MCGalaxy
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Level saving should use File.Replace
Currently the process is:
- The level is first copied from the
*.lvl
to theprev
, this may leave theprev
file corrupted as a File.Copy needs sufficient space to write the newer version of the file. - Secondly the level is written to the
*.backup
file, this may fail for many reasons leaving a corrupt*.backup
file. - Thirdly the
*.backup
file is copied to the*.lvl
file, this can leave the*.lvl
file corrupted due to the disk filling up.
The file is loaded from *.lvl
, *.backup
or prev
in that order.
Because of the way this is saved none of the files are guaranteed to be undamaged.
The usual way of doing a "safe replace" like this is to write the data to a temporary file then rename it to it's correct name once it has been completely written. That way no file that will be read ever exists in a partially written state.
Looking at the files you are writing and the available DotNET libraries I think the correct one is to use is File.Replace
.
In this way ...
- Write the level to a
*.temp
file (use Strm.Flush(true) if possible). - Use File.Replace (temp, level, prev) to place the new
*.lvl
andprev
files. - Use File.Copy to recreate the
*.temp
file from the*.lvl
file. - Use File.Replace (temp, backup, null) to update the
*.backup
file.
The File.Replace
operation just has to do metadata updates and so should succeed or fail as a single operation leaving the filesystem in either the new or previous states, both of which have a complete and undamaged level file.
The File.Copy
and Export functions only write to a temp so if they fail part way through nothing important is damaged.
On Mono the 3 argument Replace becomes two independent renames; this is ok.
All three files will always be complete (or missing) and the current load order will get the most recent of them.
The prev
file is not rewritten so will be on the physical disk even on a hardware failure.
Looking back at this I feel that I should mention that the loading of the *.lvl
file will not detect a truncated file as the format does not contain an end of file marker. This means that a truncated *.backup
file will override a fully intact prev
file.
See #779 for patch