pkg/web: closure-style middlewares (#51238)

* 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handlers to operate on outdated fields.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

pkg/web/webtest/webtest.go

@@ -37,7 +37,7 @@ func NewServer(t testing.TB, routeRegister routing.RouteRegister) *Server {
 		c.Req = c.Req.WithContext(ctxkey.Set(c.Req.Context(), initCtx))
 	})
 
-	m.Use(requestContextMiddleware())
+	m.UseMiddleware(requestContextMiddleware())
 
 	routeRegister.Register(m.Router)
 	testServer := httptest.NewServer(m)
@@ -126,24 +126,25 @@ func requestContextFromRequest(req *http.Request) *models.ReqContext {
 	return val
 }
 
-func requestContextMiddleware() web.Handler {
-	return func(res http.ResponseWriter, req *http.Request) {
-		c := ctxkey.Get(req.Context()).(*models.ReqContext)
+func requestContextMiddleware() web.Middleware {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			c := ctxkey.Get(r.Context()).(*models.ReqContext)
 
-		ctx := requestContextFromRequest(req)
-		if ctx == nil {
-			c.Next()
-			return
-		}
+			ctx := requestContextFromRequest(r)
+			if ctx != nil {
+				c.SignedInUser = ctx.SignedInUser
+				c.UserToken = ctx.UserToken
+				c.IsSignedIn = ctx.IsSignedIn
+				c.IsRenderCall = ctx.IsRenderCall
+				c.AllowAnonymous = ctx.AllowAnonymous
+				c.SkipCache = ctx.SkipCache
+				c.RequestNonce = ctx.RequestNonce
+				c.PerfmonTimer = ctx.PerfmonTimer
+				c.LookupTokenErr = ctx.LookupTokenErr
+			}
 
-		c.SignedInUser = ctx.SignedInUser
-		c.UserToken = ctx.UserToken
-		c.IsSignedIn = ctx.IsSignedIn
-		c.IsRenderCall = ctx.IsRenderCall
-		c.AllowAnonymous = ctx.AllowAnonymous
-		c.SkipCache = ctx.SkipCache
-		c.RequestNonce = ctx.RequestNonce
-		c.PerfmonTimer = ctx.PerfmonTimer
-		c.LookupTokenErr = ctx.LookupTokenErr
+			next.ServeHTTP(w, r)
+		})
 	}
 }