[Unison-hackers] Experimental "faster check" mode

[Benjamin C. Pierce]

[Continuing a conversation from almost a year ago...!]

Jerome wrote...
> But "ignorearchives" is set to true when the "ignorearchive"
> preference is set, not when "fastercheckUNSAFE" preference is set...

Sorry, that was a thinko.

>> Would it be better to change Fpcache.save to drop
>> individual fingeprints when they are pseudos?
> 
> The check "Prefs.read fastercheckUNSAFE && newfile" should be at the
> beginning of Fpcache.fingerprint rather than in Os.fingerprint.

OK.

>>> - You should check that the Xferhint code still works properly.
>>> I suspect we don't want to store pseudo-fingerprints there either...
>> 
>> Pseudo-fingerprints include the fingerprinted path, so we should
>> never have a hit in the Xferhint table on a pseudo-fp.
> 
> We should include a fingerprint of the path, not the whole path.
> Otherwise, we will end up with very large archive files.

Good point.

> At the moment, paranoidCheck returns a status: either Success or
> Failure (with some explanation of the failure).  In Copy.file,
> checkContentsChange is then called with a different boolean argument
> depending on the status.  The function paranoidCheck could return a
> fingerprint instead of the explanation when it cannot decide whether
> the file was correctly transferred.  Somethink like:
>  let paranoidCheck fspathTo pathTo realPathTo desc fp ress =
>    let info = Fileinfo.get false fspathTo pathTo in
>    let fp' = Os.fingerprint fspathTo pathTo info in
>    if fp' <> fp then begin
>      Lwt.return (info, Undecided fp')
>    end else
>      Lwt.return (info, Success)
> This fingerprint would be passed to checkContentsChangeLocal which
> compute the fingerprint of the source file and can thus make sure the
> two fingerprint matches.
> 
> Function checkContentsChangeLocal will have three fingerprints:
> the one from update detection which may be a pseudo-fingerprint
> (FPU), and the fingerprints of the source (FPS) and destination (FPD)
> files.  If FPU is not a pseudo-fingerprint, the behavior of the
> function should remain unchanged.  If it is a pseudo-fingerprint and
> FPS <> FPD, we should report that the file is not correctly
> transferred.

OK, after a few attempts, I've gotten this bit implemented.  Please have a look at the code in copy.ml and see if it makes sense...

   - Benjamin