Snap.Util.FileUploads

This module contains primitives and helper functions for handling requests with Content-type: multipart/form-data, i.e. HTML forms and file uploads.

Typically most users will want to use handleFileUploads, which writes uploaded files to a temporary directory before sending them on to a handler specified by the user.

Users who wish to handle their file uploads differently can use the lower-level interface called handleMultipart. That function takes uploaded files and streams them to a consumer of the user's choosing.

Using these functions requires making "policy" decisions which Snap can't really make for users, such as "what's the largest PDF file a user is allowed to upload?" and "should we read form inputs into the parameters mapping?". Policy is specified on a "global" basis (using UploadPolicy), and on a per-file basis (using PartUploadPolicy, which allows you to reject or limit the size of certain uploaded Content-types).

Example usage:

{-# LANGUAGE OverloadedStrings #-}

module Main where

import qualified Data.ByteString.Char8 as B8
import           Data.Functor          ((<$>))
import           Snap.Core             (Snap, route, writeBS)
import           Snap.Http.Server      (quickHttpServe)
import           Snap.Util.FileUploads
import           System.Posix          (FileOffset, fileSize, getFileStatus)

uploadForm :: Snap ()
uploadForm = writeBS "<form enctype=\"multipart/form-data\" action=\"/do-upload\" method=\"POST\">\
    \<input name=\"file\" type=\"file\" />\
    \<input type=\"submit\" value=\"Send File\" />\
    \</form>"

getFileSize :: FilePath -> IO FileOffset
getFileSize path = fileSize <$> getFileStatus path

-- Upload handler that prints out the uploaded file's size.
doUpload :: Snap ()
doUpload = do
  l <- handleFileUploads "/tmp" defaultUploadPolicy
       (const $ allowWithMaximumSize (getMaximumFormInputSize defaultUploadPolicy))
       (\pinfo mbfname -> do fsize <- either (const $ return 0) getFileSize mbfname
                             return (partFileName pinfo, fsize))
  writeBS . B8.pack . show $ l

site :: Snap ()
site = route
  [ ("/upload",    uploadForm)
  , ("/do-upload", doUpload)]

main :: IO ()
main = quickHttpServe site

Functions

handleFileUploads

Arguments

:: MonadSnap m

=> FilePath

temporary directory

-> UploadPolicy

general upload policy

-> (PartInfo -> PartUploadPolicy)

per-part upload policy

-> (PartInfo -> Either PolicyViolationException FilePath -> IO a)

user handler (see function description)

-> m [a]

Reads uploaded files into a temporary directory and calls a user handler to process them.

Note: THE REQUEST MUST BE CORRECTLY ENCODED. If the request's Content-type is not "multipart/formdata", this function skips processing using pass.

Given a temporary directory, global and file-specific upload policies, and a user handler, this function consumes a request body uploaded with Content-type: multipart/form-data. Each file is read into the temporary directory, and is then passed to the user handler. After the user handler runs (but before the Response body is streamed to the client), the files are deleted from disk; so if you want to retain or use the uploaded files in the generated response, you need to move or otherwise process them.

The argument passed to the user handler is a tuple:

(PartInfo, Either PolicyViolationException FilePath)

The first half of this tuple is a PartInfo, which contains the information the client browser sent about the given upload part (like filename, content-type, etc). The second half of this tuple is an Either stipulating that either:

1. the file was rejected on a policy basis because of the provided PartUploadPolicy handler
2. the file was accepted and exists at the given path.

Exceptions

If the client's upload rate passes below the configured minimum (see setMinimumUploadRate and setMinimumUploadSeconds), this function terminates the connection. This setting is there to protect the server against slowloris-style denial of service attacks.

If the given UploadPolicy stipulates that you wish form inputs to be placed in the rqParams parameter map (using setProcessFormInputs), and a form input exceeds the maximum allowable size, this function will throw a PolicyViolationException.

