[PATCH] Add two-arg for of current_setting(NAME, FALLBACK)

The two-arg form of the current_setting() function will allow a
fallback value to be returned instead of throwing an error when an
unknown GUC is provided. This would come in most useful when using
custom GUCs; e.g.:

-- errors out if the 'foo.bar' setting is unset
SELECT current_setting('foo.bar');

-- returns current setting of foo.bar, or 'default' if not set
SELECT current_setting('foo.bar', 'default')

This would save you having to wrap the use of the function in an
exception block just to catch and utilize a default setting value
within a function.
---
src/backend/utils/misc/guc.c | 50 ++++++++++++++++++++++++++++++
++++++---
src/include/catalog/pg_proc.h | 2 ++
src/include/utils/builtins.h | 1 +
src/include/utils/guc.h | 1 +
src/test/regress/expected/guc.out | 19 +++++++++++++++
src/test/regress/sql/guc.sql | 12 ++++++++++
6 files changed, 82 insertions(+), 3 deletions(-)

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 26275bd..002a926 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -7703,13 +7703,32 @@ ShowAllGUCConfig(DestReceiver *dest)
char *
GetConfigOptionByName(const char *name, const char **varname)
{
+       return GetConfigOptionByNameFallback(name, NULL, varname);
+}
+
+/*
+ * Return GUC variable value by name; optionally return canonical form of
+ * name.  If GUC is NULL then optionally return a fallback value instead
of an
+ * error.  Return value is palloc'd.
+ */
+char *
+GetConfigOptionByNameFallback(const char *name, const char
*default_value, const char **varname)
+{
struct config_generic *record;

record = find_option(name, false, ERROR);
if (record == NULL)
-               ereport(ERROR,
-                               (errcode(ERRCODE_UNDEFINED_OBJECT),
-                          errmsg("unrecognized configuration parameter
\"%s\"", name)));
+       {
+               if (default_value) {
+                       return pstrdup(default_value);
+               }
+               else
+               {
+                       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),
@@ -8008,6 +8027,31 @@ show_config_by_name(PG_FUNCTION_ARGS)
}

/*
+ * show_config_by_name_fallback - equiv to SHOW X command but implemented
as
+ * a function.  If X does not exist, return a fallback datum instead of
erroring
+ */
+Datum
+show_config_by_name_fallback(PG_FUNCTION_ARGS)
+{
+       char       *varname;
+       char       *varfallback;
+       char       *varval;
+
+       /* Get the GUC variable name */
+       varname = TextDatumGetCString(PG_GETARG_DATUM(0));
+
+       /* Get the fallback value */
+       varfallback = TextDatumGetCString(PG_GETARG_DATUM(1));
+
+       /* Get the value */
+       varval = GetConfigOptionByNameFallback(varname, varfallback,
NULL);
+
+       /* Convert to text */
+       PG_RETURN_TEXT_P(cstring_to_text(varval));
+}
+
+
+/*
* show_all_settings - equiv to SHOW ALL command but implemented as
* a Table Function.
*/
diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index 6a757f3..71efed2 100644
--- a/src/include/catalog/pg_proc.h
+++ b/src/include/catalog/pg_proc.h
@@ -3025,6 +3025,8 @@ 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_ show_config_by_name
_null_ _null_ _null_ ));
DESCR("SHOW X as a function");
+DATA(insert OID = 3280 (  current_setting      PGNSP PGUID 12 1 0 0 0 f f
f f t f s 2 0 25 "25 25" _null_ _null_ _null_ _null_
show_config_by_name_fallback _null_ _null_ _null_ ));
+DESCR("SHOW X as a function");
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_ 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}"
"{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}" _null_
show_all_settings _null_ _null_ _null_ ));
diff --git a/src/include/utils/builtins.h b/src/include/utils/builtins.h
index bc4517d..e3d9fbb 100644
--- a/src/include/utils/builtins.h
+++ b/src/include/utils/builtins.h
@@ -1088,6 +1088,7 @@ extern Datum quote_nullable(PG_FUNCTION_ARGS);

/* guc.c */
extern Datum show_config_by_name(PG_FUNCTION_ARGS);
+extern Datum show_config_by_name_fallback(PG_FUNCTION_ARGS);
extern Datum set_config_by_name(PG_FUNCTION_ARGS);
extern Datum show_all_settings(PG_FUNCTION_ARGS);

diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index d3100d1..145ad5d 100644
--- a/src/include/utils/guc.h
+++ b/src/include/utils/guc.h
@@ -352,6 +352,7 @@ extern int set_config_option(const char *name, const
char *value,
bool is_reload);
extern void AlterSystemSetConfigFile(AlterSystemStmt *setstmt);
extern char *GetConfigOptionByName(const char *name, const char
**varname);
+extern char *GetConfigOptionByNameFallback(const char *name, const char
*fallback, const char **varname);
extern void GetConfigOptionByNum(int varnum, const char **values, bool
*noshow);
extern int     GetNumConfigOptions(void);

diff --git a/src/test/regress/expected/guc.out
b/src/test/regress/expected/guc.out
index 4f0065c..905969b 100644
--- a/src/test/regress/expected/guc.out
+++ b/src/test/regress/expected/guc.out
@@ -736,3 +736,22 @@ NOTICE:  text search configuration "no_such_config"
does not exist
select func_with_bad_set();
ERROR:  invalid value for parameter "default_text_search_config":
"no_such_config"
reset check_function_bodies;
+-- check multi-arg custom current_setting
+-- this should error:
+select current_setting('nosuch.setting');
+ERROR:  unrecognized configuration parameter "nosuch.setting"
+-- this should return 'fallback'
+select current_setting('nosuch.setting','fallback');
+ current_setting
+-----------------
+ fallback
+(1 row)
+
+-- this should return 'nada', not 'fallback'
+set nosuch.setting = 'nada';
+select current_setting('nosuch.setting','fallback');
+ current_setting
+-----------------
+ nada
+(1 row)
+
diff --git a/src/test/regress/sql/guc.sql b/src/test/regress/sql/guc.sql
index 3de8a6b..48c0bed 100644
--- a/src/test/regress/sql/guc.sql
+++ b/src/test/regress/sql/guc.sql
@@ -275,3 +275,15 @@ set default_text_search_config = no_such_config;
select func_with_bad_set();

reset check_function_bodies;
+
+-- check multi-arg custom current_setting
+
+-- this should error:
+select current_setting('nosuch.setting');
+
+-- this should return 'fallback'
+select current_setting('nosuch.setting','fallback');
+
+-- this should return 'nada', not 'fallback'
+set nosuch.setting = 'nada';
+select current_setting('nosuch.setting','fallback');
--
2.3.3

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