Add allow_alter_system GUC.

This is marked PGC_SIGHUP, so it can only be set in a configuration
file, not anywhere else; and it is also marked GUC_DISALLOW_IN_AUTO_FILE,
so it can't be set using ALTER SYSTEM. When set to false, the
ALTER SYSTEM command is disallowed.

There was considerable concern that this would be misinterpreted as
a security feature, which it is not, because a determined superuser
has various ways of bypassing it. Hence, a lot of work has gone into
wordsmithing the documentation, in the hopes of avoiding any such
confusion.

Jelte Fennemia-Nio and Gabriele Bartolini, with wording suggestions
for the documentation from many others.

Discussion: http://postgr.es/m/CA%2BVUV5rEKt2%2BCdC_KUaPoihMu%2Bi5ChT4WVNTr4CD5-xXZUfuQw%40mail.gmail.com

src/backend/utils/misc/guc.c

@@ -4563,6 +4563,11 @@ AlterSystemSetConfigFile(AlterSystemStmt *altersysstmt)
 	 */
 	name = altersysstmt->setstmt->name;
 
+	if (!AllowAlterSystem)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("ALTER SYSTEM is not allowed in this environment")));
+
 	switch (altersysstmt->setstmt->kind)
 	{
 		case VAR_SET_VALUE:

src/backend/utils/misc/guc_tables.c

@@ -494,6 +494,7 @@ extern const struct config_enum_entry dynamic_shared_memory_options[];
 /*
  * GUC option variables that are exported from this module
  */
+bool		AllowAlterSystem = true;
 bool		log_duration = false;
 bool		Debug_print_plan = false;
 bool		Debug_print_parse = false;
@@ -1040,6 +1041,22 @@ struct config_bool ConfigureNamesBool[] =
 		false,
 		NULL, NULL, NULL
 	},
+	{
+		/*
+		 * This setting itself cannot be set by ALTER SYSTEM to avoid an
+		 * operator turning this setting off by using ALTER SYSTEM, without a
+		 * way to turn it back on.
+		 */
+		{"allow_alter_system", PGC_SIGHUP, COMPAT_OPTIONS_OTHER,
+			gettext_noop("Allows running the ALTER SYSTEM command."),
+			gettext_noop("Can be set to off for environments where global configuration "
+						 "changes should be made using a different method."),
+			GUC_DISALLOW_IN_AUTO_FILE
+		},
+		&AllowAlterSystem,
+		true,
+		NULL, NULL, NULL
+	},
 	{
 		{"bonjour", PGC_POSTMASTER, CONN_AUTH_SETTINGS,
 			gettext_noop("Enables advertising the server via Bonjour."),

src/backend/utils/misc/postgresql.conf.sample

@@ -805,6 +805,7 @@
 # - Other Platforms and Clients -
 
 #transform_null_equals = off
+#allow_alter_system = on
 
 
 #------------------------------------------------------------------------------