/*
Copyright 2011 Google Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

     http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

// This is a somewhat cut back version of webfront, available at
// http://github.com/nf/webfront

/*
webfront is an HTTP server and reverse proxy.

It reads a JSON-formatted rule file like this:

[
	{"Host": "example.com", "Serve": "/var/www"},
	{"Host": "example.org", "Forward": "localhost:8080"}
]

For all requests to the host example.com (or any name ending in
".example.com") it serves files from the /var/www directory.

For requests to example.org, it forwards the request to the HTTP
server listening on localhost port 8080.

Usage of webfront:
  -http=":80": HTTP listen address
  -poll=10s: file poll interval
  -rules="": rule definition file

webfront was written by Andrew Gerrand <adg@golang.org>
*/
package main

import (
	"encoding/json"
	"flag"
	"fmt"
	"log"
	"net/http"
	"net/http/httputil"
	"os"
	"strings"
	"sync"
	"time"
)

var (
	httpAddr     = flag.String("http", ":80", "HTTP listen address")
	ruleFile     = flag.String("rules", "", "rule definition file")
	pollInterval = flag.Duration("poll", time.Second*10, "file poll interval")
)

func main() {
	flag.Parse()

	s, err := NewServer(*ruleFile, *pollInterval)
	if err != nil {
		log.Fatal(err)
	}

	err = http.ListenAndServe(*httpAddr, s)
	if err != nil {
		log.Fatal(err)
	}
}

// Server implements an http.Handler that acts as either a reverse proxy or
// a simple file server, as determined by a rule set.
type Server struct {
	mu    sync.RWMutex // guards the fields below
	mtime time.Time    // when the rule file was last modified
	rules []*Rule
}

// Rule represents a rule in a configuration file.
type Rule struct {
	Host    string // to match against request Host header
	Forward string // non-empty if reverse proxy
	Serve   string // non-empty if file server
}

// Match returns true if the Rule matches the given Request.
func (r *Rule) Match(req *http.Request) bool {
	return req.Host == r.Host || strings.HasSuffix(req.Host, "."+r.Host)
}

// Handler returns the appropriate Handler for the Rule.
func (r *Rule) Handler() http.Handler {
	if h := r.Forward; h != "" {
		return &httputil.ReverseProxy{
			Director: func(req *http.Request) {
				req.URL.Scheme = "http"
				req.URL.Host = h
			},
		}
	}
	if d := r.Serve; d != "" {
		return http.FileServer(http.Dir(d))
	}
	return nil
}

// NewServer constructs a Server that reads rules from file with a period
// specified by poll.
func NewServer(file string, poll time.Duration) (*Server, error) {
	s := new(Server)
	if err := s.loadRules(file); err != nil {
		return nil, err
	}
	go s.refreshRules(file, poll)
	return s, nil
}

// ServeHTTP matches the Request with a Rule and, if found, serves the
// request with the Rule's handler.
func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	if h := s.handler(r); h != nil {
		h.ServeHTTP(w, r)
		return
	}
	http.Error(w, "Not found.", http.StatusNotFound)
}

// handler returns the appropriate Handler for the given Request,
// or nil if none found.
func (s *Server) handler(req *http.Request) http.Handler {
	s.mu.RLock()
	defer s.mu.RUnlock()
	for _, r := range s.rules {
		if r.Match(req) {
			return r.Handler()
		}
	}
	return nil
}

// refreshRules polls file periodically and refreshes the Server's rule
// set if the file has been modified.
func (s *Server) refreshRules(file string, poll time.Duration) {
	for {
		if err := s.loadRules(file); err != nil {
			log.Println(err)
		}
		time.Sleep(poll)
	}
}

// loadRules tests whether file has been modified
// and, if so, loads the rule set from file.
func (s *Server) loadRules(file string) error {
	fi, err := os.Stat(file)
	if err != nil {
		return err
	}
	mtime := fi.ModTime()
	if mtime.Before(s.mtime) && s.rules != nil {
		return nil // no change
	}
	rules, err := parseRules(file)
	if err != nil {
		return fmt.Errorf("parsing %s: %v", file, err)
	}
	s.mu.Lock()
	s.mtime = mtime
	s.rules = rules
	s.mu.Unlock()
	return nil
}

// parseRules reads rule definitions from file returns the resultant Rules.
func parseRules(file string) ([]*Rule, error) {
	f, err := os.Open(file)
	if err != nil {
		return nil, err
	}
	defer f.Close()
	var rules []*Rule
	err = json.NewDecoder(f).Decode(&rules)
	if err != nil {
		return nil, err
	}
	return rules, nil
}