Add comment to File copy on linux/android.

runtime/bin/file_linux.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_LINUX)
 
 #include "bin/file.h"
 
 #include <errno.h>  // NOLINT
@@ skipping 227 matching lines @@
       VOID_TEMP_FAILURE_RETRY(close(old_fd));
       return false;
     }
     off64_t offset = 0;
     int result = 1;
     while (result > 0) {
       // Loop to ensure we copy everything, and not only up to 2GB.
       result = TEMP_FAILURE_RETRY(
           sendfile64(new_fd, old_fd, &offset, kMaxUint32));
     }
+    // From sendfile man pages:
+    //   Applications may wish to fall back to read(2)/write(2) in the case
+    //   where sendfile() fails with EINVAL or ENOSYS.
     if (result < 0 && (errno == EINVAL || errno == ENOSYS)) {
       const intptr_t kBufferSize = 8 * 1024;
       uint8_t buffer[kBufferSize];
       while ((result = TEMP_FAILURE_RETRY(
           read(old_fd, buffer, kBufferSize))) > 0) {
         int wrote = TEMP_FAILURE_RETRY(write(new_fd, buffer, result));
         if (wrote != result) {
           result = -1;
           break;
         }
@@ skipping 148 matching lines @@
   return (file_1_info.st_ino == file_2_info.st_ino &&
           file_1_info.st_dev == file_2_info.st_dev) ?
       File::kIdentical :
       File::kDifferent;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_LINUX)