[MultipartReaderStream] Misleading exception message

[Eilon]

From @mabakay on November 16, 2016 22:53

When multipart/form-data request is malformed for instance (missing double dash at boundary start):

9051914041544843365972754266
Content-Disposition: form-data; name="text"

text default
--9051914041544843365972754266--

and request body is greater than default limit for HeadersLengthLimit (which is 16KB now), user get exception message of "Multipart body length limit 16384 exceeded."

MultipartReaderStream.cs #166

Copied from original issue: aspnet/Mvc#5534

[Eilon]

https://github.com/aspnet/HttpAbstractions/blob/dev/src/Microsoft.AspNetCore.WebUtilities/MultipartReaderStream.cs#L166

[muratg]

Assigning to @Tratcher for investigation.

[Tratcher]

The confusion here is that multipart forms don't start with the first boundary, there can be an arbitrary amount of data prior to the first boundary.

Hello world, how are you? This is preamble data
--9051914041544843365972754266
Content-Disposition: form-data; name="text"

text default
--9051914041544843365972754266--

Because the boundary is not correctly formatted in your example, that data is all parsed as part of the initial 'body' and so the body limit applies. Note that the limits can't currently be configured for this initial section.

HttpAbstractions/src/Microsoft.AspNetCore.WebUtilities/MultipartReader.cs

Lines 48 to 50 in 594f559

	// This stream will drain any preamble data and remove the first boundary marker.
	// TODO: HeadersLengthLimit can't be modified until after the constructor.
	_currentStream = new MultipartReaderStream(_stream, _boundary) { LengthLimit = HeadersLengthLimit };