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Finish "git annex watch" command, which runs, in the background, watching via
inotify for changes, and automatically annexing new files, etc.
There is a `watch` branch in git that adds the command.
[[!toc]]
## known bugs
* If a file is checked into git as a normal file and gets modified
(or merged, etc), it will be converted into an annexed file.
See [[blog/day_7__bugfixes]]
* When you `git annex unlock` a file, it will immediately be re-locked.
* With kqueue, if a while is created and still has a writer, it'll
give up adding it, and it will never get added. This is because kqueue
cannot track file closes. Need to go back and check these files every
second or something.
## beyond Linux
I'd also like to support OSX and if possible the BSDs.
* kqueue ([haskell bindings](http://hackage.haskell.org/package/kqueue))
is supported by FreeBSD, OSX, and other BSDs.
In kqueue, to watch for changes to a file, you have to have an open file
descriptor to the file. This wouldn't scale.
Apparently, a directory can be watched, and events are generated when
files are added/removed from it. You then have to scan to find which
files changed. [example](https://developer.apple.com/library/mac/#samplecode/FileNotification/Listings/Main_c.html#//apple_ref/doc/uid/DTS10003143-Main_c-DontLinkElementID_3)
Gamin does the best it can with just kqueue, supplimented by polling.
The source file `server/gam_kqueue.c` makes for interesting reading.
Using gamin to do the heavy lifting is one option.
([haskell bindings](http://hackage.haskell.org/package/hlibfam) for FAM;
gamin shares the API)
kqueue does not seem to provide a way to tell when a file gets closed,
only when it's initially created. Poses problems..
* [man page](http://www.freebsd.org/cgi/man.cgi?query=kqueue&apropos=0&sektion=0&format=html)
* <https://github.com/gorakhargosh/watchdog/blob/master/src/watchdog/observers/kqueue.py> (good example program)
* hfsevents ([haskell bindings](http://hackage.haskell.org/package/hfsevents))
is OSX specific.
Originally it was only directory level, and you were only told a
directory had changed and not which file. Based on the haskell
binding's code, from OSX 10.7.0, file level events were added.
This will be harder for me to develop for, since I don't have access to
OSX machines..
hfsevents does not seem to provide a way to tell when a file gets closed,
only when it's initially created. Poses problems..
* <https://developer.apple.com/library/mac/#documentation/Darwin/Conceptual/FSEvents_ProgGuide/Introduction/Introduction.html>
* <http://pypi.python.org/pypi/MacFSEvents/0.2.8> (good example program)
* <https://github.com/gorakhargosh/watchdog/blob/master/src/watchdog/observers/fsevents.py> (good example program)
* Windows has a Win32 ReadDirectoryChangesW, and perhaps other things.
## todo
- Run niced and ioniced? Seems to make sense, this is a background job.
- configurable option to only annex files meeting certian size or
filename criteria
- option to check files not meeting annex criteria into git directly,
automatically
- honor .gitignore, not adding files it excludes (difficult, probably
needs my own .gitignore parser to avoid excessive running of git commands
to check for ignored files)
- Possibly, when a directory is moved out of the annex location,
unannex its contents. (Does inotify tell us where the directory moved
to so we can access it?)
## the races
Many races need to be dealt with by this code. Here are some of them.
* File is added and then removed before the add event starts.
Not a problem; The add event does nothing since the file is not present.
* File is added and then removed before the add event has finished
processing it.
**Minor problem**; When the add's processing of the file (checksum and so
on) fails due to it going away, there is an ugly error message, but
things are otherwise ok.
* File is added and then replaced with another file before the annex add
moves its content into the annex.
Fixed this problem; Now it hard links the file to a temp directory and
operates on the hard link, which is also made unwritable.
* File is added and then replaced with another file before the annex add
makes its symlink.
**Minor problem**; The annex add will fail creating its symlink since
the file exists. There is an ugly error message, but the second add
event will add the new file.
* File is added and then replaced with another file before the annex add
stages the symlink in git.
Now fixed; `git annex watch` avoids running `git add` because of this
race. Instead, it stages symlinks directly into the index, without
looking at what's currently on disk.
* Link is moved, fixed link is written by fix event, but then that is
removed by the user and replaced with a file before the event finishes.
Now fixed; same fix as previous race above.
* File is removed and then re-added before the removal event starts.
Not a problem; The removal event does nothing since the file exists,
and the add event replaces it in git with the new one.
* File is removed and then re-added before the removal event finishes.
Not a problem; The removal event removes the old file from the index, and
the add event adds the new one.
## done
- on startup, add any files that have appeared since last run **done**
- on startup, fix the symlinks for any renamed links **done**
- on startup, stage any files that have been deleted since last run
(seems to require a `git commit -a` on startup, or at least a
`git add --update`, which will notice deleted files) **done**
- notice new files, and git annex add **done**
- notice renamed files, auto-fix the symlink, and stage the new file location
**done**
- handle cases where directories are moved outside the repo, and stop
watching them **done**
- when a whole directory is deleted or moved, stage removal of its
contents from the index **done**
- notice deleted files and stage the deletion
(tricky; there's a race with add since it replaces the file with a symlink..)
**done**
- Gracefully handle when the default limit of 8192 inotified directories
is exceeded. This can be tuned by root, so help the user fix it.
**done**
- periodically auto-commit staged changes (avoid autocommitting when
lots of changes are coming in) **done**
- coleasce related add/rm events for speed and less disk IO **done**
- don't annex `.gitignore` and `.gitattributes` files **done**
- run as a daemon **done**
- A process has a file open for write, another one closes it,
and so it's added. Then the first process modifies it.
Or, a process has a file open for write when `git annex watch` starts
up, it will be added to the annex. If the process later continues
writing, it will change content in the annex.
This changes content in the annex, and fsck will later catch
the inconsistency.
Possible fixes:
* Somehow track or detect if a file is open for write by any processes.
`lsof` could be used, although it would be a little slow.
Here's one way to avoid the slowdown: When a file is being added,
set it read-only, and hard-link it into a quarantine directory,
remembering both filenames.
Then use the batch change mode code to detect batch adds and bundle
them together.
Just before committing, lsof the quarantine directory. Any files in
it that are still open for write can just have their write bit turned
back on and be deleted from quarantine, to be handled when their writer
closes. Files that pass quarantine get added as usual. This avoids
repeated lsof calls slowing down adds, but does add a constant factor
overhead (0.25 seconds lsof call) before any add gets committed. **done**
* Or, when possible, making a copy on write copy before adding the file
would avoid this.
* Or, as a last resort, make an expensive copy of the file and add that.
* Tracking file opens and closes with inotify could tell if any other
processes have the file open. But there are problems.. It doesn't
seem to differentiate between files opened for read and for write.
And there would still be a race after the last close and before it's
injected into the annex, where it could be opened for write again.
Would need to detect that and undo the annex injection or something.
|
