Support table errors.

LuaSocket wraps error messages raised by newtry() in a table and unpacks
them later so that (string) errors raised by 3rd-party code can be
passed through as-is. This obviously didn't work when the 3rd-party code
raised a table as an error message. This change sets a private metatable
on all wrapped LuaSocket exceptions to distinguish them from 3rd-party
table errors.

src/except.c

@@ -12,7 +12,7 @@
 
 #if LUA_VERSION_NUM < 502
 #define lua_pcallk(L, na, nr, err, ctx, cont) \
-    ((void)ctx,(void)cont,lua_pcall(L, na, nr, err))
+    (((void)ctx),((void)cont),lua_pcall(L, na, nr, err))
 #endif
 
 #if LUA_VERSION_NUM < 503
@@ -39,12 +39,11 @@ static luaL_Reg func[] = {
 * Try factory
 \*-------------------------------------------------------------------------*/
 static void wrap(lua_State *L) {
-    lua_newtable(L);
-    lua_pushnumber(L, 1);
-    lua_pushvalue(L, -3);
-    lua_settable(L, -3);
-    lua_insert(L, -2);
-    lua_pop(L, 1);
+    lua_createtable(L, 1, 0);
+    lua_pushvalue(L, -2);
+    lua_rawseti(L, -2, 1);
+    lua_pushvalue(L, lua_upvalueindex(2));
+    lua_setmetatable(L, -2);
 }
 
 static int finalize(lua_State *L) {
@@ -58,15 +57,16 @@ static int finalize(lua_State *L) {
     } else return lua_gettop(L);
 }
 
-static int do_nothing(lua_State *L) { 
+static int do_nothing(lua_State *L) {
     (void) L;
-    return 0; 
+    return 0;
 }
 
 static int global_newtry(lua_State *L) {
     lua_settop(L, 1);
     if (lua_isnil(L, 1)) lua_pushcfunction(L, do_nothing);
-    lua_pushcclosure(L, finalize, 1);
+    lua_pushvalue(L, lua_upvalueindex(1));
+    lua_pushcclosure(L, finalize, 2);
     return 1;
 }
 
@@ -74,13 +74,16 @@ static int global_newtry(lua_State *L) {
 * Protect factory
 \*-------------------------------------------------------------------------*/
 static int unwrap(lua_State *L) {
-    if (lua_istable(L, -1)) {
-        lua_pushnumber(L, 1);
-        lua_gettable(L, -2);
-        lua_pushnil(L);
-        lua_insert(L, -2);
-        return 1;
-    } else return 0;
+    if (lua_istable(L, -1) && lua_getmetatable(L, -1)) {
+        int r = lua_rawequal(L, -1, lua_upvalueindex(2));
+        lua_pop(L, 1);
+        if (r) {
+            lua_pushnil(L);
+            lua_rawgeti(L, -2, 1);
+            return 1;
+        }
+    }
+    return 0;
 }
 
 static int protected_finish(lua_State *L, int status, lua_KContext ctx) {
@@ -110,7 +113,9 @@ static int protected_(lua_State *L) {
 }
 
 static int global_protect(lua_State *L) {
-    lua_pushcclosure(L, protected_, 1);
+    lua_settop(L, 1);
+    lua_pushvalue(L, lua_upvalueindex(1));
+    lua_pushcclosure(L, protected_, 2);
     return 1;
 }
 
@@ -118,6 +123,9 @@ static int global_protect(lua_State *L) {
 * Init module
 \*-------------------------------------------------------------------------*/
 int except_open(lua_State *L) {
-    luaL_setfuncs(L, func, 0);
+    lua_newtable(L); /* metatable for wrapped exceptions */
+    lua_pushboolean(L, 0);
+    lua_setfield(L, -2, "__metatable");
+    luaL_setfuncs(L, func, 1);
     return 0;
 }