pkg/web: restrict handler types (#48495)

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`.

pkg/services/accesscontrol/middleware_test.go

@@ -11,6 +11,7 @@ import (
 	"github.com/grafana/grafana/pkg/models"
 	"github.com/grafana/grafana/pkg/services/accesscontrol"
 	"github.com/grafana/grafana/pkg/services/accesscontrol/mock"
+	"github.com/grafana/grafana/pkg/services/contexthandler/ctxkey"
 	"github.com/grafana/grafana/pkg/web"
 )
 
@@ -90,5 +91,8 @@ func contextProvider() web.Handler {
 			SkipCache:    true,
 		}
 		c.Map(reqCtx)
+
+		c.Req = c.Req.WithContext(ctxkey.Set(c.Req.Context(), reqCtx))
+		c.Map(c.Req)
 	}
 }

pkg/services/accesscontrol/resourcepermissions/api_test.go

@@ -18,6 +18,7 @@ import (
 	"github.com/grafana/grafana/pkg/infra/log"
 	"github.com/grafana/grafana/pkg/models"
 	"github.com/grafana/grafana/pkg/services/accesscontrol"
+	"github.com/grafana/grafana/pkg/services/contexthandler/ctxkey"
 	"github.com/grafana/grafana/pkg/services/sqlstore"
 	"github.com/grafana/grafana/pkg/setting"
 	"github.com/grafana/grafana/pkg/web"
@@ -500,6 +501,9 @@ func contextProvider(tc *testContext) web.Handler {
 			Logger:       log.New("test"),
 		}
 		c.Map(reqCtx)
+
+		c.Req = c.Req.WithContext(ctxkey.Set(c.Req.Context(), reqCtx))
+		c.Map(c.Req)
 	}
 }
 

pkg/services/contexthandler/contexthandler.go

@@ -19,6 +19,7 @@ import (
 	"github.com/grafana/grafana/pkg/middleware/cookies"
 	"github.com/grafana/grafana/pkg/models"
 	"github.com/grafana/grafana/pkg/services/contexthandler/authproxy"
+	"github.com/grafana/grafana/pkg/services/contexthandler/ctxkey"
 	"github.com/grafana/grafana/pkg/services/login"
 	"github.com/grafana/grafana/pkg/services/rendering"
 	"github.com/grafana/grafana/pkg/services/sqlstore"
@@ -69,7 +70,7 @@ type ContextHandler struct {
 	GetTime func() time.Time
 }
 
-type reqContextKey struct{}
+type reqContextKey = ctxkey.Key
 
 // FromContext returns the ReqContext value stored in a context.Context, if any.
 func FromContext(c context.Context) *models.ReqContext {
@@ -94,7 +95,7 @@ func (h *ContextHandler) Middleware(mContext *web.Context) {
 	}
 
 	// Inject ReqContext into a request context and replace the request instance in the macaron context
-	mContext.Req = mContext.Req.WithContext(context.WithValue(mContext.Req.Context(), reqContextKey{}, reqContext))
+	mContext.Req = mContext.Req.WithContext(ctxkey.Set(mContext.Req.Context(), reqContext))
 	mContext.Map(mContext.Req)
 
 	traceID := tracing.TraceIDFromContext(mContext.Req.Context(), false)

pkg/services/contexthandler/ctxkey/key.go

@@ -0,0 +1,13 @@
+package ctxkey
+
+import "context"
+
+type Key struct{}
+
+func Set(ctx context.Context, data interface{}) context.Context {
+	return context.WithValue(ctx, Key{}, data)
+}
+
+func Get(ctx context.Context) interface{} {
+	return ctx.Value(Key{})
+}

pkg/services/ldap/testing.go

@@ -4,6 +4,9 @@ import (
 	"crypto/tls"
 
 	"gopkg.in/ldap.v3"
+
+	//TODO(sh0rez): remove once import cycle resolved
+	_ "github.com/grafana/grafana/pkg/api/response"
 )
 
 type searchFunc = func(request *ldap.SearchRequest) (*ldap.SearchResult, error)

pkg/services/multildap/multildap_test.go

@@ -8,6 +8,9 @@ import (
 	"github.com/grafana/grafana/pkg/services/ldap"
 
 	"github.com/stretchr/testify/require"
+
+	//TODO(sh0rez): remove once import cycle resolved
+	_ "github.com/grafana/grafana/pkg/api/response"
 )
 
 func TestMultiLDAP(t *testing.T) {

pkg/services/serviceaccounts/api/api_test.go

@@ -15,6 +15,7 @@ import (
 	"github.com/grafana/grafana/pkg/models"
 	"github.com/grafana/grafana/pkg/services/accesscontrol"
 	accesscontrolmock "github.com/grafana/grafana/pkg/services/accesscontrol/mock"
+	"github.com/grafana/grafana/pkg/services/contexthandler/ctxkey"
 	"github.com/grafana/grafana/pkg/services/featuremgmt"
 	"github.com/grafana/grafana/pkg/services/serviceaccounts"
 	"github.com/grafana/grafana/pkg/services/serviceaccounts/database"
@@ -238,6 +239,9 @@ func setupTestServer(t *testing.T, svc *tests.ServiceAccountMock,
 			Logger:       log.New("serviceaccounts-test"),
 		}
 		c.Map(ctx)
+
+		c.Req = c.Req.WithContext(ctxkey.Set(c.Req.Context(), ctx))
+		c.Map(c.Req)
 	})
 	a.RouterRegister.Register(m.Router)
 	return m, a