                                                                -*- Text -*-


Content
=======

 * Context
 * Issue Description
 * Pre-Resolution State of Affairs
   - Single platform
   - Multi-platform: Windows + MacOS X
 * Proposed Support Library
   - Assumptions
   - Options
 * Proposed Normal Form
 * Possible Solutions
   - Normalization of path-input on MacOS X
   - Normalization of path-input everywhere
   - Comparison routines (client side)
   - Comparison routines (everywhere)
 * Short Term (ie before 2.0) solution
 * Long Term Solution (ie 2.0+)
 * Additional Information
 * References


Context
=======

Within Unicode, some characters - with diacritical marks - can be
represented in 2 forms: Normal Form Composed (NFC) or Normal Form
Decomposed (NFD).  A string of unicode characters can contain any
mixture of both forms.

This problem explicitly does not concern itself with invisible
characters, spaces or other characters unlikely to be present in
filenames.  Please note that this issue is explicitly excluding
NFKC/NFKD (compatibility) normal forms, because they remove for
example formatting (meaning they are lossy?).

Because there are 2 forms for representing (some) characters in
Unicode, it's possible to produce different sequences of codepoints
meaning to indicate the same sequence of characters [1].  UTF-8, the
internal Unicode encoding of choice for Subversion, encodes codepoints
in (a series of) bytes (octets).  Because the sequences of codepoints
specifying a character may differ, so may the resulting UTF-8.  Hence,
we end up with more than one way to specify the same path.

The following table specifies behaviour of OSes related to handling of
Unicode filenames:

   OS           Accepts   Gives back
   ----------   -------   ----------
   MacOS X[2]     all        NFD*
   Linux          all      <input>
   Windows        all      <input>
   Others          ?          ?

   *) There are some remarks to be made regarding full or partial NFD
      here, but the essential thing is:  if you send in NFC, don't
      expect it back!


Issue Description
=================

From the above issue description, two problems follow:

First, we can't generally depend on the OS to give us back the exact
filename we gave it.  This is mainly a client side issue, something
which might be resolved in the client side libraries (client/subr/wc).

Secondly, the same filename may be encoded in different codepoints.
This issue is much broader than the first, especially given the fact
that we already have lots of populated repositories "out there".  We
cannot depend on a filename coming from the operating system -- even
though different from the one in the repository -- to name a different
file.  This has repository (ie. server-side) impact.


Pre-Resolution State of Affairs
===============================

This section serves to describe the problems to be expected in different
combinations of client/server OSes.  As indicated in the table in the
context section, Linux and Windows are expected to behave equally. This
section therefor leaves out the consideration of Linux as a separate
system.

The platforms below are strictly client side: the server side problems
mentioned in the issue description section solely relates to the repository,
which can be located at any server platform.


   Single platform
   ---------------

   This can be multiple MacOSX machines or multiple Windows machines.
   In this scenario, no interoperability problems are to be expected.


   Multi-platform: Windows + MacOSX
   --------------------------------

   Consider a filename which contains one or more precomposed (NFC)
   characters being committed from Windows.  When the MacOSX developer
   updates, a file is written in NFC form, but as stated in the
   context section, Mac recodes that to NFD.  Now, when comparing what
   comes from the disk (NFD) with what's in the entries file (NFC),
   results in a missing file (the NFC encoded one) and an unversioned
   file (the NFD encoded one).  Both of the filenames look exactly the
   same to the person reading the Subversion output on the
   screen. [==> confusion!]

   Committing a file the other way around might be less problematic,
   since Windows is capable of storing NFD filenames.


Proposed Support Library
========================

   Assumptions
   -----------

   The main assumption is that we'll keep using APR for character set
   conversion, meaning that the recoding solution to choose would not
   need to provide any other functionality than recoding.


   Options
   -------

   There are two options (that I'm aware of [dionisos]) for choosing a
   library which supports the required functionality:

   1) International Component for Unicode (ICU)[3] -- a library with a
      very wide range of targeted functions, but with a memory
      footprint to match.  In order to be able to use it, we'd need to
      trim this library down significantly.

   2) utf8proc -- a library for processing UTF-8 encoded unicode
      strings.  A library specifically targeted at a limited number of
      operations to be performed on UTF-8 encoded strings.  It
      consists of two .c and a single .h file, with a total source
      size of 1MB (compiled less than 0.5MB).

   From these two, under the given assumption, it only makes sense to
   use utf8proc.


Proposed Normal Form
====================

The proposed internal 'normal form' should be NFC, if only if
it were because it's the most compact form of the two:  when allocating
memory to store a conversion result, it won't be necessary (ever) to
allocate more than the size of the input buffer.

This would give the maximum performance from utf8proc, which requires
two recoding runs when the buffer is too small:  one to retrieve the
required buffer size, the second to actually store the result.


