Add an optional missing_ok argument to SQL function current_setting().

This allows convenient checking for existence of a GUC from SQL, which is
particularly useful when dealing with custom variables.

David Christensen, reviewed by Jeevan Chalke

src/test/regress/expected/guc.out

@@ -720,6 +720,37 @@ select myfunc(1), current_setting('work_mem');
  2MB    | 2MB
 (1 row)
 
+-- check current_setting()'s behavior with invalid setting name
+select current_setting('nosuch.setting');  -- FAIL
+ERROR:  unrecognized configuration parameter "nosuch.setting"
+select current_setting('nosuch.setting', false);  -- FAIL
+ERROR:  unrecognized configuration parameter "nosuch.setting"
+select current_setting('nosuch.setting', true) is null;
+ ?column? 
+----------
+ t
+(1 row)
+
+-- after this, all three cases should yield 'nada'
+set nosuch.setting = 'nada';
+select current_setting('nosuch.setting');
+ current_setting 
+-----------------
+ nada
+(1 row)
+
+select current_setting('nosuch.setting', false);
+ current_setting 
+-----------------
+ nada
+(1 row)
+
+select current_setting('nosuch.setting', true);
+ current_setting 
+-----------------
+ nada
+(1 row)
+
 -- Normally, CREATE FUNCTION should complain about invalid values in
 -- function SET options; but not if check_function_bodies is off,
 -- because that creates ordering hazards for pg_dump

src/test/regress/sql/guc.sql

@@ -258,6 +258,19 @@ select myfunc(0);
 select current_setting('work_mem');
 select myfunc(1), current_setting('work_mem');
 
+-- check current_setting()'s behavior with invalid setting name
+
+select current_setting('nosuch.setting');  -- FAIL
+select current_setting('nosuch.setting', false);  -- FAIL
+select current_setting('nosuch.setting', true) is null;
+
+-- after this, all three cases should yield 'nada'
+set nosuch.setting = 'nada';
+
+select current_setting('nosuch.setting');
+select current_setting('nosuch.setting', false);
+select current_setting('nosuch.setting', true);
+
 -- Normally, CREATE FUNCTION should complain about invalid values in
 -- function SET options; but not if check_function_bodies is off,
 -- because that creates ordering hazards for pg_dump