fix(open): return an error on open/new

The method would not return an error message, now it does,
typically if a conversion is not supported.

README.md

@@ -55,14 +55,29 @@ call to load up the library (that, of course, must be installed in a directory f
 ## API documentation
 
 ```lua
-cd = iconv.new(to, from)
-cd = iconv.open(to, from)
+cd, err = iconv.new(to, from)
+cd, err = iconv.open(to, from)
 ```
 
 Opens a new conversion descriptor, from the 'from' charset to the 'to' charset.
 Concatenating "//TRANSLIT" to the first argument will enable character transliteration and concatenating "//IGNORE" to the first argument will cause iconv to ignore any invalid characters found in the input string.
 
-This function returns a new converter or nil on error.
+The error code, may have any of the following values:
+
+* `nil`
+
+    No error. Creation was successful.
+
+* `iconv.ERROR_INVALID`
+
+    The conversion from `from` to `to` is not supported by the implementation.
+
+* `iconv.ERROR_UNKNOWN`
+
+    There was an unknown error.
+
+
+This function returns a new converter or nil+err on error.
 
 
 ```lua

luaiconv.c

@@ -92,11 +92,17 @@ static int Liconv_open(lua_State *L)
     const char *tocode = luaL_checkstring(L, 1);
     const char *fromcode = luaL_checkstring(L, 2);
     iconv_t cd = iconv_open(tocode, fromcode);
-    if (cd != (iconv_t)(-1))
+    if (cd != (iconv_t)(-1)) {
         push_iconv_t(L, cd);    /* ok */
-    else
-        lua_pushnil(L);         /* error */
         return 1;
+    } else {
+        lua_pushnil(L);         /* error */
+        if (errno == EINVAL)
+            lua_pushnumber(L, ERROR_INVALID);
+        else
+            lua_pushnumber(L, ERROR_UNKNOWN);
+        return 2;
+    };
 }
 
 /* Use a fixed-size buffer in the stack to avoid a lot of small mallocs
@@ -207,7 +213,7 @@ static int Liconv_close(lua_State *L)
         lua_pushboolean(L, 1);  /* ok */
     }
     else
-        lua_pushnil(L);         /* error */
+        lua_pushnil(L);         /* error, called from __gc, so no need for error messsage */
     return 1;
 }
 

test_iconv.lua

@@ -76,20 +76,30 @@ local ebcdic = "\193\150\64\147\150\149\135\133\107\64\129\150\64\147\164\129"
 .. "\37"
 
 
+local function ErrMsg(errno)
+  if errno == iconv.ERROR_INCOMPLETE then
+    return "Incomplete input."
+  elseif errno == iconv.ERROR_INVALID then
+    return "Invalid input."
+  elseif errno == iconv.ERROR_NO_MEMORY then
+    return "Failed to allocate memory."
+  elseif errno == iconv.ERROR_UNKNOWN then
+    return "There was an unknown error."
+  elseif errno == iconv.FINALIZED then
+    return "Handle was already finalized."
+  end
+  return "Unknown error: "..tostring(errno)
+end
+
+
 local function check_one(to, from, text)
   print("\n-- Testing conversion from " .. from .. " to " .. to)
-  local cd = iconv.new(to .. "//TRANSLIT", from)
-  assert(cd, "Failed to create a converter object.")
+  local cd, errno = iconv.new(to .. "//TRANSLIT", from)
+  assert(cd, "Failed to create a converter object: " .. ErrMsg(errno))
   local ostr, err = cd:iconv(text)
 
-  if err == iconv.ERROR_INCOMPLETE then
-    print("ERROR: Incomplete input.")
-  elseif err == iconv.ERROR_INVALID then
-    print("ERROR: Invalid input.")
-  elseif err == iconv.ERROR_NO_MEMORY then
-    print("ERROR: Failed to allocate memory.")
-  elseif err == iconv.ERROR_UNKNOWN then
-    print("ERROR: There was an unknown error.")
+  if err then
+    print("ERROR: " .. ErrMsg(err))
   end
   print(ostr)
 end