Re: [web-devel] Generating, serving, and deleting temporary files with Wai/Yesod

I think it's great; I've just released wai 0.2.2 with those code changes. Thanks, Michael On Sat, Oct 23, 2010 at 11:56 PM, Matt Brown wrote:

Thanks for the feedback.  I reverted my last commit and pushed a rewrite, trying to follow your advice.  Seems much cleaner now; what do you think?

http://github.com/softmechanics/wai/commit/b37747ab9e2fd031512661ea57dc52c96...

-matt

On Sat, Oct 23, 2010 at 10:54 AM, Michael Snoyman wrote:

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On Fri, Oct 22, 2010 at 8:21 PM, Matt Brown wrote:

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Hello all,

I've been experimenting with generating graphs from log files using timeplot and serving them with yesod.  Each generated graph is only served once, so I need a way to clean them up.  A cron job would be ok, but it's one more thing I'd have to remember to do, and I'm lazy.

Instead, I'd like to pass a "clean up action" to the enumerator sending the file, to be performed after EOF is reached.  For most files, this would just close the handle.  For temp files, it would also delete the file.  Here's the gist of it: http://gist.github.com/640902

The first revision defines a monolithic fromTempFile :: FilePath -> Enumerator.  The second imports a factored version from Network.Wai.Handler (see my wai commit: http://github.com/softmechanics/wai/commit/2cb5ea47ca458f4b91a73e01a1dd0a29e...). Not sure wai is the place for it.  Perhaps wai-extra would be a better?

Does this seem like a good approach?

This does seem like the right approach. Just as fair warning: using an enumerator for serving files loses out on the possible optimization of a sendfile system call. Another possibility may be to keep a pool of file names to use as temporary files and keep rotating their usage. But I *do* think your approach is the correct one.

Regarding the API changes to wai: they seem reasonable, and I'd probably accept them. Some concerns about the implementation:

* Instead of fromHandle' and fromFile', perhaps fromHandleFinally and fromFileFinally? * In fromFile', why are you calling hClose? withBinaryFile automatically closes the file handle. Are you worried about the temporary file getting deleted while the file handle is still open? * In fromHandle', onEOF won't get called if the iteratee returns Left. Is this intended behavior? * Your code won't work in the presence of exceptions. Ideally the body of fromHandle' would be wrapped with a call to finally. This point is related to the previous one.

Michael