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# Translator short-circuiting

In traditional Unix filesystems contain special files.  These are:
symbolic links, character devices, block devices, named pipes, and
named sockets.  Naturally the Hurd also support these.

However, if you take a look at `hurd/io.defs` and `hurd/fs.defs`, you'll
find that there are no RPCs that deal specifically with these types.
Sure, you can get the type of the file through `io_stat` (among other
things), but there are none that e.g.  lets you create a symbolic link.

If you take a look at how glibc implements `symlink`, you'll notice
that all it does is create a new file and sets its passive translator to
`/hurd/symlink DEST`.  You can verify this yourself by creating a symlink
with `ln -s foo bar` and print its translator with `showtrans bar`.

This is how the other special files are implemented as well.  The header
`hurd/paths.h` contains a list of paths that are used to implement
special files:

* `/hurd/symlink`
* `/hurd/chrdev`
* `/hurd/blkdev`
* `/hurd/fifo`
* `/hurd/ifsock`

So all special files implemented through special purpose translators,
right?  Well, actually there are no implementations of either `chrdev` or
`blkdev` in the Hurd.  Instead these are implemented purely by /translator
short-circuiting/.

Translator short-circuiting is when a translator, instead of starting a
passive translator, implements the functionality itself.  For instance,
by continuing a file name look up by simply appending the path of a
symlink without returning to the client.

In fact the translators that are implemented are only used as a default
implementation if the underlying translator does not implement the
functionality itself.

To make sure that you use one of these translators, you can start it as
an active translator, or you can use a different path from the one in
`hurd/path.h`, e.g. `settrans bar /hurd/./symlink foo`.

The best example of how short-circuiting is implemented can be found
in `libdiskfs`.  Notice how it detects if a translator is a special
file in `diskfs_S_file_set_translator` and marks the node's `stat`
structure.  And how it later checks the node's `stat` structure in
`diskfs_S_dir_lookup` and handles special file types appropriately.