Last-Modified header added twice (once by Warp)

[creichert]

I noticed today that on a few of my apps, I was receiving the Last-Modified header twice (and the Vary header).

I pinned down a change that is the reason this header is displayed twice to this line of code.

Which lead me down a rabbit hole trying to determine:

• Is the file read twice to calculate the file info metadata? Once by warp and once by wai-middleware-static, independently?

  • https://github.com/yesodweb/wai/blob/7819ee605324496e31fd86408e521dc0f484744a/warp/Network/Wai/Handler/Warp/Response.hs#L274

  • wai-middleware-static/Network/Wai/Middleware/Static.hs

    Line 229 in 248f20a

    callback $ responseFile status200 headers fp Nothing

I am going to dig into this issue a bit more and hopefully come up with a patch. However, I wanted to see if anyone has seen anything like this before? Am I missing some communication that happens between the middleware and Warp that informs Warp not to read this file again?

---

Not locally reproducible, but here is one service I have up which does this:

$ curl 'https://assertible.com/css/commons.cce252e7dd827e502280.css' -D - -o/dev/null -s
HTTP/1.1 200 OK
Accept-Ranges: bytes
Cache-Control: no-transform,public,max-age=300,s-maxage=900
Content-Type: text/css
Date: Tue, 03 Jul 2018 18:35:24 GMT
ETag: d34c427941055bdfa2009d02b89701c2ae1bccd1
Last-Modified: Wed, 27 Jun 2018 22:36:59 GMT
Last-Modified: Wed, 27-Jun-2018 22:36:59 UTC
Server: nginx/1.8.1
Vary: Accept-Encoding
Vary: Accept-Encoding
Content-Length: 105282
Connection: keep-alive

[creichert]

Additionally, looking through wai-app-static, it seems they don't add Last-Modified there:
https://github.com/yesodweb/wai/blob/7819ee605324496e31fd86408e521dc0f484744a/wai-app-static/Network/Wai/Application/Static.hs#L244