[Haskell-cafe] Windows: openFile gives permission denied whenfilein use

[Antoine Latter]

On Thu, Dec 29, 2011 at 11:49 AM, Donn Cave <donn at avvanta.com> wrote:
> Quoth Antoine Latter <aslatter at gmail.com>,
> ...
>>> http://www.haskell.org/pipermail/libraries/2011-October/016978.html
>>>
>>> ... wherein Ian Lynagh proposed to remove this feature and let the
>>> programmer enforce locking or not, as in other programming languages'
>>> base I/O libraries.  This met with enthusiastic universal support,
>>> so whatever the Report may say, it looks to me like the GHC libraries
>>> will eventually not do this.
>>>
>>
>> Wouldn't this lead to 'getContents' and friends being much less safe
>> than they already are? Or would we have to do something at the GHC
>> runtime level of things to add locking?
>
> Interesting question.  I tend to steer clear of that function, rather
> than try to keep track of all the things that can go wrong with it!
> but I would guess, the effect is the same on any I/O strategy that
> executes the same way: e.g. if you were to read pieces of the file
> iteratively you would be similarly exposed to the possibility of
> concurrent modifications.
>
> That risk may be less obvious with getContents if you take the naive
> view that it returns the contents of the file in the same way that normal
> I/O operations return data, but then you will eventually be punished for
> that naivety anyway!
>
> Are you aware of some possible special issue with getContents?
>

Well, the issue would more be with 'hGetContents'.

Would this program then loop:

append fromFilePath toFilePath = do
  str <- readFile fromFile
  writeFile toFile str

if 'from' and 'to' where the same file?

Currently the locking prevents this.

Antoine