[Unison-hackers] Stasher code...

[Benjamin Pierce]

Hi Jerome,

> I'm trying to understand the stasher code, but I'm a bit puzzled.

I've never been completely satisfied with the stasher stuff.  It would  
be great to get it cleaned up.

> In files.ml, we have the following piece of code:
>
>    let deleteLocal (fspath, (workingDirOpt, path)) =
>    [...]
>          Stasher.backup fspath path `AndRemove
>    [...]
>
> If "path" contains a followed link, this will backup and delete the
> *link*, not the link target as expected.  Stasher.backup should call
> Fspath.findWorkingDir to get the right file to rename or delete.

Right.

> Again in file.ml, we have the following:
>
>  let make_backup =
>    (* Perform (asynchronously) a backup of the destination files *)
>    Update.updateArchive rootTo pathTo uiTo id
>
> The function Update.updateArchive calls the function
> Stasher.stashCurrentVersion, but only after the destination has been
> updated (the call is in the archive commit action)...
>
>
> Still, the destination files are backed-up by the call to
> Stasher.backup in Files.renameLocal.

This seems right.

> There is also a call to Stasher.stashCurrentVersion in
> Update.replaceArchive, which is supposed to perform backups using the
> temporary files before they replace the destination files, but perform
> backups afterwards.  So it does nothing (this is somewhat coherent
> with the implementation of stashCurrentVersion which does not support
> recursive traversal of a temporary directory).
>
> Stasher.stashCurrentVersion is also called from transport.ml, on both
> replicas, after updates.
>
> As a summary, as far as I can see, Stasher.stashCurrentVersion is
> called twice on the source files: once from Update.updateArchive
> (called from File.copy) and then once from Transport.doAction.  It is
> also called twice on the destination files once modified, from the
> same functions.  It is called once on an empty location from
> Update.replaceArchive.  Finally, destination files are backed-up
> before modification through the call to Stasher.backup in
> Files.rename.
>
> If this analysis is correct, I suspect we can safely remove all calls
> to Stasher.stashCurrentVersion from file update.ml.

I'm not completely confident that I've correctly re-understood how all  
this is supposed to work, but in any case failing to stash the current  
version is less serious than failing to back up the old version -- it  
will only result in late merge

Best,

      - Benjamin


>
> -- Jerome
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