bbc-on-ice/main.go

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package main
import (
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"context"
"encoding/json"
"errors"
"fmt"
"io"
"io/ioutil"
"log"
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"net"
"net/http"
"os"
"path"
"regexp"
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"strconv"
"strings"
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"syscall"
"time"
"unicode/utf8"
)
const (
targetURI = "http://a.files.bbci.co.uk/media/live/manifesto/" +
"audio/simulcast/hls/%s/%s/ak/%s.m3u8"
metaBaseURI = "http://polling.bbc.co.uk/radio/nhppolling/"
)
type meta struct {
title string // what's playing right now
timeout uint // timeout for the next poll in ms
}
// getMeta retrieves and decodes metadata info from an independent webservice.
func getMeta(name string) (*meta, error) {
resp, err := http.Get(metaBaseURI + name)
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if resp != nil {
defer resp.Body.Close()
}
if err != nil {
return nil, err
}
b, err := ioutil.ReadAll(resp.Body)
if len(b) < 2 {
// There needs to be an enclosing () pair
return nil, errors.New("invalid metadata response")
}
type broadcast struct {
Title string // title of the broadcast
Percentage int // how far we're in
}
var v struct {
Packages struct {
OnAir struct {
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Broadcasts []broadcast
BroadcastNowIndex uint
} `json:"on-air"`
Richtracks []struct {
Artist string
Title string
IsNowPlaying bool `json:"is_now_playing"`
}
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}
Timeouts struct {
PollingTimeout uint `json:"polling_timeout"`
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}
}
err = json.Unmarshal(b[1:len(b)-1], &v)
if err != nil {
return nil, errors.New("invalid metadata response")
}
onAir := v.Packages.OnAir
if onAir.BroadcastNowIndex >= uint(len(onAir.Broadcasts)) {
return nil, errors.New("no active broadcast")
}
title := onAir.Broadcasts[onAir.BroadcastNowIndex].Title
for _, rt := range v.Packages.Richtracks {
if rt.IsNowPlaying {
title = rt.Artist + " - " + rt.Title + " / " + title
}
}
return &meta{timeout: v.Timeouts.PollingTimeout, title: title}, nil
}
// resolveM3U8 resolves an M3U8 playlist to the first link that seems to
// be playable, possibly recursing.
func resolveM3U8(target string) (out []string, err error) {
resp, err := http.Get(target)
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if resp != nil {
defer resp.Body.Close()
}
if err != nil {
return nil, err
}
b, err := ioutil.ReadAll(resp.Body)
if !utf8.Valid(b) {
return nil, errors.New("invalid UTF-8")
}
lines := strings.Split(string(b), "\n")
for _, line := range lines {
if strings.HasPrefix(line, "#") {
continue
}
if !strings.Contains(line, "/") {
// Seems to be a relative link, let's make it absolute.
dir, _ := path.Split(target)
line = dir + line
}
if strings.HasSuffix(line, "m3u8") {
// The playlist seems to recurse, and so will we.
// XXX: This should be bounded, not just by the stack.
return resolveM3U8(line)
}
out = append(out, line)
}
return out, nil
}
// metaProc periodically polls the sub-URL given by name for titles and sends
// them out the given channel. Never returns prematurely.
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func metaProc(ctx context.Context, name string, out chan<- string) {
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defer close(out)
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// "polling_timeout" seems to normally be 25 seconds, which is a lot,
// especially considering all the possible additional buffering.
const maxInterval = 5 * time.Second
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var current, last string
var interval time.Duration
for {
meta, err := getMeta(name)
if err != nil {
current = name + " - " + err.Error()
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interval = maxInterval
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} else {
current = meta.title
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interval = time.Duration(meta.timeout) * time.Millisecond
if interval > maxInterval {
interval = maxInterval
}
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}
if current != last {
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select {
case out <- current:
case <-ctx.Done():
return
}
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last = current
}
select {
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case <-time.After(interval):
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case <-ctx.Done():
return
}
}
}
// urlProc periodically checks the playlist for yet unseen URLs and sends them
// over the channel. Assumes that URLs are incremental for simplicity, although
// there doesn't seem to be any such gaurantee by the HLS protocol.
func urlProc(ctx context.Context, playlistURL string, out chan<- string) {
defer close(out)
highest := ""
for {
target, err := resolveM3U8(playlistURL)
if err != nil {
return
}
for _, url := range target {
if url <= highest {
continue
}
select {
case out <- url:
highest = url
case <-ctx.Done():
return
}
}
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// Media players will happily buffer the whole playlist at once,
// a small (less than target duration) additional pause is appropriate.
time.Sleep(3 * time.Second)
}
}
// https://tools.ietf.org/html/rfc8216
// http://www.gpac-licensing.com/2014/12/08/apple-hls-technical-depth/
func dataProc(ctx context.Context, playlistURL string, maxChunk int,
out chan<- []byte) {
defer close(out)
// The channel is buffered so that the urlProc can fetch in advance.
