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

contrib/tsearch2/tsearch2.c

@@ -363,7 +363,7 @@ tsa_tsearch2(PG_FUNCTION_ARGS)
 		tgargs[i + 1] = trigger->tgargs[i];
 
 	tgargs[1] = pstrdup(GetConfigOptionByName("default_text_search_config",
-											  NULL));
+											  NULL, false));
 	tgargs_old = trigger->tgargs;
 	trigger->tgargs = tgargs;
 	trigger->tgnargs++;

doc/src/sgml/func.sgml

@@ -16444,7 +16444,7 @@ SELECT collation for ('foo' COLLATE "de_DE");
         <indexterm>
          <primary>current_setting</primary>
         </indexterm>
-        <literal><function>current_setting(<parameter>setting_name</parameter>)</function></literal>
+        <literal><function>current_setting(<parameter>setting_name</parameter> [, <parameter>missing_ok</parameter> ])</function></literal>
        </entry>
        <entry><type>text</type></entry>
        <entry>get current value of setting</entry>
@@ -16492,6 +16492,11 @@ SELECT current_setting('datestyle');
  ISO, MDY
 (1 row)
 </programlisting>
+
+    If there is no setting named <parameter>setting_name</parameter>,
+    <function>current_setting</function> throws an error
+    unless <parameter>missing_ok</parameter> is supplied and is
+    <literal>true</literal>.
    </para>
 
    <para>

src/backend/utils/misc/guc.c

@@ -7131,7 +7131,7 @@ ExtractSetVariableArgs(VariableSetStmt *stmt)
 		case VAR_SET_VALUE:
 			return flatten_set_variable_args(stmt->name, stmt->args);
 		case VAR_SET_CURRENT:
-			return GetConfigOptionByName(stmt->name, NULL);
+			return GetConfigOptionByName(stmt->name, NULL, false);
 		default:
 			return NULL;
 	}
@@ -7200,7 +7200,7 @@ set_config_by_name(PG_FUNCTION_ARGS)
 							 true, 0, false);
 
 	/* get the new current value */
-	new_value = GetConfigOptionByName(name, NULL);
+	new_value = GetConfigOptionByName(name, NULL, false);
 
 	/* Convert return string to text */
 	PG_RETURN_TEXT_P(cstring_to_text(new_value));
@@ -7627,7 +7627,7 @@ GetPGVariableResultDesc(const char *name)
 		const char *varname;
 
 		/* Get the canonical spelling of name */
-		(void) GetConfigOptionByName(name, &varname);
+		(void) GetConfigOptionByName(name, &varname, false);
 
 		/* need a tuple descriptor representing a single TEXT column */
 		tupdesc = CreateTemplateTupleDesc(1, false);
@@ -7650,7 +7650,7 @@ ShowGUCConfigOption(const char *name, DestReceiver *dest)
 	char	   *value;
 
 	/* Get the value and canonical spelling of name */
-	value = GetConfigOptionByName(name, &varname);
+	value = GetConfigOptionByName(name, &varname, false);
 
 	/* need a tuple descriptor representing a single TEXT column */
 	tupdesc = CreateTemplateTupleDesc(1, false);
@@ -7734,19 +7734,30 @@ ShowAllGUCConfig(DestReceiver *dest)
 }
 
 /*
- * Return GUC variable value by name; optionally return canonical
+ * Return GUC variable value by name; optionally return canonical form of
- * form of name.  Return value is palloc'd.
+ * name.  If the GUC is unset, then throw an error unless missing_ok is true,
+ * in which case return NULL.  Return value is palloc'd (but *varname isn't).
  */
 char *
-GetConfigOptionByName(const char *name, const char **varname)
+GetConfigOptionByName(const char *name, const char **varname, bool missing_ok)
 {
 	struct config_generic *record;
 
 	record = find_option(name, false, ERROR);
 	if (record == NULL)
+	{
+		if (missing_ok)
+		{
+			if (varname)
+				*varname = NULL;
+			return NULL;
+		}
+
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
 			   errmsg("unrecognized configuration parameter \"%s\"", name)));
+	}
+
 	if ((record->flags & GUC_SUPERUSER_ONLY) && !superuser())
 		ereport(ERROR,
 				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
@@ -8033,14 +8044,34 @@ GetNumConfigOptions(void)
 Datum
 show_config_by_name(PG_FUNCTION_ARGS)
 {
-	char	   *varname;
+	char	   *varname = TextDatumGetCString(PG_GETARG_DATUM(0));
 	char	   *varval;
 
