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Re: [RFC] Non-normalizing Unicode Composition Awareness

From: Julian Foad <julianfoad_at_btopenworld.com>
Date: Tue, 14 Feb 2012 10:25:03 +0000 (GMT)

Hi Thomas.  It's fantastic that you're taking the trouble to write up this proposal.  That's just what we need.  Just a few initial comments below...

Thomas Åkesson wrote:

> Context
> ===
>
> [...] A unicode string (e.g. a file name) can be represented
> in 2 normalized forms (NFC/NFD) or mixed, i.e. multiple such
> characters where some are composed and others decomposed (rare).

What's "rare"?  We have to assume that input is in mixed composition in any system that doesn't explicitly normalize it, which (I think) includes most operating systems.  While it may be rare for any single string to contain characters in both compositions, it is very common to be processing a string that *might* have characters in both compositions -- in other words, that is not guaranteed to be normalized.  I think it would be clearer to drop the "(rare)" and just say "... normalized forms (NFC/NFD) or mixed (not normalized).".

> A minority of file systems (currently Mac OS X HFS+ only) will
> normalize the paths. In the case of HFS+, the path will be
> normalized into NFD and it will even be given back that way when
> listing the filesystem. 

Drop the word "even"?  The statement is not surprising.

[...]

> Similarities to case-sensitivity
> ===
>
>  - If two Unicode strings differ only by letter case/composition,

Drop "/composition" -- it's the subject of the following sentence.

> on some
computer systems they refer to the same file, while on
> other systems
they refer to different files.  The same applies
> if two Unicode strings
differ only by composition.

> [...]

> Client Changes
> ===
>
> [...] An abstraction between the repository path and the file
> system path can be achieved by ensuring that there is a column
> in wc.db that contains the file system path in exactly the same
> form that the file system gives back. APIs in wc needs to be
> extended to ensure that all interaction with the file system is
> performed with the file system path.

[...]

This part seems to be the heart of the whole proposal.  You describe the data that we need, but the behaviour will also need to be described in detail.  Presumably much of the behaviour is boring and obvious (when we check out a new path and create it on disk, we store the disk path), but I'm sure there will be some less obvious parts (do we need to find out what the disk path of an 'excluded' node would be, even though we're not actually creating it on disk, for example).

> Use Cases
> ===
>
> This change will only affect use cases which rely on creating
> paths that look like duplicates but use different unicode
> composition. It is highly unlikely anyone is relying on this..

Uh... it sounds like you are saying there are no interesting use cases for this proposal!  No, on the contrary, this proposal also affects checking out and using a WC on Mac HFS+ where the repository paths were created on another system and are not in NFD, and it allows that case to work.  That's the more interesting use case, is it not?  It's definitely worth writing out the interesting case in full, including steps like checkout (or update) that brings in a non-NFD path, create a new file on the Mac, and commit.

- Julian
Received on 2012-02-14 12:56:00 CET

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