Re: [PATCH] t3910: show failure of core.precomposeunicode with decomposed filenames

[Jeff King <peff@peff.net>]

On Mon, Apr 28, 2014 at 12:17:28PM -0700, Junio C Hamano wrote:

> >   3. Convert index filenames to their precomposed form when
> >      we read the index from disk. This would be efficient,
> >      but we would have to be careful not to write the
> >      precomposed forms back out to disk.
> 
> I think this may be the right approach, especially if you are going
> to do this only when core.precomposeunicode is set.
> 
> the reasoning behind "we would have to be careful not to write"
> part, is unclear to me, though.  Don't decomposing filesystems
> perform the manglig from the precomposed form without even being
> asked to do so, just like a case insensitive filesystem will
> overwrite an existing "makefile" on a request to write to
> "Makefile"?

Sorry, I meant "do not write the precomposed forms back out to the
on-disk index". And by extension, do not update cache-tree and write
them out to git trees.

IOW, it is not enough to just set cache_entry->name to the normalized
form. You'd need to store both.

Since such entries are in the minority, and because cache_entry is
already a variable-length struct, I think you could get away with
sticking it after the "name" field, and then comparing like:

  const char *ce_normalized_name(struct cache_entry *ce, size_t *len)
  {
	const char *ret;

	/* Normal, fast path */
	if (!(ce->ce_flags & CE_NORMALIZED_NAME)) {
		len = ce_namelen(ce);
		return ce->name;
	}

	/* Slow path for normalized names */
	ret = ce->name + ce->namelen + 1;
	*len = strlen(name);
	return ret;
  }

The strlen is probably OK since such paths are presumably in the
minority (even for UTF-8 paths, we can avoid storing the extra copy if
they do not need any normalization). Or we could get fancy and encode
the length in front, but I am not sure it is worth the complexity.

Anyway, the tricky part is then making sure that all cache_entry name
comparisons use ce_normalized_name instead of ce->name.

-Peff