Sergiu Ivanov has been working voluntarily on this task an inofficial GSoC 2008 participant. Not all the desired functionality is in place yet, though.
The main idea behind the Hurd is to make (almost) all system functionality user-modifiable (extensible system). This includes a user-modifiable filesystem: the whole filesystem is implemented decentrally, by a set of filesystem servers forming the directory tree together, a virtual file system. These filesystem servers are called translators, and are the most visible feature of the Hurd.
The reason they are called translators is because when you set a translator on a filesystem node, the underlying node(s) are hidden by the translator, but the translator itself can access them, and present their contents in a different format -- translate them. A simple example is a gunzip translator, which can be set on a gzipped file, and presents a virtual file with the uncompressed contents. Or the other way around. Or a translator that presents an XML file as a directory tree. Or an mbox as a set of individual files for each mail (mboxfs); or ever further breaking it down into headers, body, attachments...
This gets even more powerful when translators are used as building blocks for larger applications: A mail reader for example doesn't need backends for understanding various mailbox formats anymore. All formats can be parsed by special translators, and the mail reader gets the data as a uniform, directly usable filesystem structure. Translators can also be stacked: If you have a compressed mailbox for example, first apply a gunzip translator, and then an mbox translator on top of that.
There are a few problems with the way translators are set, though. For one, once a translator is set on a node, you always see the translated content. If you need the untranslated contents again, to do a backup for example, you first need to remove the translator again. Also, having to set a translator explicitly before accessing the contents is pretty cumbersome, making this feature almost useless.
A possible solution is implementing a mechanism for selecting translators
through special filename attributes. For example you could use
index.html.gz,,- to choose between translated and
untranslated versions of a file. Or you could use
index.html.gz,,u to get
the contents of the file with a gunzip translator applied automatically. You
could also use attributes on whole directory trees:
.,,0/ would give you a
directory tree corresponding to the current directory, but with any translators
disabled, for doing a backup. And
site,,u/*.html.gz would present a whole
directory tree of compressed HTML files as uncompressed files.
One benefit of the Hurd's flexibility is that it should be possible to implement such a mechanism without touching the existing Hurd components: Rather, just implement a special proxy, that mirrors the normal filesystem, but is able to interpret the special extensions and present transformed files in place of the original ones.
In the long run it's probably desirable to have the mechanism implemented in the standard name lookup mechanism, so it will be available globally, and avoid the overhead of a proxy; but for the beginning the proxy solution is much more flexible.
The goal of this project is implementing a prototype proxy; perhaps also a first version of the global variant as proof of concept, if time permits. It requires good understanding of the name lookup mechanism, and translator programming; but the implementation should not be too hard. Perhaps the hardest part is finding a convenient, flexible, elegant, hurdish method for mapping the special extensions to actual translators...
Possible mentors: Olaf Buddenhagen (antrik)
Exercise: Try to make some modification to the existing unionfs and/or firmlink translators. (More specific suggestions welcome... )