LibertyXDR: add support for default in unions

Have a common way of catching Lua errors for resource cleanup purposes.

libertyxdr.adoc

@@ -93,10 +93,12 @@ and always-present field, which must be a tag *enum*:
  case CAR:   void;
  case LORRY: i8 axles;
  case PLANE: i8 engines;
+ default:    void;
  };
 
-All possible enumeration values must be named, and there is no *case*
-fall-through.
+There is no *case* fall-through.
+Unless *default* is present, only the listed enumeration values are valid.
+Any *default* must currently be empty.
 
 Framing
 -------

tests/lxdrgen.lxdr

@@ -25,5 +25,7 @@ struct Struct {
 	union Onion switch (Enum tag) {
 	case NOTHING:
 		void;
+	default:
+		void;
 	} o;
 };

tools/lxdrgen.awk

@@ -1,6 +1,6 @@
 # lxdrgen.awk: an XDR-derived code generator for network protocols.
 #
-# Copyright (c) 2022 - 2023, Přemysl Eric Janouch <p@janouch.name>
+# Copyright (c) 2022 - 2025, Přemysl Eric Janouch <p@janouch.name>
 # SPDX-License-Identifier: 0BSD
 #
 # Usage: env LC_ALL=C awk -f lxdrgen.awk -f lxdrgen-{c,go,mjs}.awk \
@@ -218,7 +218,7 @@ function defstruct(    name, d, cg) {
 }
 
 function defunion(    name, tag, tagtype, tagvalue, cg, scg, d, a, i,
-		unseen, exhaustive) {
+		unseen, defaulted, exhaustive) {
 	delete cg[0]
 	delete scg[0]
 	delete d[0]
@@ -249,9 +249,22 @@ function defunion(    name, tag, tagtype, tagvalue, cg, scg, d, a, i,
 			if (!unseen[tagvalue]--)
 				fatal("no such value or duplicate case: " tagtype "." tagvalue)
 			codegen_struct_tag(tag, scg)
+		} else if (accept("default")) {
+			if (tagvalue)
+				codegen_union_struct(name, tagvalue, cg, scg)
+
+			expect(accept(":"))
+			if (defaulted)
+				fatal("duplicate default")
+
+			tagvalue = ""
+			defaulted = 1
 		} else if (tagvalue) {
 			if (readfield(d))
 				codegen_struct_field(d, scg)
+		} else if (defaulted) {
+			if (readfield(d))
+				fatal("default must not contain fields")
 		} else {
 			fatal("union fields must fall under a case")
 		}
@@ -259,11 +272,17 @@ function defunion(    name, tag, tagtype, tagvalue, cg, scg, d, a, i,
 	if (tagvalue)
 		codegen_union_struct(name, tagvalue, cg, scg)
 
-	# Unseen cases are simply not recognized/allowed.
+	# Unseen cases are only recognized/allowed when default is present.
 	exhaustive = 1
 	for (i in unseen)
-		if (i && unseen[i])
-			exhaustive = 0
+		if (i && unseen[i]) {
+			if (defaulted) {
+				codegen_struct_tag(tag, scg)
+				codegen_union_struct(name, i, cg, scg)
+			} else {
+				exhaustive = 0
+			}
+		}
 
 	Types[name] = "union"
 	codegen_union(name, cg, exhaustive)

tools/wdye/wdye.c

@@ -890,10 +890,11 @@ environ_map_serialize (struct str_map *env, struct strv *envv)
 static int
 spawn_protected (lua_State *L)
 {
-	struct spawn_context *ctx = lua_touserdata (L, 1);
+	struct spawn_context *ctx = lua_touserdata (L, lua_upvalueindex (1));
+	luaL_checktype (L, 1, LUA_TTABLE);
 
 	// Step 1: Prepare process environment.
-	if (xlua_getfield (L, 2, "environ", LUA_TTABLE, true))
+	if (xlua_getfield (L, 1, "environ", LUA_TTABLE, true))
 	{
 		environ_map_update (&ctx->env, L);
 		lua_pop (L, 1);
@@ -913,11 +914,11 @@ spawn_protected (lua_State *L)
 #endif
 
 	// Step 3: Prepare process command line.
-	size_t argc = lua_rawlen (L, 2);
+	size_t argc = lua_rawlen (L, 1);
 	for (size_t i = 1; i <= argc; i++)
 	{
 		lua_pushinteger (L, i);
-		lua_rawget (L, 2);
+		lua_rawget (L, 1);
 		const char *arg = lua_tostring (L, -1);
 		if (!arg)
 			return luaL_error (L, "spawn arguments must be strings");
@@ -1004,23 +1005,24 @@ spawn_protected (lua_State *L)
 static int
 xlua_spawn (lua_State *L)
 {
-	luaL_checktype (L, 1, LUA_TTABLE);
-
 	lua_pushcfunction (L, xlua_error_handler);
-	lua_pushcfunction (L, spawn_protected);
+	lua_insert (L, 1);
+
+	struct spawn_context ctx = {};
+	lua_pushlightuserdata (L, &ctx);
+	lua_pushcclosure (L, spawn_protected, 1);
+	lua_insert (L, 2);
 
 	// There are way too many opportunities for Lua to throw,
 	// so maintain a context to clean up in one go.
-	struct spawn_context ctx = spawn_context_make ();
-	lua_pushlightuserdata (L, &ctx);
-	lua_rotate (L, 1, -1);
-	int result = lua_pcall (L, 2, 1, -4);
+	ctx = spawn_context_make ();
+	int result = lua_pcall (L, lua_gettop (L) - 2, 1, 1);
 	spawn_context_free (&ctx);
 	if (result)
 		return lua_error (L);
 
 	// Remove the error handler ("good programming practice").
-	lua_remove (L, -2);
+	lua_remove (L, 1);
 	return 1;
 }