Merge commit 'intrigeri/po' into po

[ikiwiki] / doc / plugins / po.mdwn

[[!template id=plugin name=po core=0 author="[[intrigeri]]"]]
[[!tag type/format]]

This plugin adds support for multi-lingual wikis, translated with
gettext, using [po4a](http://po4a.alioth.debian.org/).

It depends on the Perl `Locale::Po4a::Po` library (`apt-get install po4a`).

[[!toc levels=2]]

Introduction
============

A language is chosen as the "master" one, and any other supported
language is a "slave" one.

A page written in the "master" language is a "master" page. It can be
of any page type supported by ikiwiki, except `po`. It does not have to be
named a special way: migration to this plugin does not imply any page
renaming work.

Example: `bla/page.mdwn` is a "master" Markdown page written in
English; if `usedirs` is enabled, it is rendered as
`bla/page/index.en.html`, else as `bla/page.en.html`.

Any translation of a "master" page into a "slave" language is called
a "slave" page; it is written in the gettext PO format. `po` is now
a page type supported by ikiwiki.

Example: `bla/page.fr.po` is the PO "message catalog" used to
translate `bla/page.mdwn` into French; if `usedirs` is enabled, it is
rendered as `bla/page/index.fr.html`, else as `bla/page.fr.html`


Configuration
=============

Supported languages
-------------------

`po_master_language` is used to set the "master" language in
`ikiwiki.setup`, such as:

        po_master_language => { 'code' => 'en', 'name' => 'English' }

`po_slave_languages` is used to set the list of supported "slave"
languages, such as:

        po_slave_languages => { 'fr' => 'Français',
                                'es' => 'Castellano',
                                'de' => 'Deutsch',
        }

Decide which pages are translatable
-----------------------------------

The `po_translatable_pages` setting configures what pages are
translatable. It is a [[ikiwiki/PageSpec]], so you have lots of
control over what kind of pages are translatable.

The `.po` files are not considered as being translatable, so you don't need to
worry about excluding them explicitly from this [[ikiwiki/PageSpec]].

Internal links
--------------

The `po_link_to` option in `ikiwiki.setup` is used to decide how
internal links should be generated, depending on web server features
and site-specific preferences.

### Default linking behavior

If `po_link_to` is unset, or set to `default`, ikiwiki's default
linking behavior is preserved: `\[[destpage]]` links to the master
language's page.

### Link to current language

If `po_link_to` is set to `current`, `\[[destpage]]` links to the
`destpage`'s version written in the current page's language, if
available, *i.e.*:

- `foo/destpage/index.LL.html` if `usedirs` is enabled
- `foo/destpage.LL.html` if `usedirs` is disabled

### Link to negotiated language

If `po_link_to` is set to `negotiated`, `\[[page]]` links to the
negotiated preferred language, *i.e.* `foo/page/`.

(In)compatibility notes:

- if `usedirs` is disabled, it does not make sense to set `po_link_to`
  to `negotiated`; this option combination is neither implemented
  nor allowed.
- if the web server does not support Content Negotiation, setting
  `po_link_to` to `negotiated` will produce a unusable website.


Server support
==============

Apache
------

Using Apache `mod_negotiation` makes it really easy to have Apache
serve any page in the client's preferred language, if available.
This is the default Debian Apache configuration.

When `usedirs` is enabled, one has to set `DirectoryIndex index` for
the wiki context.

Setting `DefaultLanguage LL` (replace `LL` with your default MIME
language code) for the wiki context can help to ensure
`bla/page/index.en.html` is served as `Content-Language: LL`.

lighttpd
--------

lighttpd unfortunately does not support content negotiation.

**FIXME**: does `mod_magnet` provide the functionality needed to
 emulate this?


Usage
=====

Templates
---------

When `po_link_to` is not set to `negotiated`, one should replace some
occurrences of `BASEURL` with `HOMEPAGEURL` to get correct links to
the wiki homepage.

The `ISTRANSLATION` and `ISTRANSLATABLE` variables can be used to
display things only on translatable or translation pages.

### Display page's versions in other languages

The `OTHERLANGUAGES` loop provides ways to display other languages'
versions of the same page, and the translations' status.

