50 lines
2.7 KiB
Markdown
50 lines
2.7 KiB
Markdown
I've been investigating how to make `git annex watch` work on
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FreeBSD, and by extension, OSX.
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One option is kqueue, which works on both operating systems, and allows
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very basic monitoring of file changes. There's also an OSX specific
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hfsevents interface.
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Kqueue is far from optimal for `git annex watch`, because it provides even
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less information than inotify (which didn't really provide everything I
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needed, thus the lsof hack). Kqueue doesn't have events for files being
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closed, only an event when a file is created. So it will be difficult for
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`git annex watch` to know when a file is done being written to and can be
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annexed. git annex will probably need to run lsof periodically to check when
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recently added files are complete. (hsevents shares this limitation)
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Kqueue also doesn't provide specific events when a file or directory is
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moved. Indeed, it doesn't provide specific events about what changed at
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all. All you get with kqueue is a generic "oh hey, the directory you're
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watching changed in some way", and it's up to you to scan it to work out
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how. So git annex will probably need to run `git ls-tree --others`
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to find changes in the directory tree. This could be expensive with large
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trees. (hsevents has per-file events on current versions of OSX)
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Despite these warts, I want to try kqueue first, since it's more portable
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than hfsevents, and will surely be easier for me to develop support for,
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since I don't have direct access to OSX.
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So I went to a handy Debian kFreeBSD porter box, and tried some kqueue
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stuff to get a feel for it. I got a python program that does basic
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directory monitoring with kqueue to work, so I know it's usable there.
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Next step was getting kqueue working from Haskell. Should be easy, there's
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a Haskell library already. I spent a while trying to get it to work on
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Debian kFreeBSD, but ran into a
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[problem](https://github.com/hesselink/kqueue/issues/1) that could be
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caused by the Debian kFreeBSD being different, or just a bug in the Haskell
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library. I didn't want to spend too long shaving this yak; I might install
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"real" FreeBSD on a spare laptop and try to get it working there instead.
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But for now, I've dropped down to C instead, and have a simple C program
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that can monitor a directory with kqueue. Next I'll turn it into a simple
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library, which can easily be linked into my Haskell code. The Haskell code
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will pass it a set of open directory descriptors, and it'll return the
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one that it gets an event on. This is necessary because kqueue doesn't
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recurse into subdirectories on its own.
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I've generally had good luck with this approach to adding stuff in Haskell;
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rather than writing a bit-banging and structure packing low level interface
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in Haskell, write it in C, with a simpler interface between C and
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Haskell.
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