urls := make(chan string, 3)
go urlProc(ctx, playlistURL, urls)
for url := range urls {
resp, err := http.Get(url)
if resp != nil {
defer resp.Body.Close()
}
if err != nil {
return
}
for {
chunk := make([]byte, maxChunk)
n, err := resp.Body.Read(chunk)
select {
case out <- chunk[:n]:
case <-ctx.Done():
return
}
if err == io.EOF {
break
}
if err != nil {
return
}
}
}
}
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var pathRE = regexp.MustCompile(`^/(.*?)/(.*?)/(.*?)$`)
func proxy(w http.ResponseWriter, req *http.Request) {
const metaint = 1 << 15
m := pathRE.FindStringSubmatch(req.URL.Path)
if m == nil {
http.NotFound(w, req)
return
}
hijacker, ok := w.(http.Hijacker)
if !ok {
// We're not using TLS where HTTP/2 could have caused this.
panic("cannot hijack connection")
}
// E.g. `nonuk`, `sbr_low` `bbc_radio_one`, or `uk`, `sbr_high`, `bbc_1xtra`
region, quality, name := m[1], m[2], m[3]
// TODO: We probably shouldn't poll the top level playlist.
mainPlaylistURL := fmt.Sprintf(targetURI, region, quality, name)
// This validates the parameters as a side-effect.
target, err := resolveM3U8(mainPlaylistURL)
if err == nil && len(target) == 0 {
err = errors.New("cannot resolve playlist")
}
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
wantMeta := req.Header.Get("Icy-MetaData") == "1"
resp, err := http.Head(target[0])
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
conn, bufrw, err := hijacker.Hijack()
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
defer conn.Close()
// TODO: Retrieve some general information from somewhere?
// There's nothing interesting in the playlist files.
fmt.Fprintf(bufrw, "ICY 200 OK\r\n")
fmt.Fprintf(bufrw, "icy-name:%s\r\n", name)
// BBC marks this as a video type, maybe just force audio/mpeg.
fmt.Fprintf(bufrw, "content-type:%s\r\n", resp.Header["Content-Type"][0])
fmt.Fprintf(bufrw, "icy-pub:%d\r\n", 0)
if wantMeta {
fmt.Fprintf(bufrw, "icy-metaint: %d\r\n", metaint)
}
fmt.Fprintf(bufrw, "\r\n")
metaChan := make(chan string)
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go metaProc(req.Context(), name, metaChan)
chunkChan := make(chan []byte)
go dataProc(req.Context(), mainPlaylistURL, metaint, chunkChan)
// dataProc may return less data near the end of a subfile, so we give it
// a maximum count of bytes to return at once and do our own buffering.
var queuedMetaUpdate, queuedData []byte
writeMeta := func() error {
if !wantMeta {
return nil
}
var meta [1 + 16*255]byte
meta[0] = byte((copy(meta[1:], queuedMetaUpdate) + 15) / 16)
queuedMetaUpdate = nil
_, err := bufrw.Write(meta[:1+int(meta[0])*16])
return err
}
for {
select {
case title := <-metaChan:
queuedMetaUpdate = []byte(fmt.Sprintf("StreamTitle='%s'",
strings.Replace(title, "'", "", -1)))
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case chunk, ok := <-chunkChan:
if !ok {
return
}
missing := metaint - len(queuedData)
if len(chunk) < missing {
queuedData = append(queuedData, chunk...)
continue
}
queuedData = append(queuedData, chunk[:missing]...)
if _, err := bufrw.Write(queuedData); err != nil {
return
}
queuedData = chunk[missing:]
if writeMeta() != nil || bufrw.Flush() != nil {
return
}
}
}
}
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// https://www.freedesktop.org/software/systemd/man/sd_listen_fds.html
func socketActivationListener() net.Listener {
pid, err := strconv.Atoi(os.Getenv("LISTEN_PID"))
if err != nil || pid != os.Getpid() {
return nil
}
nfds, err := strconv.Atoi(os.Getenv("LISTEN_FDS"))
if err != nil || nfds == 0 {
log.Println("LISTEN_FDS unworkable")
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return nil
} else if nfds > 1 {
log.Fatalln("not supporting more than one listening socket")
}
const firstListenFd = 3
syscall.CloseOnExec(firstListenFd)
ln, err := net.FileListener(os.NewFile(firstListenFd, "socket activation"))
if err != nil {
log.Fatalln(err)
}
return ln
}
// Had to copy this from Server.ListenAndServe.
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type tcpKeepAliveListener struct{ *net.TCPListener }
func (ln tcpKeepAliveListener) Accept() (c net.Conn, err error) {
tc, err := ln.AcceptTCP()
if err != nil {
return
}
_ = tc.SetKeepAlive(true)
_ = tc.SetKeepAlivePeriod(3 * time.Minute)
return tc, nil
}
func main() {
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var listener net.Listener
if ln := socketActivationListener(); ln != nil {
// Keepalives can be set in the systemd unit, see systemd.socket(5).
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listener = ln
} else {
if len(os.Args) < 2 {
log.Fatalf("usage: %s LISTEN-ADDR\n", os.Args[0])
}
if ln, err := net.Listen("tcp", os.Args[1]); err != nil {
log.Fatalln(err)
} else {
listener = tcpKeepAliveListener{ln.(*net.TCPListener)}
}
}
http.HandleFunc("/", proxy)
// We don't need to clean up properly since we store no data.
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log.Fatalln(http.Serve(listener, nil))
}