One typically adds the following code to `templates/page.tmpl`:

        <TMPL_IF NAME="OTHERLANGUAGES">
        <div id="otherlanguages">
          <ul>
          <TMPL_LOOP NAME="OTHERLANGUAGES">
            <li>
              <a href="<TMPL_VAR NAME="URL">"><TMPL_VAR NAME="LANGUAGE"></a>
              <TMPL_UNLESS NAME="MASTER">
                (<TMPL_VAR NAME="PERCENT">&nbsp;%)
              </TMPL_UNLESS>
            </li>
          </TMPL_LOOP>
          </ul>
        </div>
        </TMPL_IF>

The following variables are available inside the loop (for every page in):

- `URL` - url to the page
- `CODE` - two-letters language code
- `LANGUAGE` - language name (as defined in `po_slave_languages`)
- `MASTER` - is true (1) if, and only if the page is a "master" page
- `PERCENT` - for "slave" pages, is set to the translation completeness, in percents

### Display the current translation status

The `PERCENTTRANSLATED` variable is set to the translation
completeness, expressed in percent, on "slave" pages.

One can use it this way:

        <TMPL_IF NAME="ISTRANSLATION">
        <div id="percenttranslated">
          <TMPL_VAR NAME="PERCENTTRANSLATED">
        </div>
        </TMPL_IF>

Additional PageSpec tests
-------------------------

This plugin enhances the regular [[ikiwiki/PageSpec]] syntax with some
additional tests that are documented [[here|ikiwiki/pagespec/po]].

Automatic PO file update
------------------------

Committing changes to a "master" page:

1. updates the POT file and the PO files for the "slave" languages;
   the updated PO files are then put under version control;
2. triggers a refresh of the corresponding HTML slave pages.

Also, when the plugin has just been enabled, or when a page has just
been declared as being translatable, the needed POT and PO files are
created, and the PO files are checked into version control.

Discussion pages
----------------

Discussion should happen in the language in which the pages are
written for real, *i.e.* the "master" one. If discussion pages are
enabled, "slave" pages therefore link to the "master" page's
discussion page.

Translating
-----------

One can edit the PO files using ikiwiki's CGI (a message-by-message
interface could also be implemented at some point).

If [[tips/untrusted_git_push]] is setup, one can edit the PO files in one's
preferred `$EDITOR`, without needing to be online.

TODO
====

Security checks
---------------

### Security history

The only past security issues I could find in GNU gettext and po4a
are:

- [CVE-2004-0966](http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2004-0966),
  *i.e.* [Debian bug #278283](http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=278283):
  the autopoint and gettextize scripts in the GNU gettext package
  1.14 and later versions, as used in Trustix Secure Linux 1.5
  through 2.1 and other operating systems, allows local users to
  overwrite files via a symlink attack on temporary files.
- [CVE-2007-4462](http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2007-4462):
  `lib/Locale/Po4a/Po.pm` in po4a before 0.32 allows local users to
  overwrite arbitrary files via a symlink attack on the
  gettextization.failed.po temporary file.

**FIXME**: check whether this plugin would have been a possible attack
vector to exploit these vulnerabilities.

Depending on my mood, the lack of found security issues can either
indicate that there are none, or reveal that no-one ever bothered to
find (and publish) them.

### PO file features

Can any sort of directives be put in po files that will cause mischief
(ie, include other files, run commands, crash gettext, whatever)?

> No [documented](http://www.gnu.org/software/gettext/manual/gettext.html#PO-Files)
> directive is supposed to do so. [[--intrigeri]]

### Running po4a on untrusted content

Are there any security issues on running po4a on untrusted content?

To say the least, this issue is not well covered, at least publicly:

- the documentation does not talk about it;
- grep'ing the source code for `security` or `trust` gives no answer.

On the other hand, a po4a developer answered my questions in
a convincing manner, stating that processing untrusted content was not
an initial goal, and analysing in detail the possible issues.

#### Already checked

- the core (`Po.pm`, `Transtractor.pm`) should be safe
- po4a source code was fully checked for other potential symlink
  attacks, after discovery of one such issue
- the only external program run by the core is `diff`, in `Po.pm` (in
  parts of its code we don't use)
- `Locale::gettext`: only used to display translated error messages
- Nicolas François "hopes" `DynaLoader` is safe, and has "no reason to
  think that `Encode` is not safe"
- Nicolas François has "no reason to think that `Encode::Guess` is not
  safe". The po plugin nevertheless avoids using it by defining the
  input charset (`file_in_charset`) before asking `Transtractor` to
  read any file. NB: this hack depends on po4a internals to stay
  the same.

