Refactor web route (#24080)

The old code is unnecessarily complex, and has many misuses.

Old code "wraps" a lot, wrap wrap wrap, it's difficult to understand
which kind of handler is used.

The new code uses a general approach, we do not need to write all kinds
of handlers into the "wrapper", do not need to wrap them again and
again.

New code, there are only 2 concepts:

1. HandlerProvider: `func (h any) (handlerProvider func (next)
http.Handler)`, it can be used as middleware
2. Use HandlerProvider to get the final HandlerFunc, and use it for
`r.Get()`


And we can decouple the route package from context package (see the
TODO).

# FAQ

## Is `reflect` safe?

Yes, all handlers are checked during startup, see the `preCheckHandler`
comment. If any handler is wrong, developers could know it in the first
time.

## Does `reflect` affect performance?

No. https://github.com/go-gitea/gitea/pull/24080#discussion_r1164825901

1. This reflect code only runs for each web handler call, handler is far
more slower: 10ms-50ms
2. The reflect is pretty fast (comparing to other code): 0.000265ms
3. XORM has more reflect operations already

modules/web/handler.go

@@ -0,0 +1,200 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package web
+
+import (
+	goctx "context"
+	"fmt"
+	"net/http"
+	"reflect"
+	"strings"
+
+	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/web/routing"
+)
+
+// ResponseStatusProvider is an interface to check whether the response has been written by the handler
+type ResponseStatusProvider interface {
+	Written() bool
+}
+
+// TODO: decouple this from the context package, let the context package register these providers
+var argTypeProvider = map[reflect.Type]func(req *http.Request) ResponseStatusProvider{
+	reflect.TypeOf(&context.APIContext{}):     func(req *http.Request) ResponseStatusProvider { return context.GetAPIContext(req) },
+	reflect.TypeOf(&context.Context{}):        func(req *http.Request) ResponseStatusProvider { return context.GetContext(req) },
+	reflect.TypeOf(&context.PrivateContext{}): func(req *http.Request) ResponseStatusProvider { return context.GetPrivateContext(req) },
+}
+
+// responseWriter is a wrapper of http.ResponseWriter, to check whether the response has been written
+type responseWriter struct {
+	respWriter http.ResponseWriter
+	status     int
+}
+
+var _ ResponseStatusProvider = (*responseWriter)(nil)
+
+func (r *responseWriter) Written() bool {
+	return r.status > 0
+}
+
+func (r *responseWriter) Header() http.Header {
+	return r.respWriter.Header()
+}
+
+func (r *responseWriter) Write(bytes []byte) (int, error) {
+	if r.status == 0 {
+		r.status = http.StatusOK
+	}
+	return r.respWriter.Write(bytes)
+}
+
+func (r *responseWriter) WriteHeader(statusCode int) {
+	r.status = statusCode
+	r.respWriter.WriteHeader(statusCode)
+}
+
+var (
+	httpReqType    = reflect.TypeOf((*http.Request)(nil))
+	respWriterType = reflect.TypeOf((*http.ResponseWriter)(nil)).Elem()
+	cancelFuncType = reflect.TypeOf((*goctx.CancelFunc)(nil)).Elem()
+)
+
+// preCheckHandler checks whether the handler is valid, developers could get first-time feedback, all mistakes could be found at startup
+func preCheckHandler(fn reflect.Value, argsIn []reflect.Value) {
+	hasStatusProvider := false
+	for _, argIn := range argsIn {
+		if _, hasStatusProvider = argIn.Interface().(ResponseStatusProvider); hasStatusProvider {
+			break
+		}
+	}
+	if !hasStatusProvider {
+		panic(fmt.Sprintf("handler should have at least one ResponseStatusProvider argument, but got %s", fn.Type()))
+	}
+	if fn.Type().NumOut() != 0 && fn.Type().NumIn() != 1 {
+		panic(fmt.Sprintf("handler should have no return value or only one argument, but got %s", fn.Type()))
+	}
+	if fn.Type().NumOut() == 1 && fn.Type().Out(0) != cancelFuncType {
+		panic(fmt.Sprintf("handler should return a cancel function, but got %s", fn.Type()))
+	}
+}
