RE: [boost] Re: Filesystem library: remove / etc

From: Brey, Edward D (EdwardDBrey_at_[hidden])
Date: 2002-03-20 09:35:04

> From: Beman Dawes [mailto:bdawes_at_[hidden]]
> 
> Like all I/O, directory operations are subject to unexpected 
> errors.  Consider the following assert, which may fail even in a 
> non-threaded environment:
> 
>     assert( exists( "foo" ) == exists( "foo" ) );
> 
> I don't see anything to do about these problems except to 
> warn people that 
> the problems may occur.
> 
> Does anyone have a better idea?

I think the answer is to rely on the built-in atomic capabilities of the
operating system.  For example, when you proposed, IIRC,

void remove_if_exists(string file) {
  if (exists(file))
      remove(file);
}

this thought came to mind, because I assumed that we were just ignoring the
issue for sake of simplicity of conversation.  To eliminate the race
condition, one would really implement it more like this:

void remove_if_exists(string file)
{
  switch (error_type os_specific error = os_specific_remove(file))
  {
      default: throw file_exception(os_specific_error);
      case success:
      case file_does_not_exist:
  }
}

Am I right in understanding that a function like Win32 DeleteFile can be
counted on to serialize its operations with other OS operations that affect
the same file, so that if it says that the file does not exist at remove
time, then that truly is the case?

I think that you're remove_if_exists idea is a great way to handle the
common case of not caring whether a deleted file was present without having
to resort to the error-prone ways of old of returning a (too often
incorrectly left unchecked) success/failure code.