Control max open dirs in Golang

index.go

// src/golang.org/x/tools/godoc/index.go, L993 

// NewIndex creates a new index for the .go files provided by the corpus.
func (c *Corpus) NewIndex() *Index {
	// initialize Indexer
	// (use some reasonably sized maps to start)
	x := &Indexer{
		c:           c,
		fset:        token.NewFileSet(),
		fsOpenGate:  make(chan bool, maxOpenFiles),
		strings:     make(map[string]string),
		packages:    make(map[Pak]*Pak, 256),
		words:       make(map[string]*IndexResult, 8192),
		throttle:    util.NewThrottle(c.throttle(), 100*time.Millisecond), // run at least 0.1s at a time
		importCount: make(map[string]int),
		packagePath: make(map[string]map[string]bool),
		exports:     make(map[string]map[string]SpotKind),
		idents:      make(map[SpotKind]map[string][]Ident, 4),
	}

	// index all files in the directories given by dirnames
	var wg sync.WaitGroup // outstanding ReadDir + visitFile
	dirGate := make(chan bool, maxOpenDirs)
	for dirname := range c.fsDirnames() {
		if c.IndexDirectory != nil && !c.IndexDirectory(dirname) {
			continue
		}
		dirGate <- true
		wg.Add(1)
		go func(dirname string) {
			defer func() { <-dirGate }()
			defer wg.Done()

			list, err := c.fs.ReadDir(dirname)
			if err != nil {
				log.Printf("ReadDir(%q): %v; skipping directory", dirname, err)
				return // ignore this directory
			}
			for _, fi := range list {
				wg.Add(1)
				go func(fi os.FileInfo) {
					defer wg.Done()
					x.visitFile(dirname, fi)
				}(fi)
			}
		}(dirname)
	}
	wg.Wait()
  
  // ...
}