notes/l10n-problems

Issues (and their resolutions) when using gettext for message translation

Contents
========

 * Windows issues
 * Automatic characterset conversion
 * Translations on the client
 * No translations on the server



Windows issues
==============

On Windows, Subversion is linked against a modified version of GNU gettext.
This resolves several issues:

 - Eliminated need to link against libiconv (which would be the second
   iconv library, since we already link against apr-iconv)
 - No automatic charset conversion (guaranteed UTF-8 strings returned by
   gettext() calls without performance penalties)

More in the paragraphs below...


Automatic characterset conversion
=================================

Some gettext implementations automatically convert the strings in the
message catalogue to the active system characterset.  The source encoding
is stored in the "" message id.  The message string looks somewhat like
a mime header and contains a "Content-Encoding" line. It's typically GNU's
gettext which does this.

Subversion uses UTF-8 to encode strings internally, which may not be the
systems default character encoding.  To prevent internal corruption,
libsvn_subr:svn_cmdline_init2() explicitly tells gettext to return UTF-8
encoded strings if it has bind_textdomain_codeset().

Some gettext implementations don't contain automatic string recoding.  In
order to work with both recoding and non-recoding implementations, the
source strings must be UTF-8 encoded.  This is achieved by requiring .po
files to be UTF-8 encoded.  [Note: a pre-commit hook has been installed to
ensure this.]

On Windows Subversion links against a version of GNU gettext, which has
been modified not to do character conversions.  This eliminates the
requirement to link against libiconv which would mean Subversion being
linked against 2 iconv libraries (apr_iconv as well as libiconv).


Translations on the client
==========================

The translation effort is to translate most error messages generated on
the system on which the user has invoked his subversion command (svnadmin,
svnlook, svndumpfilter, svnversion or svn).

This means that in all layers of the libraries strings have been marked for
translation, either with _(), N_() or Q_().

Parameters are sprintf-ed straight into errorstrings at the time they are
added to the error structure, so most strings are marked with _() and
translated directly into the language for which the client was set up.
[Note: The N_() macro marks strings for delayed translation.]



Translations on the server
==========================

On systems which define the LC_MESSAGES constant, setlocale() can be used
to set string translation for all (error) strings even those outside
the Subversion domain.

Windows doesn't define LC_MESSAGES.  Instead GNU gettext uses the environ-
ment variables LANGUAGE, LC_ALL, LC_MESSAGES and LANG (in that order) to
find out what language to translate to.  If none of these are defined, the
system and user default locales are queried.  Though setting one of
the aforementioned variables before starting the server will avoid
localization by Subversion to the default locale, messages generated
by the system itself are likely to still be in its default locale
(they are on Windows).

While systems which have the LC_MESSAGES flag (or setenv() - of which
Windows has neither) allow languages to be switched at run time, this cannot
be done portably.

Any attempt to use setlocale() in an Apache environment may conflict
with settings other modules expect to be setup (even when using a
prefork MPM).  On the svnserve side, having no portable way to change
languages dynamically means that the environment has to be set up
correctly from the start.  Futhermore, the svnserve protocol doesn't
yet support content negotiation.

In other words, there is no way -- programmatically -- to ensure that
messages are served in any specific language using a traditional
gettext implementation.  Current consensus is that gettext must be
replaced on the server side with a more flexible implementation.

Server requirement(s):
 - Language negotiation on a per-client session basis.  For a
   stateless protocol like HTTP, this means per-request.  For a
   stateful protocol like the one used by svnserve, this means
   per-connection.
 - Avoid contamination of environment used by other code (e.g. other
   Apache modules running in the same server as mod_dav_svn).
 - Allow for propagation of the language to use to hook scripts.
 - Continue to inter-op with generic HTTP/DAV clients, and stay
   compatible with SVN clients of various versions (as per existing
   compatibility rules).

I18N module requirement(s):
 - Cross-platform.
 - Interoperable with gettext tools (e.g. for .po files).
 - Non-viral license which allows for any necessary modifications.
 - gettext-like API (needn't be an exact match).

Implementation guidelines:
 - The L10N API will be uniform across all libraries, clients, and
   servers.  Server-negotiated language will be recorded in either a
   context baton (e.g. apr_pool_t.userdata), or in thread-local
   storage (TLS).
 - Implemented on top of a new gettext-like module with per-struct or
   per-thread locale mutator functions and storage for name/value
   pairs (a glorified apr_hash_t).  (See implementation from Nicolás
   Lichtmaier noted below.)
 - Language chosen by the server will be negotiated based on a ranked
   list of preferences provided by the client.
 - Language used by httpd/mod_dav_svn will be derived from the
   Accept-Language HTTP header, and setup by mod_negotiation (when
   available), or by mod_dav_svn on a per-request basis.
 - Language used by svnserve derived from additions to the protocol
   which allow for HTTP-style content negotiation on a per-connection
   basis.  The protocol extension would use the same sort of q-value
   list found in the Accept-Language header to specify user language
   preferences.

Investigation: A brief canvasing of developers (on IRC) indicated that
no thorough investigation of existing solutions which might meet the
above requirements has been done.  This incomplete canvasing may not
paint an accurate picture, however.

A branch <http://svn.apache.org/repos/asf/subversion/branches/server-l10n/>
has been created to explore a solution to the above requirements.  While
the L10N module is important, how that module is applied to both the
server-side and client-side is possibly even more so; an
implementation which meets the requirements should not dramatically
impact the solution used across the code base for the general L10N
API, nor the necessary server-side machinations.

Nicolás Lichtmaier wrote something along the lines of the module
referenced in the "Possible implementation" section
<http://svn.haxx.se/dev/archive-2004-04/0788.shtml>, which has been
committed to the server-l10n branch.  However, it depends upon the GNU
gettext .mo format, and the GNU implementation may not be available on
all platforms (unless re-implemented).  This module will need to be
enhanced or replaced, ideally completely obviating the need for
linkage against a platform's own gettext implementation.

Whether to use TLS or a context baton for the L10N API is under
discussion.  TLS can provide a more friendly API (albeit somewhat
underhanded), while use of a context baton more resilient to change
(e.g. if httpd someday allowed more than one thread to service a
request).  Here's a sample:
 - No localization:                          "A message to localize"
 - Localization w/ TLS or global preference: _("A message to localize")
 - Localization w/ a context baton:          _("A message to localize", pool)

Historical note: Original consensus indicated that messages from the
server side should stay untranslated for transmission to the client.
However, client side localization is not an option, because by then
the parameter values have been inserted into the string, meaning that
it can't be looked up in the messages catalogue anymore.  So any
localization must occur on the server, or significantly increase the
complexity of marshalling messages from the server as
unlocalized/unformatted data structures and localizing them on the
client side using some additional wrapper APIs to handle the
unmarshalling and message formatting.  Additionally, client and server
versions may not match up, meaning that message keys and format string
values provided by the server may not correspond to what's available
on the client.

Paul Querna suggested a variation on this scheme involving requesting
(once) and caching the localizations (to the local disk) for each
server version, along with sending the message key (for lookup of
localized text) and an already formatted text (to use as the default
when no localization bundle is available).  In addition to the
complications mentioned previously, this has the downside of crippling
the localization of server-generated messages when no write access to
the local disk is available to the client.