[Geany-devel] Changed file saving implementation for systems with GIO

[Colomban Wendling]

Le 05/11/2010 21:33, Dimitar Zhekov a écrit :
> On Fri, 05 Nov 2010 19:50:59 +0100
> Colomban Wendling <lists.ban at herbesfolles.org> wrote:
> 
>> Le 05/11/2010 20:08, Dimitar Zhekov a ecrit :
>>>
>>> a. Create filename-foo, write data to it, abort and unlink on failure.
>>> [...]
>> Hey, that's quite clever :) We would then avoid to have to read the
>> original file.
> 
> It's a well-known algorithm, but there is a better one, assuming that
> the underlying I/O system supports open for read-write and truncate. I
> checked GIO, and it does: g_file_open_readwrite, g_seekable_truncate. :D
> 
> 1. Open the file for reading and writing.
> 2. If the new data is longer, append the extra part only (thus claiming
> the required disk space).
> If that fails, truncate to the original size and abort.
> 3. Write the data (without the extra part, if any).
> 4. If the new data is shorter, truncate.
> 
> That's almost 100% safe (there is no apparent reason for truncate to a
> smaller size to fail), preserves everything, no extra disk space, not
> even extra I/O.
Even if it seems really interesting, I'm not sure it is really reliable.
What if the program terminates (power loss, whatever) during the final
write? What if a network connection is broken before we truncate?
Since the whole operation is not atomic, it's not 100% safe, and I still
believe that the backup is needed.

This said, it could be a good implementation for non-safe file saving,
that would be safer than without. But that's probably much work for less
gain IMHO.

Regards,
Colomban