_cost TO 0;
+        SET parallel_tuple_cost TO 0;
+        SET max_parallel_workers_per_gather TO 2;
+
+        SELECT bool_and(SYSTEM_USER IS NOT DISTINCT FROM n) FROM sysuser_data;
+    ",
+	connstr => "user=md5_role");
+is($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');
diff --git a/src/test/kerberos/t/001_auth.pl b/src/test/kerberos/t/001_auth.pl
index 62e0542639..f7d8228b24 100644
--- a/src/test/kerberos/t/001_auth.pl
+++ b/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
@@ -176,6 +176,13 @@ $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.
@@ -307,6 +314,23 @@ 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(
+	$node,
+	'test1',
+	"SET min_parallel_table_scan_size TO 0;\n"
+	  . "SET parallel_setup_cost TO 0;\n"
+	  . "SET parallel_tuple_cost TO 0;\n"
+	  . "SET max_parallel_workers_per_gather TO 2;\n"
+	  . "SELECT bool_and(SYSTEM_USER = id) FROM ids;",
+	qr/^t$/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});