SQL Error [42601]: ERROR: syntax error at or near "(" - postgresql

Question

I am trying to call this function
CREATE or replace FUNCTION get_column_names(param text)
RETURNS text AS $get_column_names$
DECLARE
return_value text;
x record;
y int;
begin
return_value := '';
y := 0;
for x in SELECT *
FROM information_schema.columns
WHERE table_schema = 'public'
AND table_name = param
ORDER BY ordinal_position
loop
if (y = 0)
then
return_value = x.column_name;
else
return_value := return_value ||','|| x.column_name;
end if;
Y = Y+1;
end loop;
return return_value;
END;
$get_column_names$ LANGUAGE plpgsql;
Which this function works here
select get_column_names('users');
Results
first_name,middle_name,last_name,gender,locale,auth_user_id,identity_id,active,date_created,provisioned,user_id,timezone,last_seen
However when I use it in another function I get an error.
Here is the other function I am calling it from.
CREATE OR REPLACE FUNCTION process_users_audit() RETURNS TRIGGER AS $users_audit$
BEGIN
--
-- Create a row in users_audit to reflect the operation performed on users
--
IF (TG_OP = 'DELETE') THEN
INSERT INTO users_audit (user_audit_id, stamp, operation, db_user, get_column_names(TG_TABLE_NAME)) values (uuid_generate_v4(), now(), 'D', user, OLD.*);
RETURN OLD;
ELSIF (TG_OP = 'UPDATE') THEN
INSERT INTO users_audit (user_audit_id, stamp, operation, db_user, get_column_names(TG_TABLE_NAME)) values (uuid_generate_v4(), now(), 'U', user, NEW.*);
RETURN NEW;
ELSIF (TG_OP = 'INSERT') THEN
INSERT INTO users_audit (user_audit_id, stamp, operation, db_user, get_column_names(TG_TABLE_NAME)) values (uuid_generate_v4(), now(), 'I', user, NEW.*);
RETURN NEW;
END IF;
RETURN NULL; -- result is ignored since this is an AFTER trigger
END;
$users_audit$ LANGUAGE plpgsql;
The error I am receiving is this (same as title).
SQL Error [42601]: ERROR: syntax error at or near "("
Position: 406
UPDATE
#Ed Brook's answer worked except for had to change how NEW was being used. Will remove this update once he has updated his answer but here is what worked.
CREATE OR REPLACE FUNCTION process_audit_table() RETURNS TRIGGER AS $audit_table$
BEGIN
--
-- Create a row in the requesting table's audit to reflect the operation performed on users,
-- make use of the special variable TG_OP to work out the operation.
--
IF (TG_OP = 'DELETE') then
EXECUTE 'INSERT INTO ' || concat(TG_TABLE_NAME, '_audit') || ' (' || get_primary_key_name(concat(TG_TABLE_NAME, '_audit')) || ', stamp, operation, db_user, ' || get_column_names(TG_TABLE_NAME) || ') values (uuid_generate_v4(), now(), ''D'', user, $1.*);'
USING OLD;
ELSIF (TG_OP = 'UPDATE') then
EXECUTE 'INSERT INTO ' || concat(TG_TABLE_NAME, '_audit') || ' (' || get_primary_key_name(concat(TG_TABLE_NAME, '_audit')) || ', stamp, operation, db_user, ' || get_column_names(TG_TABLE_NAME) || ') values (uuid_generate_v4(), now(), ''U'', user, $1.*);'
USING NEW;
ELSIF (TG_OP = 'INSERT') then
EXECUTE 'INSERT INTO ' || concat(TG_TABLE_NAME, '_audit') || ' (' || get_primary_key_name(concat(TG_TABLE_NAME, '_audit')) || ', stamp, operation, db_user, ' || get_column_names(TG_TABLE_NAME) || ') values (uuid_generate_v4(), now(), ''I'', user, $1.*);'
USING NEW;
END IF;
RETURN NULL; -- result is ignored since this is an AFTER trigger
END;
$audit_table$ LANGUAGE plpgsql;
Also added some extra bits in there like dynamically getting the audit table name and then get_primary_key_name below is that function.
CREATE OR REPLACE FUNCTION get_primary_key_name(table_name text)
RETURNS text AS $primary_key_name$
DECLARE
return_value text;
BEGIN
SELECT pg_attribute.attname INTO return_value
FROM pg_index, pg_class, pg_attribute, pg_namespace
WHERE pg_class.oid = table_name::regclass
AND indrelid = pg_class.oid
AND nspname = 'public'
AND pg_class.relnamespace = pg_namespace.oid
AND pg_attribute.attrelid = pg_class.oid
AND pg_attribute.attnum = any(pg_index.indkey)
AND indisprimary;
RETURN return_value;
END
$primary_key_name$ LANGUAGE plpgsql;

Answer 1

You may need to use EXECUTE, as I don't think you can have function calls in the column list?
ref: https://www.postgresql.org/docs/current/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN
So, something like this?
EXECUTE 'INSERT INTO users_audit (user_audit_id, stamp, operation, db_user, ' || get_column_names(TG_TABLE_NAME) || ') values (uuid_generate_v4(), now(), ''U'', user, NEW.*)';