searcher: make indexing of repos concurrent

This patch attempts to improve the startup time of hound vastly in cases
when we have huge number of repositories and hound would generally take
long time to start because of its sequential nature of indexing.

Now, we have the startup indexing in a concurrent way while respecting
the config.MaxConcurrentIndexers parameter set by users in config.json.

Fixes hound-search#250

searcher/searcher.go

@@ -34,6 +34,24 @@ type Searcher struct {
 	doneCh            chan empty
 }
 
+// Wrapper around searcher - used for sending the searcher struct to channel once
+// successfully returned from `newSearcher` function.
+// We need this wrapper because we need to have the `name` of a repo which a
+// particular searcher belongs to.
+type searcherWrapper struct {
+	searcher *Searcher
+	name     string
+}
+
+// Wrapper around error - used for sending error to channel once returned from
+// `newSearcher` function.
+// We need this wrapper because we need to have the `name` of a repo which a
+// particular error belongs to.
+type errorWrapper struct {
+	error error
+	name  string
+}
+
 type empty struct{}
 type limiter chan bool
 
@@ -277,21 +295,49 @@ func MakeAll(cfg *config.Config) (map[string]*Searcher, map[string]error, error)
 
 	lim := makeLimiter(cfg.MaxConcurrentIndexers)
 
-	for name, repo := range cfg.Repos {
-		s, err := newSearcher(cfg.DbPath, name, repo, refs, lim)
-		if err != nil {
-			log.Print(err)
-			errs[name] = err
-			continue
-		}
+	// Channel to receive the successfully created searchers.
+	searcherCh := make(chan searcherWrapper, 1)
+	// Channel to receive the errors in creation of searchers.
+	errorCh := make(chan errorWrapper, 1)
+	// Channel to quit the go routine listening for new successful searchers or errors.
+	quitCh := make(chan struct{}, 1)
+
+	// Create a wait group for total number of repos so that we can proceed once the
+	// go routines to create searchers for all repos have returned.
+	var wg sync.WaitGroup
+	wg.Add(len(cfg.Repos))
 
-		searchers[name] = s
+	for name, repo := range cfg.Repos {
+		go newSearcherConcurrent(cfg.DbPath, name, repo, refs, lim, searcherCh, errorCh)
 	}
 
+	// Create a listener on searcherCh and errorCh.
+	// It also listens on quitCh to decide when to return.
+	go func() {
+		for {
+			select {
+			case sw := <-searcherCh:
+				searchers[sw.name] = sw.searcher
+				wg.Done()
+			case ew := <-errorCh:
+				log.Print(ew.error)
+				errs[ew.name] = ew.error
+				wg.Done()
+			case <-quitCh:
+				return
+			}
+		}
+	}()
+
 	if err := refs.removeUnclaimed(); err != nil {
 		return nil, nil, err
 	}
 
+	// Wait for all go routines to finish.
+	wg.Wait()
+	// Send close signal to quitCh to return the listening go routine.
+	close(quitCh)
+
 	// after all the repos are in good shape, we start their polling
 	for _, s := range searchers {
 		s.begin()
@@ -464,3 +510,33 @@ func newSearcher(
 
 	return s, nil
 }
+
+// This function is a wrapper around `newSearcher` function.
+// It respects the parameter `cfg.MaxConcurrentIndexers` while making the
+// creation of searchers for various repositories concurrent.
+func newSearcherConcurrent(
+	dbpath, name string,
+	repo *config.Repo,
+	refs *foundRefs,
+	lim limiter,
+	searcherCh chan searcherWrapper,
+	errorCh chan errorWrapper) {
+
+	// acquire a token from the rate limiter
+	lim.Acquire()
+	defer lim.Release()
+
+	s, err := newSearcher(dbpath, name, repo, refs, lim)
+	if err != nil {
+		errorCh <- errorWrapper{
+			error: err,
+			name: name,
+		}
+		return
+	}
+
+	searcherCh <- searcherWrapper{
+		searcher: s,
+		name: name,
+	}
+}