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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// HTTP reverse proxy handler

package httputil

import (
        "io"
        "log"
        "net"
        "net/http"
        "net/url"
        "strings"
        "sync"
        "time"
)

// ReverseProxy is an HTTP Handler that takes an incoming request and
// sends it to another server, proxying the response back to the
// client.
type ReverseProxy struct {
        // Director must be a function which modifies
        // the request into a new request to be sent
        // using Transport. Its response is then copied
        // back to the original client unmodified.
        Director func(*http.Request)

        // The transport used to perform proxy requests.
        // If nil, http.DefaultTransport is used.
        Transport http.RoundTripper

        // FlushInterval specifies the flush interval
        // to flush to the client while copying the
        // response body.
        // If zero, no periodic flushing is done.
        FlushInterval time.Duration
}

func singleJoiningSlash(a, b string) string {
        aslash := strings.HasSuffix(a, "/")
        bslash := strings.HasPrefix(b, "/")
        switch {
        case aslash && bslash:
                return a + b[1:]
        case !aslash && !bslash:
                return a + "/" + b
        }
        return a + b
}

// NewSingleHostReverseProxy returns a new ReverseProxy that rewrites
// URLs to the scheme, host, and base path provided in target. If the
// target's path is "/base" and the incoming request was for "/dir",
// the target request will be for /base/dir.
func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
        targetQuery := target.RawQuery
        director := func(req *http.Request) {
                req.URL.Scheme = target.Scheme
                req.URL.Host = target.Host
                req.URL.Path = singleJoiningSlash(target.Path, req.URL.Path)
                if targetQuery == "" || req.URL.RawQuery == "" {
                        req.URL.RawQuery = targetQuery + req.URL.RawQuery
                } else {
                        req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
                }
        }
        return &ReverseProxy{Director: director}
}

func copyHeader(dst, src http.Header) {
        for k, vv := range src {
                for _, v := range vv {
                        dst.Add(k, v)
                }
        }
}

func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
        transport := p.Transport
        if transport == nil {
                transport = http.DefaultTransport
        }

        outreq := new(http.Request)
        *outreq = *req // includes shallow copies of maps, but okay

        p.Director(outreq)
        outreq.Proto = "HTTP/1.1"
        outreq.ProtoMajor = 1
        outreq.ProtoMinor = 1
        outreq.Close = false

        // Remove the connection header to the backend.  We want a
        // persistent connection, regardless of what the client sent
        // to us.  This is modifying the same underlying map from req
        // (shallow copied above) so we only copy it if necessary.
        if outreq.Header.Get("Connection") != "" {
                outreq.Header = make(http.Header)
                copyHeader(outreq.Header, req.Header)
                outreq.Header.Del("Connection")
        }

        if clientIp, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
                outreq.Header.Set("X-Forwarded-For", clientIp)
        }

        res, err := transport.RoundTrip(outreq)
        if err != nil {
                log.Printf("http: proxy error: %v", err)
                rw.WriteHeader(http.StatusInternalServerError)
                return
        }

        copyHeader(rw.Header(), res.Header)

        rw.WriteHeader(res.StatusCode)

        if res.Body != nil {
                var dst io.Writer = rw
                if p.FlushInterval != 0 {
                        if wf, ok := rw.(writeFlusher); ok {
                                dst = &maxLatencyWriter{dst: wf, latency: p.FlushInterval}
                        }
                }
                io.Copy(dst, res.Body)
        }
}

type writeFlusher interface {
        io.Writer
        http.Flusher
}

type maxLatencyWriter struct {
        dst     writeFlusher
        latency time.Duration

        lk   sync.Mutex // protects init of done, as well Write + Flush
        done chan bool
}

func (m *maxLatencyWriter) Write(p []byte) (n int, err error) {
        m.lk.Lock()
        defer m.lk.Unlock()
        if m.done == nil {
                m.done = make(chan bool)
                go m.flushLoop()
        }
        n, err = m.dst.Write(p)
        if err != nil {
                m.done <- true
        }
        return
}

func (m *maxLatencyWriter) flushLoop() {
        t := time.NewTicker(m.latency)
        defer t.Stop()
        for {
                select {
                case <-t.C:
                        m.lk.Lock()
                        m.dst.Flush()
                        m.lk.Unlock()
                case <-m.done:
                        return
                }
        }
        panic("unreached")
}