#### To be checked

##### Locale::Po4a modules

The modules we want to use have to be checked, as not all are safe
(e.g. the LaTeX module's behaviour is changed by commands included in
the content); they may use regexps generated from the content.

`Chooser.pm` only loads the plugin we tell it too: currently, this
means the `Text` module only.

`Text` module (I checked the CVS version):

- it does not run any external program
- only `do_paragraph()` builds regexp's that expand untrusted
  variables; they seem safe to me, but someone more expert than me
  will need to check. Joey?

##### Text::WrapI18N

`Text::WrapI18N` can cause DoS (see the
[Debian bug #470250](http://bugs.debian.org/470250)), but it is
optional and we do not need the features it provides.

It is loaded if available by `Locale::Po4a::Common`; looking at the
code, I'm not sure we can prevent this at all, but maybe some symbol
table manipulation tricks could work; overriding
`Locale::Po4a::Common::wrapi18n` may be easier. I'm no expert at all
in this field. Joey? [[--intrigeri]]

> Update: Nicolas François suggests we add an option to po4a to
> disable it. It would do the trick, but only for people running
> a brand new po4a (probably too late for Lenny). Anyway, this option
> would have to take effect in a `BEGIN` / `eval` that I'm not
> familiar with. I can learn and do it, in case no Perl wizard
> volunteers to provide the po4a patch. [[--intrigeri]]

##### Term::ReadKey

`Term::ReadKey` is not a hard dependency in our case, *i.e.* po4a
works nicely without it. But the po4a Debian package recommends
`libterm-readkey-perl`, so it will probably be installed on most
systems using the po plugin.

If `$ENV{COLUMNS}` is not set, `Locale::Po4a::Common` uses
`Term::ReadKey::GetTerminalSize()` to get the terminal size. How safe
is this?

Part of `Term::ReadKey` is written in C. Depending on the runtime
platform, this function use ioctl, environment, or C library function
calls, and may end up running the `resize` command (without
arguments).

IMHO, using Term::ReadKey has too far reaching implications for us to
be able to guarantee anything wrt. security. Since it is anyway of no
use in our case, I suggest we define `ENV{COLUMNS}` before loading
`Locale::Po4a::Common`, just to be on the safe side. Joey?
[[--intrigeri]]

> Update: adding an option to disable `Text::WrapI18N`, as Nicolas
> François suggested, would as a bonus disable `Term::ReadKey`
> as well. [[--intrigeri]]

### msgmerge

`refreshpofiles()` runs this external program. A po4a developer
answered he does "not expect any security issues from it".

### Fuzzing input

I was not able to find any public information about gettext or po4a
having been tested with a fuzzing program, such as `zzuf` or `fusil`.
Moreover, some gettext parsers seem to be quite
[easy to crash](http://fusil.hachoir.org/trac/browser/trunk/fuzzers/fusil-gettext),
so it might be useful to bang msgmerge/po4a's heads against such
a program in order to easily detect some of the most obvious DoS.
[[--intrigeri]]

> po4a was not fuzzy-tested, but according to one of its developers,
> "it would be really appreciated". [[--intrigeri]]

Test conditions:

- a 21M file containing 100 concatenated copies of all the files in my
  `/usr/share/common-licenses/`; I had no existing PO file or
  translated versions at hand, which renders these tests
  quite incomplete.
- po4a was the Debian 0.34-2 package; the same tests were also run
  after replacing the `Text` module with the CVS one (the core was not
  changed in CVS since 0.34-2 was released), without any significant
  difference in the results.
- Perl 5.10.0-16

#### po4a-gettextize

`po4a-gettextize` uses more or less the same po4a features as our
`refreshpot` function.

Without specifying an input charset, zzuf'ed `po4a-gettextize` quickly
errors out, complaining it was not able to detect the input charset;
it leaves no incomplete file on disk.

So I had to pretend the input was in UTF-8, as does the po plugin.

Two ways of crashing were revealed by this command-line:

        zzuf -vc -s 0:100 -r 0.1:0.5 \
          po4a-gettextize -f text -o markdown -M utf-8 -L utf-8 \
            -m LICENSES >/dev/null

They are:

        Malformed UTF-8 character (UTF-16 surrogate 0xdcc9) in substitution iterator at /usr/share/perl5/Locale/Po4a/Po.pm line 1443.
        Malformed UTF-8 character (fatal) at /usr/share/perl5/Locale/Po4a/Po.pm line 1443.

and

        Malformed UTF-8 character (UTF-16 surrogate 0xdcec) in substitution (s///) at /usr/share/perl5/Locale/Po4a/Po.pm line 1443.
        Malformed UTF-8 character (fatal) at /usr/share/perl5/Locale/Po4a/Po.pm line 1443.

