UPnP/SSDP: Worker, ping Manager URLs in parallel

Ping all Manager URLs at once, and wait until they've all responded (or
caused an error), instead of pinging them one by one sequentially.

cmd/flamenco-worker/main.go

@@ -11,6 +11,7 @@ import (
 	"os"
 	"os/signal"
 	"runtime"
+	"sync"
 	"syscall"
 	"time"
 
@@ -226,29 +227,53 @@ func autodiscoverManager(ctx context.Context) (string, error) {
 	}
 
 	// Try out the URLs to see which one responds.
-	// TODO: parallelise this.
+	startTime := time.Now()
-	usableURLs := make([]string, 0)
+	numUsableURLs := 0
-	for _, url := range urls {
+	wg := new(sync.WaitGroup)
-		if pingManager(ctx, url) {
+	wg.Add(len(urls))
-			usableURLs = append(usableURLs, url)
+	mutex := new(sync.Mutex)
-		}
+	for idx, url := range urls {
-	}
+		go func(idx int, url string) {
+			defer wg.Done()
+			ok := pingManager(ctx, url)
 
-	switch len(usableURLs) {
+			mutex.Lock()
-	case 0:
+			defer mutex.Unlock()
+
+			if ok {
+				numUsableURLs++
+			} else {
+				// Erase the URL from the usable list.
+				urls[idx] = ""
+			}
+		}(idx, url)
+	}
+	wg.Wait()
+	log.Debug().Str("pingTime", time.Since(startTime).String()).Msg("pinging all Manager URLs done")
+
+	if numUsableURLs == 0 {
 		return "", fmt.Errorf("autodetected %d URLs, but none were usable", len(urls))
-
-	case 1:
-		log.Info().Str("url", usableURLs[0]).Msg("found Manager")
-		return usableURLs[0], nil
-
-	default:
-		log.Info().
-			Strs("urls", usableURLs).
-			Str("url", usableURLs[0]).
-			Msg("found multiple usable URLs, using the first one")
-		return usableURLs[0], nil
 	}
+
+	// Find the first usable URL.
+	var firstURL string
+	for _, url := range urls {
+		if url != "" {
+			firstURL = url
+			break
+		}
+	}
+
+	if numUsableURLs == 1 {
+		log.Info().Str("url", firstURL).Msg("found Manager")
+	} else {
+		log.Info().
+			Strs("urls", urls).
+			Str("url", firstURL).
+			Msg("found multiple usable URLs, using the first one")
+	}
+
+	return firstURL, nil
 }
 
 // pingManager connects to a Manager and returns true if it responds.