Improve HINT message that FDW reports when there are no valid options.

The foreign data wrapper's validator function provides a HINT message with
list of valid options for the object specified in CREATE or ALTER command,
when the option given in the command is invalid. Previously
postgresql_fdw_validator() and the validator functions for postgres_fdw and
dblink_fdw worked in that way even there were no valid options in the object,
which could lead to the HINT message with empty list (because there were
no valid options). For example, ALTER FOREIGN DATA WRAPPER postgres_fdw
OPTIONS (format 'csv') reported the following ERROR and HINT messages.
This behavior was confusing.

    ERROR: invalid option "format"
    HINT: Valid options in this context are:

There is no such issue in file_fdw. The validator function for file_fdw
reports the HINT message "There are no valid options in this context."
instead in that case.

This commit improves postgresql_fdw_validator() and the validator functions
for postgres_fdw and dblink_fdw so that they do likewise. For example,
this change causes the above ALTER FOREIGN DATA WRAPPER command to
report the following messages.

    ERROR:  invalid option "nonexistent"
    HINT:  There are no valid options in this context.

Author: Kosei Masumura
Reviewed-by: Bharath Rupireddy, Fujii Masao
Discussion: https://postgr.es/m/557d06cebe19081bfcc83ee2affc98d3@oss.nttdata.com

contrib/dblink/dblink.c

@@ -2054,8 +2054,10 @@ dblink_fdw_validator(PG_FUNCTION_ARGS)
 			ereport(ERROR,
 					(errcode(ERRCODE_FDW_OPTION_NAME_NOT_FOUND),
 					 errmsg("invalid option \"%s\"", def->defname),
-					 errhint("Valid options in this context are: %s",
-							 buf.data)));
+					 buf.len > 0
+					 ? errhint("Valid options in this context are: %s",
+							   buf.data)
+					 : errhint("There are no valid options in this context.")));
 		}
 	}
 

contrib/dblink/expected/dblink.out

@@ -917,6 +917,10 @@ REVOKE EXECUTE ON FUNCTION dblink_connect_u(text, text) FROM regress_dblink_user
 DROP USER regress_dblink_user;
 DROP USER MAPPING FOR public SERVER fdtest;
 DROP SERVER fdtest;
+-- should fail
+ALTER FOREIGN DATA WRAPPER dblink_fdw OPTIONS (nonexistent 'fdw');
+ERROR:  invalid option "nonexistent"
+HINT:  There are no valid options in this context.
 -- test asynchronous notifications
 SELECT dblink_connect(connection_parameters());
  dblink_connect 

contrib/dblink/sql/dblink.sql

@@ -463,6 +463,9 @@ DROP USER regress_dblink_user;
 DROP USER MAPPING FOR public SERVER fdtest;
 DROP SERVER fdtest;
 
+-- should fail
+ALTER FOREIGN DATA WRAPPER dblink_fdw OPTIONS (nonexistent 'fdw');
+
 -- test asynchronous notifications
 SELECT dblink_connect(connection_parameters());
 

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -10746,7 +10746,7 @@ DROP TABLE join_tbl;
 ALTER SERVER loopback OPTIONS (DROP async_capable);
 ALTER SERVER loopback2 OPTIONS (DROP async_capable);
 -- ===================================================================
--- test invalid server and foreign table options
+-- test invalid server, foreign table and foreign data wrapper options
 -- ===================================================================
 -- Invalid fdw_startup_cost option
 CREATE SERVER inv_scst FOREIGN DATA WRAPPER postgres_fdw
@@ -10764,3 +10764,7 @@ ERROR:  invalid value for integer option "fetch_size": 100$%$#$#
 CREATE FOREIGN TABLE inv_bsz (c1 int )
 	SERVER loopback OPTIONS (batch_size '100$%$#$#');
 ERROR:  invalid value for integer option "batch_size": 100$%$#$#
+-- No option is allowed to be specified at foreign data wrapper level
+ALTER FOREIGN DATA WRAPPER postgres_fdw OPTIONS (nonexistent 'fdw');
+ERROR:  invalid option "nonexistent"
+HINT:  There are no valid options in this context.