If an uploaded part contains MIME headers longer than a fixed internal threshold (currently 32KB), this function will throw a BadPartException.

handleMultipart

Arguments

:: MonadSnap m

=> UploadPolicy

global upload policy

-> PartProcessor a

part processor

-> m [a]

Given an upload policy and a function to consume uploaded "parts", consume a request body uploaded with Content-type: multipart/form-data.

Note: THE REQUEST MUST BE CORRECTLY ENCODED. If the request's Content-type is not "multipart/formdata", this function skips processing using pass.

Most users will opt for the higher-level handleFileUploads, which writes to temporary files, rather than handleMultipart. This function should be chosen, however, if you need to stream uploaded files directly to your own processing function: e.g. to a database or a remote service via RPC.

If the client's upload rate passes below the configured minimum (see setMinimumUploadRate and setMinimumUploadSeconds), this function terminates the connection. This setting is there to protect the server against slowloris-style denial of service attacks.

Exceptions

If the given UploadPolicy stipulates that you wish form inputs to be placed in the rqParams parameter map (using setProcessFormInputs), and a form input exceeds the maximum allowable size, this function will throw a PolicyViolationException.

If an uploaded part contains MIME headers longer than a fixed internal threshold (currently 32KB), this function will throw a BadPartException.

type PartProcessor a = PartInfo -> InputStream ByteString -> IO a

A type alias for a function that will process one of the parts of a multipart/form-data HTTP request body.

Uploaded parts

data PartInfo

PartInfo contains information about a "part" in a request uploaded with Content-type: multipart/form-data.

Instances

Show PartInfo

Methods

showsPrec :: Int -> PartInfo -> ShowS

show :: PartInfo -> String

showList :: [PartInfo] -> ShowS

data PartDisposition

Represents the disposition type specified via the Content-Disposition header field. See RFC 1806.

Constructors

DispositionAttachment

Content-Disposition: attachment.

DispositionFile

Content-Disposition: file.

DispositionFormData

Content-Disposition: form-data.

DispositionOther ByteString

Any other value.

Instances

Eq PartDisposition

Methods

(==) :: PartDisposition -> PartDisposition -> Bool

(/=) :: PartDisposition -> PartDisposition -> Bool

Show PartDisposition

Methods

showsPrec :: Int -> PartDisposition -> ShowS

show :: PartDisposition -> String

showList :: [PartDisposition] -> ShowS

partFieldName :: PartInfo -> ByteString

Field name associated with this part (i.e., the name specified with <input name="partFieldName" ...).

partFileName :: PartInfo -> Maybe ByteString

Name of the uploaded file.

partContentType :: PartInfo -> ByteString

Content type of this part.

partHeaders :: PartInfo -> Headers

Remaining headers associated with this part.

partDisposition :: PartInfo -> PartDisposition

Disposition type of this part. See PartDisposition.

Policy

General upload policy

data UploadPolicy

UploadPolicy controls overall policy decisions relating to multipart/form-data uploads, specifically:

• whether to treat parts without filenames as form input (reading them into the rqParams map)
• because form input is read into memory, the maximum size of a form input read in this manner, and the maximum number of form inputs
• the minimum upload rate a client must maintain before we kill the connection; if very low-bitrate uploads were allowed then a Snap server would be vulnerable to a trivial denial-of-service using a "slowloris"-type attack
• the minimum number of seconds which must elapse before we start killing uploads for having too low an upload rate.
• the amount of time we should wait before timing out the connection whenever we receive input from the client.

defaultUploadPolicy :: UploadPolicy

A reasonable set of defaults for upload policy. The default policy is:

maximum form input size

128kB

maximum number of form inputs

10

minimum upload rate

1kB/s

seconds before rate limiting kicks in

10

inactivity timeout

20 seconds

doProcessFormInputs :: UploadPolicy -> Bool

Does this upload policy stipulate that we want to treat parts without filenames as form input?

