KDOC 713: gofmtはバッファ付きチャネルで同時に開くファイルの数を制限する

https://github.com/golang/go/blob/e3336a22ad3f0a90bd252c95d8b5544e02674205/src/cmd/gofmt/gofmt.go#L61-L69

// fdSem guards the number of concurrently-open file descriptors.
//
// For now, this is arbitrarily set to 200, based on the observation that many
// platforms default to a kernel limit of 256. Ideally, perhaps we should derive
// it from rlimit on platforms that support that system call.
//
// File descriptors opened from outside of this package are not tracked,
// so this limit may be approximate.
var fdSem = make(chan bool, 200)

https://github.com/golang/go/blob/e3336a22ad3f0a90bd252c95d8b5544e02674205/src/cmd/gofmt/gofmt.go#L302-L319

// readFile reads the contents of filename, described by info.
// If in is non-nil, readFile reads directly from it.
// Otherwise, readFile opens and reads the file itself,
// with the number of concurrently-open files limited by fdSem.
func readFile(filename string, info fs.FileInfo, in io.Reader) ([]byte, error) {
	if in == nil {
		fdSem <- true
		var err error
		f, err := os.Open(filename)
		if err != nil {
			return nil, err
		}
		in = f
		defer func() {
			f.Close()
			<-fdSem
		}()
	}