Useful for stuff like storing logs without memory going overboard, or in cases when you have multiple readers reading a single stream of data from a single writer.

This code is thread safe and allows either standard readers, or blocking readers that will wait when no more data is available (this uses sync.Cond with RLocker meaning multiple readers will resume reading in parallel, allowing the most out of goroutines).

Originally this was written as a buffer for log allowing to redirect output from logs to multiple targets at the same time (write to file and stdout at the same time) while also allowing a web API to fetch the last X MB of entries (as large as the buffer is).

See the godoc documentation

	l, err := ringbuf.New(1024*1024) // 1MB ring buffer for logs
	if err != nil {
		...
	}
	log.SetOutput(l)
	// output log to stdout
	// (duplicate this to also output to files/etc)
	go func() {
		r := l.BlockingReader()
		defer r.Close()
		io.Copy(os.Stdout, r)
	}()
	func dmesg() ([]byte, error) {
		// return up to last 1MB of log entries
		r := l.Reader()
		defer r.Close()
		return ioutil.ReadAll(r)
	}