setProcessFormInputs :: Bool -> UploadPolicy -> UploadPolicy

Set the upload policy for treating parts without filenames as form input.

getMaximumFormInputSize :: UploadPolicy -> Int64

Get the maximum size of a form input which will be read into our rqParams map.

setMaximumFormInputSize :: Int64 -> UploadPolicy -> UploadPolicy

Set the maximum size of a form input which will be read into our rqParams map.

getMaximumNumberOfFormInputs :: UploadPolicy -> Int

Get the maximum size of a form input which will be read into our rqParams map.

setMaximumNumberOfFormInputs :: Int -> UploadPolicy -> UploadPolicy

Set the maximum size of a form input which will be read into our rqParams map.

getMinimumUploadRate :: UploadPolicy -> Double

Get the minimum rate (in bytes/second) a client must maintain before we kill the connection.

setMinimumUploadRate :: Double -> UploadPolicy -> UploadPolicy

Set the minimum rate (in bytes/second) a client must maintain before we kill the connection.

getMinimumUploadSeconds :: UploadPolicy -> Int

Get the amount of time which must elapse before we begin enforcing the upload rate minimum

setMinimumUploadSeconds :: Int -> UploadPolicy -> UploadPolicy

Set the amount of time which must elapse before we begin enforcing the upload rate minimum

getUploadTimeout :: UploadPolicy -> Int

Get the "upload timeout". Whenever input is received from the client, the connection timeout is set this many seconds in the future.

setUploadTimeout :: Int -> UploadPolicy -> UploadPolicy

Set the upload timeout.

Per-file upload policy

data PartUploadPolicy

Upload policy can be set on an "general" basis (using UploadPolicy), but handlers can also make policy decisions on individual files/parts uploaded. For each part uploaded, handlers can decide:

• whether to allow the file upload at all
• the maximum size of uploaded files, if allowed

disallow :: PartUploadPolicy

Disallows the file to be uploaded.

allowWithMaximumSize :: Int64 -> PartUploadPolicy

Allows the file to be uploaded, with maximum size n.

Exceptions

data FileUploadException

All of the exceptions defined in this package inherit from FileUploadException, so if you write

foo `catch` \(e :: FileUploadException) -> ...

you can catch a BadPartException, a PolicyViolationException, etc.

Instances

Show FileUploadException

Methods

showsPrec :: Int -> FileUploadException -> ShowS

show :: FileUploadException -> String

showList :: [FileUploadException] -> ShowS

Exception FileUploadException

Methods

toException :: FileUploadException -> SomeException

fromException :: SomeException -> Maybe FileUploadException

displayException :: FileUploadException -> String

fileUploadExceptionReason :: FileUploadException -> Text

Human-readable error message corresponding to the FileUploadException.

data BadPartException

Thrown when a part is invalid in some way (e.g. the headers are too large).

Instances

Show BadPartException

Methods

showsPrec :: Int -> BadPartException -> ShowS

show :: BadPartException -> String

showList :: [BadPartException] -> ShowS

Exception BadPartException

Methods

toException :: BadPartException -> SomeException

fromException :: SomeException -> Maybe BadPartException

displayException :: BadPartException -> String

badPartExceptionReason :: BadPartException -> Text

Human-readable error message corresponding to the BadPartException.

data PolicyViolationException

Thrown when an UploadPolicy or PartUploadPolicy is violated.

Instances

Show PolicyViolationException

Methods

showsPrec :: Int -> PolicyViolationException -> ShowS

show :: PolicyViolationException -> String

showList :: [PolicyViolationException] -> ShowS

Exception PolicyViolationException

Methods

toException :: PolicyViolationException -> SomeException

fromException :: SomeException -> Maybe PolicyViolationException

displayException :: PolicyViolationException -> String

policyViolationExceptionReason :: PolicyViolationException -> Text

Human-readable error message corresponding to the PolicyViolationException.