Re: git on MacOSX and files with decomposed utf-8 file names

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Jan 22, 2008, 01:13 UTC
Message-ID: <alpine.LFD.1.00.0801211702050.2957@woody.linux-foundation.org>
In-Reply-To: <alpine.LFD.1.00.0801211656130.2957@woody.linux-foundation.org>

On Mon, 21 Jan 2008, Linus Torvalds wrote:

Show 8 quoted lines

> 
> Think about the file name "Abc", and think about what happens when you 
> create it.
> 
> Now, think about what happens if that filename is considered equivalent in 
> case..
> 
> See? The filesystem has to *corrupt* the filename.

Let me make this really clear, because I'm afraid that you won't get it when I leave out any steps of the way.

Let us say that there is a filename "xyz" that is equivalent to a filename "abc" in *any* way. It does not matter if xyz/abc is Hello/hello, or whether it's two canonically equivalent strings.

So now, do

	close(open(xyz, O_WRONLY | O_CREAT, 0666));
	close(open(abc, O_WRONLY | O_CREAT, 0666));

and then look at the directory contents afterwards.

There are two, and only two, choices here (*):
 - the filesystem created both files, and they show up as created
 - the filesystem decided they were equivalent, and munged one (or both) 
   of them

Now, let's go back to my claim:
 - munging user data is unacceptable
and realize that equivalence BY DEFINITION must do it.

So no, you do *not* get to have your cake and eat it too. You simply fundamentally *cannot* have both filename equivalence and a non-munging filesystem. See above why.

		Linus

(*) Actually, there is third choice above, which is:

 - the filesystem created the first file, and errored out on the second 
   because it noticed it was equivalent - but not identical - to one it 
   already had

   This one is actually a perfectly fine choice, but it's not "your" kind 
   of equivalence, since it actually makes a difference between two 
   equivalent but non-identical names. So the filenames aren't actually 
   interchangable, and this case is really more of a "the filesystem has 
   some very specific limitations on what it allows".