Add backwards compatibility wrappers for socket.unix

Add backwards compatibility aliases "tcp" and "udp" for the recently renamed
"stream" and "dgram" functions, as well as a wrapper function and metatable
setup so that socket.unix() calls socket.unix.stream().

src/unix.c

@@ -17,14 +17,54 @@ static const luaL_Reg mod[] = {
     {NULL, NULL}
 };
 
+static void add_alias(lua_State *L, int index, const char *name, const char *target)
+{
+    lua_getfield(L, index, target);
+    lua_setfield(L, index, name);
+}
+
+static int compat_socket_unix_call(lua_State *L)
+{
+    /* Look up socket.unix.stream in the socket.unix table (which is the first
+     * argument). */
+    lua_getfield(L, 1, "stream");
+
+    /* Replace the stack entry for the socket.unix table with the
+     * socket.unix.stream function. */
+    lua_replace(L, 1);
+
+    /* Call socket.unix.stream, passing along any arguments. */
+    int n = lua_gettop(L);
+    lua_call(L, n-1, LUA_MULTRET);
+
+    /* Pass along the return values from socket.unix.stream. */
+    n = lua_gettop(L);
+    return n;
+}
+
 /*-------------------------------------------------------------------------*\
 * Initializes module
 \*-------------------------------------------------------------------------*/
 int luaopen_socket_unix(lua_State *L)
 {
-	int i;
-	lua_newtable(L);
-    for (i = 0; mod[i].name; i++) mod[i].func(L);
-	return 1;
-}
+    int i;
+    lua_newtable(L);
+    int socket_unix_table = lua_gettop(L);
 
+    for (i = 0; mod[i].name; i++)
+        mod[i].func(L);
+
+    /* Add backwards compatibility aliases "tcp" and "udp" for the "stream" and
+     * "dgram" functions. */
+    add_alias(L, socket_unix_table, "tcp", "stream");
+    add_alias(L, socket_unix_table, "udp", "dgram");
+
+    /* Add a backwards compatibility function and a metatable setup to call it
+     * for the old socket.unix() interface. */
+    lua_pushcfunction(L, compat_socket_unix_call);
+    lua_setfield(L, socket_unix_table, "__call");
+    lua_pushvalue(L, socket_unix_table);
+    lua_setmetatable(L, socket_unix_table);
+
+    return 1;
+}