ffmpeg magic: piped mp4 output without fragments

ffmpeg.go

func ffmpeg(in io.Reader) (out io.ReadCloser) {
	// the command overwrites the output file that "already exists"
	// actually, the output file is a reference to ffmpegs own file descriptor #3
	c := exec.Command("ffmpeg", "-y", "-i", "-", "-c", "copy", "-f", "mp4", "/proc/self/fd/3")

	// standard input, buffered by us
	c.Stdin = bufio.NewReader(in)

	// create a memory backed temporary file using the MemfdCreate syscall
	// the file is managed by the kernel and doesn't need to be deleted
	tmp := ramTMP()

	// seek to the start, just in case, after the command is finished
	defer tmp.Seek(0, 0)

	// extrafiles makes ffmpeg inherit the given file descriptors, starting at 3 onward
	c.ExtraFiles = append(c.ExtraFiles, tmp)

	// run it, ffmpeg will open its own file descriptor, which is seekable (unlike pipe:3)
	c.Run()

	// return the memory backed file, which the caller will close after reading
	return tmp
}

func ramTMP() *os.File {
	// names need not be unique here, they have no meaning except for debugging in procfs
	fd, err := unix.MemfdCreate("tmp", os.O_RDWR)
	if err != nil {
		panic(err)
	}
	return os.NewFile(uintptr(fd), "tmp")
}