-	/* Get the GUC variable name */
+	/* Get the value */
-	varname = TextDatumGetCString(PG_GETARG_DATUM(0));
+	varval = GetConfigOptionByName(varname, NULL, false);
+
+	/* Convert to text */
+	PG_RETURN_TEXT_P(cstring_to_text(varval));
+}
+
+/*
+ * show_config_by_name_missing_ok - equiv to SHOW X command but implemented as
+ * a function.  If X does not exist, suppress the error and just return NULL
+ * if missing_ok is TRUE.
+ */
+Datum
+show_config_by_name_missing_ok(PG_FUNCTION_ARGS)
+{
+	char	   *varname = TextDatumGetCString(PG_GETARG_DATUM(0));
+	bool		missing_ok = PG_GETARG_BOOL(1);
+	char	   *varval;
 
 	/* Get the value */
-	varval = GetConfigOptionByName(varname, NULL);
+	varval = GetConfigOptionByName(varname, NULL, missing_ok);
+
+	/* return NULL if no such variable */
+	if (varval == NULL)
+		PG_RETURN_NULL();
 
 	/* Convert to text */
 	PG_RETURN_TEXT_P(cstring_to_text(varval));

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201506282
+#define CATALOG_VERSION_NO	201507021
 
 #endif

src/include/catalog/pg_proc.h

@@ -3067,6 +3067,8 @@ DESCR("convert bitstring to int8");
 
 DATA(insert OID = 2077 (  current_setting	PGNSP PGUID 12 1 0 0 0 f f f f t f s 1 0 25 "25" _null_ _null_ _null_ _null_ _null_ show_config_by_name _null_ _null_ _null_ ));
 DESCR("SHOW X as a function");
+DATA(insert OID = 3294 (  current_setting	PGNSP PGUID 12 1 0 0 0 f f f f t f s 2 0 25 "25 16" _null_ _null_ _null_ _null_ _null_ show_config_by_name_missing_ok _null_ _null_ _null_ ));
+DESCR("SHOW X as a function, optionally no error for missing variable");
 DATA(insert OID = 2078 (  set_config		PGNSP PGUID 12 1 0 0 0 f f f f f f v 3 0 25 "25 25 16" _null_ _null_ _null_ _null_ _null_ set_config_by_name _null_ _null_ _null_ ));
 DESCR("SET X as a function");
 DATA(insert OID = 2084 (  pg_show_all_settings	PGNSP PGUID 12 1 1000 0 0 f f f f t t s 0 0 2249 "" "{25,25,25,25,25,25,25,25,25,25,25,1009,25,25,25,23,16}" "{o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o}" "{name,setting,unit,category,short_desc,extra_desc,context,vartype,source,min_val,max_val,enumvals,boot_val,reset_val,sourcefile,sourceline,pending_restart}" _null_ _null_ show_all_settings _null_ _null_ _null_ ));

src/include/utils/builtins.h

@@ -1114,6 +1114,7 @@ extern Datum quote_nullable(PG_FUNCTION_ARGS);
 
 /* guc.c */
 extern Datum show_config_by_name(PG_FUNCTION_ARGS);
+extern Datum show_config_by_name_missing_ok(PG_FUNCTION_ARGS);
 extern Datum set_config_by_name(PG_FUNCTION_ARGS);
 extern Datum show_all_settings(PG_FUNCTION_ARGS);
 extern Datum show_all_file_settings(PG_FUNCTION_ARGS);

src/include/utils/guc.h

@@ -365,7 +365,8 @@ extern int set_config_option(const char *name, const char *value,
 				  GucAction action, bool changeVal, int elevel,
 				  bool is_reload);
 extern void AlterSystemSetConfigFile(AlterSystemStmt *setstmt);
-extern char *GetConfigOptionByName(const char *name, const char **varname);
+extern char *GetConfigOptionByName(const char *name, const char **varname,
+					  bool missing_ok);
 extern void GetConfigOptionByNum(int varnum, const char **values, bool *noshow);
 extern int	GetNumConfigOptions(void);
 

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