> I'm writign a file manager (efm) - when files get "changed" i need to
> re-evaluate them - that means if the mime type is image/* re-generate a
> thumbnail... and that means openig file, read, generate thumb, write,
> display.
>
> the problem is the change event comes in whilste the file is still
> being written it isnt finished yet - and theres no way to know if
> someone is activel ywritign to the file- yes - i suppose i could use
> fuser and check if tis opened - but it may beopened for read onyl- or
> the writes maye be finsihed but the file nto close (maybe a log file of
> some sort)
>
> so beig able tohave events like:
> file written at byte 16384 size 4096
> file read at byte 300 size 256
> etc.
> woudl be greta - then at ;eats i could pause the thumbnail generation
> until writes have stopped (ie no more write events for that file come
> in, and then wait a small amoutn of time after each event comign in
> before spawinging the thumbnail generator)
>
> the problem si some files take a while to writ e- for example saving a
> 1024x768 png in gimp actually takes like 3 or 4 seconds.... and some
> formats save instantly... so it'd be great to have a way of telling
> this.. :) and sicne fam can alreayd tell when when things execute, i'm
> sure it probabyl coudl tell me when they read/write :)
It sounds like what you want to know is not when & where someone writes,
but when someone *stops* writing. (So instead of spawning the thumbnail
generator on a Changed event, you do it on a "DoneBeingChanged" event.)
I don't know of a good way to do this. A not-so-good way for fam to do it
would be if imon reported when a file was opened & closed; fam could then
pass that on to clients. (That would take changes to imon, the interface
between imon and fam, and the fam API. The reason it's not so good is that
you still have the problem of knowing whether the process that just closed
the file is the same one that was writing to it, whether or not the writing
process is done, etc.) Failing that, a not-so-good way for the client
application to do it would be to always wait a certain amount of time
between the last Changed event on a file and starting the thumbnail
generator on the file, but that's so bad, forget I even suggested it.
Maybe the right way for this to work is, you spawn the thumbnail generator
whenever you get a Changed event, and the first thing the t.g. does is
fcntl(fd, F_SETLKW, {F_RDLCK, ...}) the file, and hope that whoever's
writing the file was polite enough to lock it while they're writing. That
way the t.g. sleeps until the writing process explicitly says "OK, I'm done
writing." I like that, but I have no idea how many image-writing processes
are that polite. (I wouldn't be surprised if the answer is "none.")
--Rusty
P.S. The problem with imon passing the offset & length of writes to fam is
that, in the current implementation, imon doesn't actually know the offset
& length. If its hooks into the fs code were rewritten, it would be able to
do this, but you'd still have to change the interface between imon & fam,
and the fam API, and I think that information still wouldn't tell you when
someone was done changing the file.
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