Consolidate all header parsing code

[muratg]

Header parsing logic is used in

• MVC
• StaticFiles
• ResponseCaching

We should investigate the usages across the board and consolidate the code here.

[muratg]

Tracking issues:

• HttpAbstractions Reduce allocations by using StringSegment in parsers #758
• ...

[Tratcher]

@Eilon #758 is complete, next we need to investigate having MVC replace their implementation with this one.

[muratg]

@Eilon Moving the rest of the work to preview3.

[muratg]

@Eilon could we do this in 2.1?

[Eilon]

Yessss... @jkotalik is actually already working on some of this, starting with the toughest of them all: MediaType header value.

[jkotalik]

@muratg @Eilon We added structured suffixes and facets to MediaTypeHeaderValue (MTHV) in HttpAbstractions. I was trying to modify MediaType in MVC to wrap MTHV, though it seemed non trivial and wouldn't have much benefit until we either remove MediaType or can deprecate it. What other header logic can be moved from StaticFiles/ResponseCaching to HttpAbstractions?

[muratg]

I think we can deprecate MediaType. @Eilon thoughts?

[Tratcher]

@jkotalik the idea is to at least remove the duplicate code and have MediaType refer to MTHV. MediaType's code has already been the source of multiple critical bugs.

As for StaticFiles and ResponseCaching you'll have to investigate yourself. Those components process lots of headers but they mostly use HttpAbstractions types to do it.

[rynowak]

@javiercn did some research on obsoleting MediaType and I think his findings were promising. @javiercn do you remember what the conclusions were?

[rynowak]

@javiercn just came by to express to me just how excited he is to share his wealth of knowledge and extensive notes in this area; alas, he had to go to an important meeting first.

[Eilon]

I would only obsolete MediaType if we actually plan to get rid of it, say, in MVC 3.0. Would that be the plan?

[javiercn]

We should have a design we are all happy with for 3.0 (one that allows efficient header non allocating iteration as a basic API and a richer strongly typed allocating API for convenience)

[Eilon]

I set up a mtg for this for tomorrow. I invited @javiercn @muratg @jkotalik @rynowak @Tratcher .

[Eilon]

Plan from the mtg:

• Change MediaType into a facade that just calls into MediaTypeHeaderValue
• Mark MediaType as obsolete
• Find all public exposure of MediaType in MVC, obsolete them, and make sure that there are non-obsolete alternatives
• Do basic before/after perf tests on MVC scenarios for conneg to assess whether there is a regression, and, if so, how much

[aspnet-hello]

This issue was moved to dotnet/aspnetcore#2705