Add file_base::sync_all_on_write flag.

Maps to:
  - O_SYNC on POSIX
  - FILE_FLAG_WRITE_THROUGH on Windows

Author: chriskohlhoff

asio/include/asio/detail/impl/win_iocp_file_service.ipp

@@ -88,6 +88,8 @@ asio::error_code win_iocp_file_service::open(
     flags |= FILE_FLAG_SEQUENTIAL_SCAN;
   else
     flags |= FILE_FLAG_RANDOM_ACCESS;
+  if ((open_flags & file_base::sync_all_on_write) != 0)
+    flags |= FILE_FLAG_WRITE_THROUGH;
 
   HANDLE handle = ::CreateFileA(path, access, 0, 0, disposition, flags, 0);
   if (handle != INVALID_HANDLE_VALUE)

asio/include/asio/file_base.hpp

@@ -58,6 +58,10 @@ class file_base
 
   /// Open the file with any existing contents truncated.
   static const flags truncate = implementation_defined;
+
+  /// Open the file so that write operations automatically synchronise the file
+  /// data and metadata to disk.
+  static const flags sync_all_on_write = implementation_defined;
 #else
   enum flags
   {
@@ -68,15 +72,17 @@ class file_base
     append = 8,
     create = 16,
     exclusive = 32,
-    truncate = 64
+    truncate = 64,
+    sync_all_on_write = 128
 #else // defined(ASIO_WINDOWS)
     read_only = O_RDONLY,
     write_only = O_WRONLY,
     read_write = O_RDWR,
     append = O_APPEND,
     create = O_CREAT,
     exclusive = O_EXCL,
-    truncate = O_TRUNC
+    truncate = O_TRUNC,
+    sync_all_on_write = O_SYNC
 #endif // defined(ASIO_WINDOWS)
   };