Match pg_user_mappings limits to information_schema.user_mapping_options.

Both views replace the umoptions field with NULL when the user does not
meet qualifications to see it.  They used different qualifications, and
pg_user_mappings documented qualifications did not match its implemented
qualifications.  Make its documentation and implementation match those
of user_mapping_options.  One might argue for stronger qualifications,
but these have long, documented tenure.  pg_user_mappings has always
exhibited this problem, so back-patch to 9.2 (all supported versions).

Michael Paquier and Feike Steenbergen.  Reviewed by Jeff Janes.
Reported by Andrew Wheelwright.

Security: CVE-2017-7486

src/backend/catalog/system_views.sql

@@ -826,11 +826,11 @@ CREATE VIEW pg_user_mappings AS
         ELSE
             A.rolname
         END AS usename,
-        CASE WHEN pg_has_role(S.srvowner, 'USAGE') OR has_server_privilege(S.oid, 'USAGE') THEN
-            U.umoptions
-        ELSE
-            NULL
-        END AS umoptions
+        CASE WHEN (U.umuser <> 0 AND A.rolname = current_user)
+                    OR (U.umuser = 0 AND pg_has_role(S.srvowner, 'USAGE'))
+                    OR (SELECT rolsuper FROM pg_authid WHERE rolname = current_user)
+                    THEN U.umoptions
+                 ELSE NULL END AS umoptions
     FROM pg_user_mapping U
          LEFT JOIN pg_authid A ON (A.oid = U.umuser) JOIN
         pg_foreign_server S ON (U.umserver = S.oid);

src/test/regress/expected/foreign_data.out

@@ -1194,7 +1194,61 @@ WARNING:  no privileges were granted for "s9"
 CREATE USER MAPPING FOR current_user SERVER s9;
 DROP SERVER s9 CASCADE;                                         -- ERROR
 ERROR:  must be owner of foreign server s9
+-- Check visibility of user mapping data
+SET ROLE regress_test_role;
+CREATE SERVER s10 FOREIGN DATA WRAPPER foo;
+CREATE USER MAPPING FOR public SERVER s10 OPTIONS (user 'secret');
+GRANT USAGE ON FOREIGN SERVER s10 TO regress_unprivileged_role;
+-- owner of server can see option fields
+\deu+
+                 List of user mappings
+ Server |         User name         |    FDW Options    
+--------+---------------------------+-------------------
+ s10    | public                    | ("user" 'secret')
+ s4     | regress_foreign_data_user | 
+ s5     | regress_test_role         | (modified '1')
+ s6     | regress_test_role         | 
+ s8     | public                    | 
+ s8     | regress_foreign_data_user | 
+ s9     | regress_unprivileged_role | 
+ t1     | public                    | (modified '1')
+(8 rows)
+
 RESET ROLE;
+-- superuser can see option fields
+\deu+
+                  List of user mappings
+ Server |         User name         |     FDW Options     
+--------+---------------------------+---------------------
+ s10    | public                    | ("user" 'secret')
+ s4     | regress_foreign_data_user | 
+ s5     | regress_test_role         | (modified '1')
+ s6     | regress_test_role         | 
+ s8     | public                    | 
+ s8     | regress_foreign_data_user | (password 'public')
+ s9     | regress_unprivileged_role | 
+ t1     | public                    | (modified '1')
+(8 rows)
+
+-- unprivileged user cannot see option fields
+SET ROLE regress_unprivileged_role;
+\deu+
+              List of user mappings
+ Server |         User name         | FDW Options 
+--------+---------------------------+-------------
+ s10    | public                    | 
+ s4     | regress_foreign_data_user | 
+ s5     | regress_test_role         | 
+ s6     | regress_test_role         | 
+ s8     | public                    | 
+ s8     | regress_foreign_data_user | 
+ s9     | regress_unprivileged_role | 
+ t1     | public                    | 
+(8 rows)
+
+RESET ROLE;
+DROP SERVER s10 CASCADE;
+NOTICE:  drop cascades to user mapping for public on server s10
 -- Triggers
 CREATE FUNCTION dummy_trigger() RETURNS TRIGGER AS $$
   BEGIN

src/test/regress/expected/rules.out

@@ -2151,7 +2151,9 @@ pg_user_mappings| SELECT u.oid AS umid,
             ELSE a.rolname
         END AS usename,
         CASE
-            WHEN (pg_has_role(s.srvowner, 'USAGE'::text) OR has_server_privilege(s.oid, 'USAGE'::text)) THEN u.umoptions
+            WHEN (((u.umuser <> (0)::oid) AND (a.rolname = "current_user"())) OR ((u.umuser = (0)::oid) AND pg_has_role(s.srvowner, 'USAGE'::text)) OR ( SELECT pg_authid.rolsuper
+               FROM pg_authid
+              WHERE (pg_authid.rolname = "current_user"()))) THEN u.umoptions
             ELSE NULL::text[]
         END AS umoptions
    FROM ((pg_user_mapping u

src/test/regress/sql/foreign_data.sql

@@ -490,7 +490,22 @@ ALTER SERVER s9 VERSION '1.2';                                  -- ERROR
 GRANT USAGE ON FOREIGN SERVER s9 TO regress_test_role;          -- WARNING
 CREATE USER MAPPING FOR current_user SERVER s9;
 DROP SERVER s9 CASCADE;                                         -- ERROR
+
+-- Check visibility of user mapping data
+SET ROLE regress_test_role;
+CREATE SERVER s10 FOREIGN DATA WRAPPER foo;
+CREATE USER MAPPING FOR public SERVER s10 OPTIONS (user 'secret');
+GRANT USAGE ON FOREIGN SERVER s10 TO regress_unprivileged_role;
+-- owner of server can see option fields
+\deu+
 RESET ROLE;
+-- superuser can see option fields
+\deu+
+-- unprivileged user cannot see option fields
+SET ROLE regress_unprivileged_role;
+\deu+
+RESET ROLE;
+DROP SERVER s10 CASCADE;
 
 -- Triggers
 CREATE FUNCTION dummy_trigger() RETURNS TRIGGER AS $$