* pkg/web: closure-style middlewares
Switches the middleware execution model from web.Handlers in a slice to
web.Middleware.
Middlewares are temporarily kept in a slice to preserve ordering, but
prior to execution they are applied, forming a giant call-stack, giving
granular control over the execution flow.
* pkg/middleware: adapt to web.Middleware
* pkg/middleware/recovery: use c.Req over req
c.Req gets updated by future handlers, while req stays static.
The current recovery implementation needs this newer information
* pkg/web: correct middleware ordering
* pkg/webtest: adapt middleware
* pkg/web/hack: set w and r onto web.Context
By adopting std middlewares, it may happen they invoke next(w,r) without
putting their modified w,r into the web.Context, leading old-style
handlers to operate on outdated fields.
pkg/web now takes care of this
* pkg/middleware: selectively use future context
* pkg/web: accept closure-style on Use()
* webtest: Middleware testing
adds a utility function to web/webtest to obtain a http.ResponseWriter,
http.Request and http.Handler the same as a middleware that runs would receive
* *: cleanup
* pkg/web: don't wrap Middleware from Router
* pkg/web: require chain to write response
* *: remove temp files
* webtest: don't require chain write
* *: cleanup
Makes `pkg/web` only accept handles from the following set:
```go
handlerStd = func(http.ResponseWriter, *http.Request)
handlerStdCtx = func(http.ResponseWriter, *http.Request, *web.Context)
handlerStdReqCtx = func(http.ResponseWriter, *http.Request, *models.ReqContext)
handlerReqCtx = func(*models.ReqContext)
handlerReqCtxRes = func(*models.ReqContext) Response
handlerCtx = func(*web.Context)
```
This is a first step to reducing above set to only `http.Handler`.
---
Due to a cyclic import situation between `pkg/models` and `pkg/web`, parts of this PR were put into `pkg/api/response`, even though they definitely do not belong there. This however is _temporary_ until we untangle `models.ReqContext`.