Perl seems to exit cleanly, and an incomplete PO file is written on
disk. I not sure whether if this is a bug in Perl or in `Po.pm`.

#### po4a-translate

`po4a-translate` uses more or less the same po4a features as our
`filter` function.

Without specifying an input charset, same behaviour as
`po4a-gettextize`, so let's specify UTF-8 as input charset as of now.

        zzuf -cv \
          po4a-translate -d -f text -o markdown -M utf-8 -L utf-8 \
            -k 0 -m LICENSES -p LICENSES.fr.po -l test.fr

... prints tons of occurences of the following error, but a complete
translated document is written (obviously with some weird chars
inside):

        Use of uninitialized value in string ne at /usr/share/perl5/Locale/Po4a/TransTractor.pm line 854.
        Use of uninitialized value in string ne at /usr/share/perl5/Locale/Po4a/TransTractor.pm line 840.
        Use of uninitialized value in pattern match (m//) at /usr/share/perl5/Locale/Po4a/Po.pm line 1002.

While:

        zzuf -cv -s 0:10 -r 0.001:0.3 \
          po4a-translate -d -f text -o markdown -M utf-8 -L utf-8 \
            -k 0 -m LICENSES -p LICENSES.fr.po -l test.fr

... seems to lose the fight, at the `readpo(LICENSES.fr.po)` step,
against some kind of infinite loop, deadlock, or any similar beast.
It does not seem to eat memory, though.

Whatever format module is used does not change anything. This is thus
probably a bug in po4a's core or in a lib it depends on.

The sub `read`, in `TransTractor.pm`, seems to be a good debugging
starting point.

#### msgmerge

`msgmerge` is run in our `refreshpofiles` function. I did not manage
to crash it with `zzuf`.

gettext/po4a rough corners
--------------------------

- fix infinite loop when synchronizing two ikiwiki (when checkouts
  live in different directories): say bla.fr.po has been updated in
  repo2; pulling repo2 from repo1 seems to trigger a PO update, that
  changes bla.fr.po in repo1; then pushing repo1 to repo2 triggers
  a PO update, that changes bla.fr.po in repo2; etc.; quickly fixed in
  `629968fc89bced6727981c0a1138072631751fee`, by disabling references
  in Pot files. Using `Locale::Po4a::write_if_needed` might be
  a cleaner solution. (warning: this function runs the external
  `diff` program, have to check security)
- new translations created in the web interface must get proper
  charset/encoding gettext metadata, else the next automatic PO update
  removes any non-ascii chars; possible solution: put such metadata
  into the Pot file, and let it propagate; should be fixed in
  `773de05a7a1ee68d2bed173367cf5e716884945a`, time will tell.

Page titles in links
--------------------

To use the page titles set with the [meta](plugins/meta) plugin when
rendering links would be very much nicer, than the current
"filename.LL" format. This is actually a duplicate for
[[bugs/pagetitle_function_does_not_respect_meta_titles]].

Page formats
------------

Markdown is well supported, great, but what about others?

The [po](plugins/po) uses `Locale::Po4a::Text` for every page format;
this can be expected to work out of the box with most other wiki-like
formats supported by ikiwiki. Some of their ad-hoc syntax might be
parsed in a strange way, but the worst problems I can imagine would be
wrapping issues; e.g. there is code in po4a dedicated to prevent
re-wrapping the underlined Markdown headers.

While it would be easy to better support formats such as [[html]] or
LaTeX, by using for each one the dedicated po4a module, this can be
problematic from a security point of view.

**TODO**: test the more popular formats and write proper documentation
about it.

Translation quality assurance
-----------------------------

Modifying a PO file via the CGI must be forbidden if the new version
is not a valid PO file. As a bonus, check that it provides a more
complete translation than the existing one.

A new `cansave` type of hook would be needed to implement this.

Note: committing to the underlying repository is a way to bypass
this check.

Broken links
------------

See [[contrib/po]].

Documentation
-------------

Maybe write separate documentation depending on the people it targets:
translators, wiki administrators, hackers. This plugin is maybe
complex enough to deserve this.