Possible Solutions
==================

Several options are available for resolution of this problem, each
with its pros and cons, to be outlined below.

1) Normalization of (path) input on MacOSX.  Since the Mac seems to be
   the only platform which mutilates its pathname input to be NFD,
   this seems like a logical (low impact) solution.

2) Normalization of (path) input on all platforms.  Since paths can't
   differ only in encoding if we standardize on encoding, this seems
   like a logical (relatively low) impact solution.

3) Normalization of path input in the client and server.  On the server
   side, non-normalized paths may have become part of the repository.
   We can achieve full in-memory standardization by converting any
   path coming from the repository as well as the client.

4) Client and server-side path comparison routines.  Because paths read
   from the repository may be used to access said repository, possibly
   by calculating hash values, paths from can't be munged
   (repository-side).  To eliminate the effect, we acknowledge we're
   not going to be 'clean': we'll always need path comparison
   routines.

Solution (1) has a very strong CON: it will break all pre-existing
MacOSX-only workshops.  Consider a client which starts sending NFC
encoded paths in an environment where all paths have been NFD encoded
until that time - without proper support in the server.  This would
result in commits with NFC encoded paths to files for which the path
in the repository is NFD encoded: breakage.

Solution (2) has the same problem as solution (1) on MacOSX, but
on the upside it prevents new NFD paths from entering into the repository
(for sufficiently broad definitions of 'client' [think mod_dav_svn]).

As already stated, solution (3) may prevent paths from being found, if
the retrieval mechanism is hash-based.  Meaning this could break any
repository backend using hashing to store information about paths.
(Don't we store locks in FSFS based on hashing?)

Solution (4) defines no internal standard representation, assuming it's
not possible to maintain a clean in-memory state, given all problems
found in the earlier solutions.  Instead, it requires all path comparisons
to be performed using special NFC/NFD encoding aware functions.


Short Term Solution
===================

Because of our interoperability guarantees, the client and server
should be considered separate universes, each of which can use its own
(internal) solution.  However, the client should at all times use the
exact path the server sent it.  The same applies the other way around.

Given the above, the short term (before 2.0) solution should be to
use path comparison routines as stated in solution (4).


Long Term Solution
==================

The long term (2.0+) solution would be to use option (2), which ensures
recoding of all input paths into the 'normal' normal form (NFC).  In that
case, it'll no longer require the use of specialised path comparison
routines (although that might still be desired for other design
considerations).


Short Term Solution Implementation Consequences
===============================================

As stated before, since we don't know whether the other side of the
equation might be a pre-normalization-aware client or server until
we break backward compat in 2.0, the client and server should be
able to talk backward compatibly with a pre-NF-aware 'other side'.

Hence, solving this problem means considering the client and the server
separate universes, each of which can employ its own internal solution.

Implementing option (4) means:

A. Comparing file names with entry paths using NFC/NFD aware
   comparison functions. Then, when there's a match, *use the pathname
   from the entries file* to communicate with the server; after all,
   the path might have been added with a different encoding than we
   got back from the disk.

B. Match working copy paths with entries-file paths using NFC/NFD
   aware comparison functions. On a match, use the entries-file path
   to communicate with the server.

The above means the client has to be very careful to preserve the
encoding from the server and use that when talking to the server
otherwise the server may not recognize the path as a versioned entity.

Locally however, we can't be sure the filesystem enforces the encoding
the server sent to the client, meaning there are (contrived) cases where
a file exists in a different encoding locally than in the repository.
Which means we have to be very careful about how we find our files and
to use the encoding we got from the local filesystem.

Implementation details:

 * The hash keys in svn_wc_adm_access_t's are hashed on the normalized
   path encoding, not the repository path, in order to be able to
   calculate the hash key from both the wc path as well as the repo
   path.

 * The same line of reasoning applies to the hash keys in the entries
   hash.

New conventions:

 * Variables containing a path as encoded in the local filesystem
   should contain the (sub)string 'wc_path'.

 * Variables containing a path as encoded in the repository should
   contain the (sub)string 'repo_path'.


Additional Information
======================

  * "UTF-8 NFC/NFD paths issue" dev@ mailing list thread:
     http://svn.haxx.se/dev/archive-2010-09/0319.shtml


References
==========

1) UAX #15: Unicode normalization forms
   http://unicode.org/reports/tr15/
2) Apple Technical Q&A: Path encodings in VFS
   http://developer.apple.com/qa/qa2001/qa1173.html
3) ICU - International Component for Unicode
   http://www-306.ibm.com/software/globalization/icu/index.jsp
4) utf8proc - a library targeted at processing UTF-8 encoded unicode strings
   http://www.flexiguided.de/publications.utf8proc.en.html