emissary/internal/proxy/proxy.go

package proxy

import (
	"fmt"
	"net/http"
	"net/http/httputil"
	"net/url"
	"sync"

	"github.com/pkg/errors"
	"gitlab.com/wpetit/goweb/logger"
)

type Proxy struct {
	reversers                sync.Map
	handler                  http.Handler
	proxyResponseTransformer ProxyResponseTransformer
	proxyRequestTransformer  ProxyRequestTransformer
}

// ServeHTTP implements http.Handler
func (p *Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	p.handler.ServeHTTP(w, r)
}

func (p *Proxy) proxyRequest(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()

	var reverser *httputil.ReverseProxy

	key := fmt.Sprintf("%s://%s", r.URL.Scheme, r.URL.Host)

	createAndStore := func() {
		target := &url.URL{
			Scheme: r.URL.Scheme,
			Host:   r.URL.Host,
		}

		reverser = httputil.NewSingleHostReverseProxy(target)

		originalDirector := reverser.Director

		if p.proxyRequestTransformer != nil {
			reverser.Director = func(r *http.Request) {
				originalURL := r.URL.String()
				originalDirector(r)
				p.proxyRequestTransformer.TransformRequest(r)
				logger.Debug(ctx, "proxying request", logger.F("targetURL", r.URL.String()), logger.F("originalURL", originalURL))
			}
		}

		if p.proxyResponseTransformer != nil {
			reverser.ModifyResponse = func(r *http.Response) error {
				if err := p.proxyResponseTransformer.TransformResponse(r); err != nil {
					return errors.WithStack(err)
				}

				return nil
			}
		}

		p.reversers.Store(key, reverser)
	}

	raw, exists := p.reversers.Load(key)
	if !exists {
		createAndStore()
	}

	reverser, ok := raw.(*httputil.ReverseProxy)
	if !ok {
		createAndStore()
	}

	reverser.ServeHTTP(w, r)
}

func New(funcs ...OptionFunc) *Proxy {
	opts := defaultOptions()
	for _, fn := range funcs {
		fn(opts)
	}

	proxy := &Proxy{}

	handler := http.HandlerFunc(proxy.proxyRequest)
	proxy.handler = createMiddlewareChain(handler, opts.Middlewares)

	proxy.proxyRequestTransformer = createProxyRequestChain(&defaultProxyRequestTransformer{}, opts.ProxyRequestMiddlewares)
	proxy.proxyResponseTransformer = createProxyResponseChain(&defaultProxyResponseTransformer{}, opts.ProxyResponseMiddlewares)

	return proxy
}

var _ http.Handler = &Proxy{}

func createMiddlewareChain(handler http.Handler, middlewares []Middleware) http.Handler {
	reverse(middlewares)

	for _, m := range middlewares {
		handler = m(handler)
	}

	return handler
}

func createProxyResponseChain(transformer ProxyResponseTransformer, middlewares []ProxyResponseMiddleware) ProxyResponseTransformer {
	reverse(middlewares)

	for _, m := range middlewares {
		transformer = m(transformer)
	}

	return transformer
}

func createProxyRequestChain(transformer ProxyRequestTransformer, middlewares []ProxyRequestMiddleware) ProxyRequestTransformer {
	reverse(middlewares)

	for _, m := range middlewares {
		transformer = m(transformer)
	}

	return transformer
}

func reverse[S ~[]E, E any](s S) {
	for i, j := 0, len(s)-1; i < j; i, j = i+1, j-1 {
		s[i], s[j] = s[j], s[i]
	}
}