+
+func prepareHandleArgsIn(resp http.ResponseWriter, req *http.Request, fn reflect.Value) []reflect.Value {
+	isPreCheck := req == nil
+
+	argsIn := make([]reflect.Value, fn.Type().NumIn())
+	for i := 0; i < fn.Type().NumIn(); i++ {
+		argTyp := fn.Type().In(i)
+		switch argTyp {
+		case respWriterType:
+			argsIn[i] = reflect.ValueOf(resp)
+		case httpReqType:
+			argsIn[i] = reflect.ValueOf(req)
+		default:
+			if argFn, ok := argTypeProvider[argTyp]; ok {
+				if isPreCheck {
+					argsIn[i] = reflect.ValueOf(&responseWriter{})
+				} else {
+					argsIn[i] = reflect.ValueOf(argFn(req))
+				}
+			} else {
+				panic(fmt.Sprintf("unsupported argument type: %s", argTyp))
+			}
+		}
+	}
+	return argsIn
+}
+
+func handleResponse(fn reflect.Value, ret []reflect.Value) goctx.CancelFunc {
+	if len(ret) == 1 {
+		if cancelFunc, ok := ret[0].Interface().(goctx.CancelFunc); ok {
+			return cancelFunc
+		}
+		panic(fmt.Sprintf("unsupported return type: %s", ret[0].Type()))
+	} else if len(ret) > 1 {
+		panic(fmt.Sprintf("unsupported return values: %s", fn.Type()))
+	}
+	return nil
+}
+
+func hasResponseBeenWritten(argsIn []reflect.Value) bool {
+	for _, argIn := range argsIn {
+		if statusProvider, ok := argIn.Interface().(ResponseStatusProvider); ok {
+			if statusProvider.Written() {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// toHandlerProvider converts a handler to a handler provider
+// A handler provider is a function that takes a "next" http.Handler, it can be used as a middleware
+func toHandlerProvider(handler any) func(next http.Handler) http.Handler {
+	if hp, ok := handler.(func(next http.Handler) http.Handler); ok {
+		return hp
+	}
+
+	funcInfo := routing.GetFuncInfo(handler)
+	fn := reflect.ValueOf(handler)
+	if fn.Type().Kind() != reflect.Func {
+		panic(fmt.Sprintf("handler must be a function, but got %s", fn.Type()))
+	}
+
+	provider := func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(respOrig http.ResponseWriter, req *http.Request) {
+			// wrap the response writer to check whether the response has been written
+			resp := respOrig
+			if _, ok := resp.(ResponseStatusProvider); !ok {
+				resp = &responseWriter{respWriter: resp}
+			}
+
+			// prepare the arguments for the handler and do pre-check
+			argsIn := prepareHandleArgsIn(resp, req, fn)
+			if req == nil {
+				preCheckHandler(fn, argsIn)
+				return // it's doing pre-check, just return
+			}
+
+			routing.UpdateFuncInfo(req.Context(), funcInfo)
+			ret := fn.Call(argsIn)
+
+			// handle the return value, and defer the cancel function if there is one
+			cancelFunc := handleResponse(fn, ret)
+			if cancelFunc != nil {
+				defer cancelFunc()
+			}
+
+			// if the response has not been written, call the next handler
+			if next != nil && !hasResponseBeenWritten(argsIn) {
+				next.ServeHTTP(resp, req)
+			}
+		})
+	}
+
+	provider(nil).ServeHTTP(nil, nil) // do a pre-check to make sure all arguments and return values are supported
+	return provider
+}
+
+// MiddlewareWithPrefix wraps a handler function at a prefix, and make it as a middleware
+// TODO: this design is incorrect, the asset handler should not be a middleware
+func MiddlewareWithPrefix(pathPrefix string, middleware func(handler http.Handler) http.Handler, handlerFunc http.HandlerFunc) func(next http.Handler) http.Handler {
+	funcInfo := routing.GetFuncInfo(handlerFunc)
+	handler := http.HandlerFunc(func(resp http.ResponseWriter, req *http.Request) {
+		routing.UpdateFuncInfo(req.Context(), funcInfo)
+		handlerFunc(resp, req)
+	})
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(resp http.ResponseWriter, req *http.Request) {
+			if !strings.HasPrefix(req.URL.Path, pathPrefix) {
+				next.ServeHTTP(resp, req)
+				return
+			}
+			if middleware != nil {
+				middleware(handler).ServeHTTP(resp, req)
+			} else {
+				handler.ServeHTTP(resp, req)
+			}
+		})
+	}
+}