contrib/postgres_fdw/option.c

@@ -107,8 +107,10 @@ postgres_fdw_validator(PG_FUNCTION_ARGS)
 			ereport(ERROR,
 					(errcode(ERRCODE_FDW_INVALID_OPTION_NAME),
 					 errmsg("invalid option \"%s\"", def->defname),
-					 errhint("Valid options in this context are: %s",
-							 buf.data)));
+					 buf.len > 0
+					 ? errhint("Valid options in this context are: %s",
+							   buf.data)
+					 : errhint("There are no valid options in this context.")));
 		}
 
 		/*

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -3409,7 +3409,7 @@ ALTER SERVER loopback OPTIONS (DROP async_capable);
 ALTER SERVER loopback2 OPTIONS (DROP async_capable);
 
 -- ===================================================================
--- test invalid server and foreign table options
+-- test invalid server, foreign table and foreign data wrapper options
 -- ===================================================================
 -- Invalid fdw_startup_cost option
 CREATE SERVER inv_scst FOREIGN DATA WRAPPER postgres_fdw
@@ -3423,3 +3423,6 @@ CREATE FOREIGN TABLE inv_fsz (c1 int )
 -- Invalid batch_size option
 CREATE FOREIGN TABLE inv_bsz (c1 int )
 	SERVER loopback OPTIONS (batch_size '100$%$#$#');
+
+-- No option is allowed to be specified at foreign data wrapper level
+ALTER FOREIGN DATA WRAPPER postgres_fdw OPTIONS (nonexistent 'fdw');

src/backend/foreign/foreign.c

@@ -670,8 +670,10 @@ postgresql_fdw_validator(PG_FUNCTION_ARGS)
 			ereport(ERROR,
 					(errcode(ERRCODE_SYNTAX_ERROR),
 					 errmsg("invalid option \"%s\"", def->defname),
-					 errhint("Valid options in this context are: %s",
-							 buf.data)));
+					 buf.len > 0
+					 ? errhint("Valid options in this context are: %s",
+							   buf.data)
+					 : errhint("There are no valid options in this context.")));
 
 			PG_RETURN_BOOL(false);
 		}

src/test/regress/expected/foreign_data.out

@@ -100,6 +100,9 @@ LINE 1: ...GN DATA WRAPPER test_fdw HANDLER test_fdw_handler HANDLER in...
 CREATE FOREIGN DATA WRAPPER test_fdw HANDLER test_fdw_handler;
 DROP FOREIGN DATA WRAPPER test_fdw;
 -- ALTER FOREIGN DATA WRAPPER
+ALTER FOREIGN DATA WRAPPER foo OPTIONS (nonexistent 'fdw');         -- ERROR
+ERROR:  invalid option "nonexistent"
+HINT:  There are no valid options in this context.
 ALTER FOREIGN DATA WRAPPER foo;                             -- ERROR
 ERROR:  syntax error at or near ";"
 LINE 1: ALTER FOREIGN DATA WRAPPER foo;

src/test/regress/sql/foreign_data.sql

@@ -59,6 +59,8 @@ CREATE FOREIGN DATA WRAPPER test_fdw HANDLER test_fdw_handler;
 DROP FOREIGN DATA WRAPPER test_fdw;
 
 -- ALTER FOREIGN DATA WRAPPER
+ALTER FOREIGN DATA WRAPPER foo OPTIONS (nonexistent 'fdw');         -- ERROR
+
 ALTER FOREIGN DATA WRAPPER foo;                             -- ERROR
 ALTER FOREIGN DATA WRAPPER foo VALIDATOR bar;               -- ERROR
 ALTER FOREIGN DATA WRAPPER foo NO VALIDATOR;