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Diffstat (limited to 'hurd/translator/short-circuiting.mdwn')
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diff --git a/hurd/translator/short-circuiting.mdwn b/hurd/translator/short-circuiting.mdwn deleted file mode 100644 index 9de9f7b8..00000000 --- a/hurd/translator/short-circuiting.mdwn +++ /dev/null @@ -1,88 +0,0 @@ -[[!meta copyright="Copyright © 2009 Free Software Foundation, Inc."]] - -[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable -id="license" text="Permission is granted to copy, distribute and/or modify this -document under the terms of the GNU Free Documentation License, Version 1.2 or -any later version published by the Free Software Foundation; with no Invariant -Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license -is included in the section entitled -[[GNU Free Documentation License|/fdl]]."]]"""]] - -In traditional [[Unix]], file systems 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 [[RPC]]s 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 set its passive translator to -`/hurd/symlink DEST`. You can verify this yourself by creating a symlink -with `ln -s foo bar` and print its passive translator setting 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 are implemented through special-purpose translators, -right? Not quite, instead the translators of this list are often -implemented in their underlying filesystem through *translator -short-circuiting*. In fact, `chrdev` and `blkdev` aren't even implemented -as translators at all. - -Translator short-circuiting is when a file system server implements the -functionality of a passive translator itself, instead of actually starting -it. For instance, all the [[`symlink`|symlink]] translator does is return -a `FS_RETRY_*` reply to the caller. So instead of starting it, the file -system server can simply continue the file name look-up internally by -appending the target of the symbolic link to the path being looked-up. - -This way, we can skip starting the `symlink` translator, skip retrying -the look-up on the newly started translator, and we might also skip a -retry to the same file system server again, if the target of the symbolic -link is in it. - -In fact, the list's translators that actually are implemented (`symlink`, -`fifo`, `ifsock`) are only used as a default implementation if the underlying -file system's translator does not implement the functionality itself, i.e., if -it doesn't short-circuit it. - -To make sure that you use one of these translators, there by bypassing the -short-circuiting mechanism, you can either start it as -an active translator, or 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`|libdiskfs]]. Notice how it detects if a translator to store -is a special file in `diskfs_S_file_set_translator` and instead -of storing a real passive translator setting on the disk, stores it as a -symlink node (using `diskfs_create_symlink_hook` or a generic implementation). - -In later look-ups to the node, it checks the node's `stat` structure in -`diskfs_S_file_get_translator`, or -`diskfs_S_dir_lookup` and handles special file types appropriately. - -Doing this translator short-circuiting has disadvantages: code duplication, or -in general adding code complexity that isn't needed for implementing the same -functionality, but it also has advantages: using functionality that the file -system's data structures nevertheless already provide -- storing symbolic links -in `ext2fs`' inodes instead of storing passive translator settings -- and thus -staying compatible with other operating systems mounting that file system. - -Also, this short-circuiting does preserve system resources, as it's no longer -required to start a `symlink` translator for resolving each symbolic link, as -well as it does reduce the [[RPC]] overhead. - -It can also confuse users who expect the passive translator to start. -For instance, if a user notices that [[`symlink`|symlink]]'s code is -lacking some functionality, but that it unexpectedly works when the user -tries to run it. |