Issue 5216 - [emscripten] Use a more reliable way of retrieving mangled function name

compiled/bindings/library.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2017 eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 "use strict";
 
 mergeInto(LibraryManager.library, {
   GetFunctionName__asm: true,
-  GetFunctionName__sig: ["iii"],

[Wladimir Palant, 2017/05/08 08:53:57]

This signature is actually wrong, it should have been "viii". It works
regardless, so it seems that the signatures specified here aren't important for
our use case. I removed them.

[sergei, 2017/05/08 09:16:29]

Wouldn't it be better to completely remove GetFunctionName__sig instead of
defining it as an empty array? BTW, in the examples in the repo they use string
for the signature, e.g. 'viii' instead of array.

[Wladimir Palant, 2017/05/08 10:17:18]

This entry is required, GetFunctionName__asm will be ignored otherwise. The
default library has both arrays and strings here. However, from what I saw in
the source code so far it appears that the value is actually unused. So maybe
worth filing an Emscripten issue on it.

+  GetFunctionName__sig: [],
   GetFunctionName: function(buffer, ptr, signature)
   {
     var table = eval("FUNCTION_TABLE_" + AsciiToString(signature));
     var name = table[HEAP32[ptr >> 2] % table.length].name;
-    Module.stringToAscii(name, buffer);

[Wladimir Palant, 2017/05/08 08:53:57]

This function doesn't need to be called via Module in this context, it can be
accessed directly.

Also, this is a potential memory safety issue as buffer size isn't checked -
better fix this while we are changing the code anyway.

+    if (buffer)
+      writeAsciiToMemory(name, buffer, true);
+    return name.length;
   }
 });