// Copyright 2020 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package node

import (
	"compress/gzip"
	"io"
	"io/ioutil"
	"net"
	"net/http"
	"strings"
	"sync"

	"github.com/ethereum/go-ethereum/log"
	"github.com/rs/cors"
)

// NewHTTPHandlerStack returns wrapped http-related handlers
func NewHTTPHandlerStack(srv http.Handler, cors []string, vhosts []string) http.Handler {
	// Wrap the CORS-handler within a host-handler
	handler := newCorsHandler(srv, cors)
	handler = newVHostHandler(vhosts, handler)
	return newGzipHandler(handler)
}

func newCorsHandler(srv http.Handler, allowedOrigins []string) http.Handler {
	// disable CORS support if user has not specified a custom CORS configuration
	if len(allowedOrigins) == 0 {
		return srv
	}
	c := cors.New(cors.Options{
		AllowedOrigins: allowedOrigins,
		AllowedMethods: []string{http.MethodPost, http.MethodGet},
		MaxAge:         600,
		AllowedHeaders: []string{"*"},
	})
	return c.Handler(srv)
}

// virtualHostHandler is a handler which validates the Host-header of incoming requests.
// Using virtual hosts can help prevent DNS rebinding attacks, where a 'random' domain name points to
// the service ip address (but without CORS headers). By verifying the targeted virtual host, we can
// ensure that it's a destination that the node operator has defined.
type virtualHostHandler struct {
	vhosts map[string]struct{}
	next   http.Handler
}

func newVHostHandler(vhosts []string, next http.Handler) http.Handler {
	vhostMap := make(map[string]struct{})
	for _, allowedHost := range vhosts {
		vhostMap[strings.ToLower(allowedHost)] = struct{}{}
	}
	return &virtualHostHandler{vhostMap, next}
}

// ServeHTTP serves JSON-RPC requests over HTTP, implements http.Handler
func (h *virtualHostHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	// if r.Host is not set, we can continue serving since a browser would set the Host header
	if r.Host == "" {
		h.next.ServeHTTP(w, r)
		return
	}
	host, _, err := net.SplitHostPort(r.Host)
	if err != nil {
		// Either invalid (too many colons) or no port specified
		host = r.Host
	}
	if ipAddr := net.ParseIP(host); ipAddr != nil {
		// It's an IP address, we can serve that
		h.next.ServeHTTP(w, r)
		return

	}
	// Not an IP address, but a hostname. Need to validate
	if _, exist := h.vhosts["*"]; exist {
		h.next.ServeHTTP(w, r)
		return
	}
	if _, exist := h.vhosts[host]; exist {
		h.next.ServeHTTP(w, r)
		return
	}
	http.Error(w, "invalid host specified", http.StatusForbidden)
}

var gzPool = sync.Pool{
	New: func() interface{} {
		w := gzip.NewWriter(ioutil.Discard)
		return w
	},
}

type gzipResponseWriter struct {
	io.Writer
	http.ResponseWriter
}

func (w *gzipResponseWriter) WriteHeader(status int) {
	w.Header().Del("Content-Length")
	w.ResponseWriter.WriteHeader(status)
}

func (w *gzipResponseWriter) Write(b []byte) (int, error) {
	return w.Writer.Write(b)
}

func newGzipHandler(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if !strings.Contains(r.Header.Get("Accept-Encoding"), "gzip") {
			next.ServeHTTP(w, r)
			return
		}

		w.Header().Set("Content-Encoding", "gzip")

		gz := gzPool.Get().(*gzip.Writer)
		defer gzPool.Put(gz)

		gz.Reset(w)
		defer gz.Close()

		next.ServeHTTP(&gzipResponseWriter{ResponseWriter: w, Writer: gz}, r)
	})
}

// NewWebsocketUpgradeHandler returns a websocket handler that serves an incoming request only if it contains an upgrade
// request to the websocket protocol. If not, serves the the request with the http handler.
func NewWebsocketUpgradeHandler(h http.Handler, ws http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if isWebsocket(r) {
			ws.ServeHTTP(w, r)
			log.Debug("serving websocket request")
			return
		}

		h.ServeHTTP(w, r)
	})
}

// isWebsocket checks the header of an http request for a websocket upgrade request.
func isWebsocket(r *http.Request) bool {
	return strings.ToLower(r.Header.Get("Upgrade")) == "websocket" &&
		strings.ToLower(r.Header.Get("Connection")) == "upgrade"
}