Improve define_custom_variable's handling of pre-existing settings.

Arrange for any problems with pre-existing settings to be reported as
WARNING not ERROR, so that we don't undesirably abort the loading of the
incoming add-on module.  The bad setting is just discarded, as though it
had never been applied at all.  (This requires a change in the API of
set_config_option.  After some thought I decided the most potentially
useful addition was to allow callers to just pass in a desired elevel.)

Arrange to restore the complete stacked state of the variable, rather than
cheesily reinstalling only the active value.  This ensures that custom GUCs
will behave unsurprisingly even when the module loading operation occurs
within nested subtransactions that have changed the active value.  Since a
module load could occur as a result of, eg, a PL function call, this is not
an unlikely scenario.

contrib/tsearch2/tsearch2.c

@@ -253,11 +253,8 @@ tsa_set_curcfg(PG_FUNCTION_ARGS)
 	name = DatumGetCString(DirectFunctionCall1(regconfigout,
 											   ObjectIdGetDatum(arg0)));
 
-	set_config_option("default_text_search_config", name,
-					  PGC_USERSET,
-					  PGC_S_SESSION,
-					  GUC_ACTION_SET,
-					  true);
+	SetConfigOption("default_text_search_config", name,
+					PGC_USERSET, PGC_S_SESSION);
 
 	PG_RETURN_VOID();
 }
@@ -271,11 +268,8 @@ tsa_set_curcfg_byname(PG_FUNCTION_ARGS)
 
 	name = text_to_cstring(arg0);
 
-	set_config_option("default_text_search_config", name,
-					  PGC_USERSET,
-					  PGC_S_SESSION,
-					  GUC_ACTION_SET,
-					  true);
+	SetConfigOption("default_text_search_config", name,
+					PGC_USERSET, PGC_S_SESSION);
 
 	PG_RETURN_VOID();
 }

src/backend/commands/extension.c

@@ -816,14 +816,14 @@ execute_extension_script(Oid extensionOid, ExtensionControlFile *control,
 	if (client_min_messages < WARNING)
 		(void) set_config_option("client_min_messages", "warning",
 								 PGC_USERSET, PGC_S_SESSION,
-								 GUC_ACTION_LOCAL, true);
+								 GUC_ACTION_LOCAL, true, 0);
 
 	save_log_min_messages =
 		pstrdup(GetConfigOption("log_min_messages", false, false));
 	if (log_min_messages < WARNING)
 		(void) set_config_option("log_min_messages", "warning",
 								 PGC_SUSET, PGC_S_SESSION,
-								 GUC_ACTION_LOCAL, true);
+								 GUC_ACTION_LOCAL, true, 0);
 
 	/*
 	 * Set up the search path to contain the target schema, then the schemas
@@ -849,7 +849,7 @@ execute_extension_script(Oid extensionOid, ExtensionControlFile *control,
 
 	(void) set_config_option("search_path", pathbuf.data,
 							 PGC_USERSET, PGC_S_SESSION,
-							 GUC_ACTION_LOCAL, true);
+							 GUC_ACTION_LOCAL, true, 0);
 
 	/*
 	 * Set creating_extension and related variables so that
@@ -915,13 +915,13 @@ execute_extension_script(Oid extensionOid, ExtensionControlFile *control,
 	 */
 	(void) set_config_option("search_path", save_search_path,
 							 PGC_USERSET, PGC_S_SESSION,
-							 GUC_ACTION_LOCAL, true);
+							 GUC_ACTION_LOCAL, true, 0);
 	(void) set_config_option("client_min_messages", save_client_min_messages,
 							 PGC_USERSET, PGC_S_SESSION,
-							 GUC_ACTION_LOCAL, true);
+							 GUC_ACTION_LOCAL, true, 0);
 	(void) set_config_option("log_min_messages", save_log_min_messages,
 							 PGC_SUSET, PGC_S_SESSION,
-							 GUC_ACTION_LOCAL, true);
+							 GUC_ACTION_LOCAL, true, 0);
 }
 
 /*

src/backend/utils/adt/ri_triggers.c

@@ -2779,7 +2779,7 @@ RI_Initial_Check(Trigger *trigger, Relation fk_rel, Relation pk_rel)
 	snprintf(workmembuf, sizeof(workmembuf), "%d", maintenance_work_mem);
 	(void) set_config_option("work_mem", workmembuf,
 							 PGC_USERSET, PGC_S_SESSION,
-							 GUC_ACTION_LOCAL, true);
+							 GUC_ACTION_LOCAL, true, 0);
 
 	if (SPI_connect() != SPI_OK_CONNECT)
 		elog(ERROR, "SPI_connect failed");
@@ -2868,7 +2868,7 @@ RI_Initial_Check(Trigger *trigger, Relation fk_rel, Relation pk_rel)
 	snprintf(workmembuf, sizeof(workmembuf), "%d", old_work_mem);
 	(void) set_config_option("work_mem", workmembuf,
 							 PGC_USERSET, PGC_S_SESSION,
-							 GUC_ACTION_LOCAL, true);
+							 GUC_ACTION_LOCAL, true, 0);
 
 	return true;
 }

src/backend/utils/misc/guc-file.l

@@ -238,7 +238,7 @@ ProcessConfigFile(GucContext context)
 		/* Now we can re-apply the wired-in default (i.e., the boot_val) */
 		if (set_config_option(gconf->name, NULL,
 							  context, PGC_S_DEFAULT,
-							  GUC_ACTION_SET, true) > 0)
+							  GUC_ACTION_SET, true, 0) > 0)
 		{
 			/* Log the change if appropriate */
 			if (context == PGC_SIGHUP)
@@ -293,7 +293,7 @@ ProcessConfigFile(GucContext context)
 
 		scres = set_config_option(item->name, item->value,
 								  context, PGC_S_FILE,
-								  GUC_ACTION_SET, true);
+								  GUC_ACTION_SET, true, 0);
 		if (scres > 0)
 		{
 			/* variable was updated, so log the change if appropriate */

src/include/utils/guc.h

@@ -315,7 +315,7 @@ extern bool parse_int(const char *value, int *result, int flags,
 extern bool parse_real(const char *value, double *result);
 extern int	set_config_option(const char *name, const char *value,
 				  GucContext context, GucSource source,
-				  GucAction action, bool changeVal);
+				  GucAction action, bool changeVal, int elevel);
 extern char *GetConfigOptionByName(const char *name, const char **varname);
 extern void GetConfigOptionByNum(int varnum, const char **values, bool *noshow);
 extern int	GetNumConfigOptions(void);