Issue 5183 - Provide async interface for FileSystem

include/AdblockPlus/FileSystem.h

 /*
  * 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
@@ skipping 20 matching lines @@
    * File system interface.
    */
   class FileSystem
   {
   public:
     virtual ~FileSystem() {}
 
     /**
      * Reads from a file.
      * @param path File path.
-     * @return Input stream with the file's contents.
+     * @return Buffer with the file content.
      */
-    virtual std::shared_ptr<std::istream>
-      Read(const std::string& path) const = 0;
+    virtual IFileSystem::IOBuffer Read(const std::string& path) const = 0;
 
     /**
      * Writes to a file.
      * @param path File path.
-     * @param data Output stream with the data to write.
+     * @param data Buffer with the data to write.
      */
     virtual void Write(const std::string& path,
-                       std::ostream& data) = 0;

[sergei, 2017/07/06 14:14:04]

What do you think about making it in a separate commit?
I think that maybe we should also switch to buffers even here.

[hub, 2017/07/06 14:33:31]

This is needed because we write to that stream buffer before passing it here.
There is no write in std::basic_istream<>.

[sergei, 2017/07/06 14:53:33]

I see, but if we changed either std::istream or std::ostream to a buffer type
now, I think it would simplify the code a bit. And I think we should actually do
it in a separate commit, it seems to be a self contained change.

[hub, 2017/07/06 21:24:29]

See https://codereview.adblockplus.org/29481704

+                       const IFileSystem::IOBuffer& data) = 0;
 
     /**
      * Moves a file (i.e.\ renames it).
      * @param fromPath Current path to the file.
      * @param toPath New path to the file.
      */
     virtual void Move(const std::string& fromPath,
                       const std::string& toPath) = 0;
 
     /**
@@ skipping 17 matching lines @@
     virtual std::string Resolve(const std::string& path) const = 0;
   };
 
   /**
    * Shared smart pointer to a `FileSystem` instance.
    */
   typedef std::shared_ptr<FileSystem> FileSystemSyncPtr;
 }
 
 #endif