Update comments concerning PGC_S_TEST.

This GUC context value was once only used by ALTER DATABASE SET and
ALTER USER SET.  That's not true anymore, though, so rewrite the
comments to be a bit more general.

Patch in HEAD only, since this is just an internal documentation issue.

src/backend/commands/tablespace.c

@@ -977,12 +977,8 @@ check_default_tablespace(char **newval, void **extra, GucSource source)
 			!OidIsValid(get_tablespace_oid(*newval, true)))
 		{
 			/*
-			 * When source == PGC_S_TEST, we are checking the argument of an
-			 * ALTER DATABASE SET or ALTER USER SET command.  pg_dumpall dumps
-			 * all roles before tablespaces, so if we're restoring a
-			 * pg_dumpall script the tablespace might not yet exist, but will
-			 * be created later.  Because of that, issue a NOTICE if source ==
-			 * PGC_S_TEST, but accept the value anyway.
+			 * When source == PGC_S_TEST, don't throw a hard error for a
+			 * nonexistent tablespace, only a NOTICE.  See comments in guc.h.
 			 */
 			if (source == PGC_S_TEST)
 			{
@@ -1112,13 +1108,8 @@ check_temp_tablespaces(char **newval, void **extra, GucSource source)
 
 			/*
 			 * In an interactive SET command, we ereport for bad info.  When
-			 * source == PGC_S_TEST, we are checking the argument of an ALTER
-			 * DATABASE SET or ALTER USER SET command.	pg_dumpall dumps all
-			 * roles before tablespaces, so if we're restoring a pg_dumpall
-			 * script the tablespace might not yet exist, but will be created
-			 * later.  Because of that, issue a NOTICE if source ==
-			 * PGC_S_TEST, but accept the value anyway.  Otherwise, silently
-			 * ignore any bad list elements.
+			 * source == PGC_S_TEST, don't throw a hard error for a
+			 * nonexistent tablespace, only a NOTICE.  See comments in guc.h.
 			 */
 			curoid = get_tablespace_oid(curname, source <= PGC_S_TEST);
 			if (curoid == InvalidOid)

src/backend/utils/cache/ts_cache.c

@@ -604,11 +604,8 @@ check_TSCurrentConfig(char **newval, void **extra, GucSource source)
 		cfgId = get_ts_config_oid(stringToQualifiedNameList(*newval), true);
 
 		/*
-		 * When source == PGC_S_TEST, we are checking the argument of an ALTER
-		 * DATABASE SET or ALTER USER SET command.	It could be that the
-		 * intended use of the setting is for some other database, so we
-		 * should not error out if the text search configuration is not
-		 * present in the current database.  We issue a NOTICE instead.
+		 * When source == PGC_S_TEST, don't throw a hard error for a
+		 * nonexistent configuration, only a NOTICE.  See comments in guc.h.
 		 */
 		if (!OidIsValid(cfgId))
 		{

src/include/utils/guc.h

@@ -72,11 +72,15 @@ typedef enum
  * dividing line between "interactive" and "non-interactive" sources for
  * error reporting purposes.
  *
- * PGC_S_TEST is used when testing values to be stored as per-database or
- * per-user defaults ("doit" will always be false, so this never gets stored
- * as the actual source of any value).	This is an interactive case, but
- * it needs its own source value because some assign hooks need to make
- * different validity checks in this case.
+ * PGC_S_TEST is used when testing values to be used later ("doit" will always
+ * be false, so this never gets stored as the actual source of any value).
+ * For example, ALTER DATABASE/ROLE tests proposed per-database or per-user
+ * defaults this way, and CREATE FUNCTION tests proposed function SET clauses
+ * this way.  This is an interactive case, but it needs its own source value
+ * because some assign hooks need to make different validity checks in this
+ * case.  In particular, references to nonexistent database objects generally
+ * shouldn't throw hard errors in this case, at most NOTICEs, since the
+ * objects might exist by the time the setting is used for real.
  *
  * NB: see GucSource_Names in guc.c if you change this.
  */