[web-devel] Upgrading to warp 2

[Tyler Huffman]

Thanks for keeping the mailing list involved. It's pretty quiet, but those
of us that develop on the many Haskell web frameworks highly appreciate it.
=)


Regards,
Tyler Huffman


On Tue, Feb 25, 2014 at 12:43 PM, Bardur Arantsson <spam at scientician.net>wrote:

> On 2014-02-25 05:49, Michael Snoyman wrote:
> > On Mon, Feb 24, 2014 at 11:00 PM, Bardur Arantsson <spam at scientician.net
> >wrote:
>
> (I've taken the liberty of CC'ing the list.)
>
> >
> >> Hi all,
> >>
> [--snip--]
> >> When I try this I get:
> >>
> >>   src/Handlers.hs:96:54:
> >>     No instance for (MonadResource IO)
> >>       arising from a use of `sourceFileRange'
> >>     Possible fix: add an instance declaration for (MonadResource IO)
> >>     In the second argument of `($)', namely
> >>       `sourceFileRange sfp (Just l') (Just n)'
> >>     In the expression:
> >>       mapOutput (Chunk . fromByteString)
> >>       $ sourceFileRange sfp (Just l') (Just n)
> >>     In an equation for `src':
> >>         src
> >>           = mapOutput (Chunk . fromByteString)
> >>             $ sourceFileRange sfp (Just l') (Just n)
> >>
> >> ... which I guess kinda makes sense -- you'd need a ResourceT (or
> >> similar) in the monad stack. I'm just not sure how or where to introduce
> >> the ResourceT to get this working.
> >>
> >> If I introduce an explicit type signature for "src",
> >>
> >>    src :: forall m . MonadResource m => Source m (Flush Builder)
> >>
> >> then that line compiles, but I get the equivalent error at the usage
> >> site of "src" in the third argument to "responseSource" (which again
> >> also makes sense).
> >>
> >> Anyone got any pointers?
> >>
> >> Regards,
> >>
> >>
> > Firstly, why aren't you just using responseFile[1]? That will be more
> > efficient than using a Source directly, since Warp is able to use the
> > sendfile system call, not to mention all of Kazu's fancy fd-caching
> logic.
> > Even on a non-Warp backend, the functionality can simply degrade to being
> > the same as using a Source. Also, not sure if it's relevant, but Warp can
> > automatically handle range requests for you, so if you simply return a
> > responseFile and the user requests part of the file, Warp will
> > automatically generate the 206 response.
> >
>
> ResponseFile is broken for some clients (with Linux host at least),
> specifically my type of client, a PlayStation 3 (UPnP media player).
> There was a Thread of Doom(TM) about it on haskell-cafe, but it caused
> FD leaks, though we never found out quite *why* it was broken that way.
>
> (Things may have changed since this was in February 2010, but frankly I
> don't even care to spend the time to find out -- I don't need the
> performance anyway. I think I remember testing the WAI/Warp sendfile
> equivalent in pre-2.0 WAI/Warp and finding that it still had the same
> problem.)
>
> > Anyway, the issue here is that in WAI 2.0, we removed ResourceT from the
> > WAI stack for efficiency reasons.
>
> Yup, I noticed that ;)
>
> > That makes the cases where you *do* need
> > to safely allocate scarce resources like a file handle a bit trickier. In
> > this case, I wouldn't bother with ResourceT at all, but instead
> explicitly
> > use the bracket pattern, via responseSourceBracket, openFile, and
> > closeFile. This would look like:
> >
> > serveFile :: Integer -> [(Maybe Integer, Maybe Integer)] -> IO Response
> > serveFile fsz [(l,h)] = do
> >   let l' = maybe 0 id l
> >   let h' = maybe (fsz-1) id h
> >   let n = (h' - l' + 1)
> >   let src h = mapOutput (Chunk . fromByteString) $ sourceHandleRange h
> > (Just l') (Just n)
> >       sfp = "foo.hs"
> >   responseSourceBracket
> >     (IO.openFile sfp IO.ReadMode) (IO.hClose) $ \h ->
> >         return (partialContent206, [], src h)
> >
> >
> > [1]
> >
> http://hackage.haskell.org/package/wai-2.0.0/docs/Network-Wai.html#v:responseFile
> >
>
> Ah, yes, this works! I had noticed responseSourceBracket, but (stupidly)
> didn't actually try it. Thank you very much.
>
> Regards,
>
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