Writing to ffpmeg stdin freezes program

I'm trying to convert a file in memory using ffmpeg to another format by using stdin and stdout, but everytime I try to write to stdin, of my ffmpeg command, it just freezes there.

package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"os/exec"
)

func test(bytes []byte) ([]byte, error) {

	cmd := exec.Command(
		"ffmpeg",
		"-i", "pipe:0", // read from stdin
		"-vcodec", "copy",
		"-acodec", "copy",
		"-f", "matroska",
		"pipe:1",
	)

	in, err := cmd.StdinPipe()
	if err != nil {
		panic(err)
	}
	out, err := cmd.StdoutPipe()
	if err != nil {
		panic(err)
	}

	fmt.Println("starting")
	err = cmd.Start()
	if err != nil {
		panic(err)
	}

	fmt.Println("writing")
	w := bufio.NewWriter(in)
	_, err = w.Write(bytes)
	if err != nil {
		panic(err)
	}

	err = w.Flush()
	if err != nil {
		panic(err)
	}

	err = in.Close()
	if err != nil {
		panic(err)
	}

	fmt.Println("reading")
	outBytes, err := io.ReadAll(out)
	if err != nil {
		panic(err)
	}

	fmt.Println("waiting")
	err = cmd.Wait()
	if err != nil {
		panic(err)
	}

	return outBytes, nil
}

func main() {
	dat, err := os.ReadFile("speech.mp4")
	if err != nil {
		panic(err)
	}

	out, err := test(dat)
	if err != nil {
		panic(err)
	}

	err = os.WriteFile("test.m4v", out, 0644)
	if err != nil {
		panic(err)
	}
}

It prints

starting
writing

and gets stuck there. I tried similar code with grep, and the everything worked fine, so this seems to be some ffmpeg specific problem.

I tried running

cat speech.mp4 | ffmpeg -i pipe:0 -vcodec copy -acodec copy -f matroska pipe:1 | cat > test.mkv

and that works fine, so it's not an ffmpeg problem, but some problem with how I'm piping/reading/writing my data.

My speech.mp4 file is around 2MB.

So the secret lied in reading stdout as you dumped the bytes into stdin, since writing to the pipe blocks. Thanks @JimB for helping me figure this out.

You just have to read as you write:

cmd := exec.Command(
	"ffmpeg",
	"-i", "pipe:0", // read from stdin
	"-vcodec", "copy",
	"-acodec", "copy",
	"-f", "matroska",
	"pipe:1",
)

out, err := cmd.StdoutPipe()
if err != nil {
	panic(err)
}

in, err := cmd.StdinPipe()
writer := bufio.NewWriter(in)
if err != nil {
	panic(err)
}

fmt.Println("starting")
err = cmd.Start()
if err != nil {
	panic(err)
}

go func() {
	defer writer.Flush()
	defer in.Close()
	fmt.Println("writing")
	_, err = writer.Write(bytes)
	if err != nil {
		panic(err)
	}
}()

var outBytes []byte

defer out.Close()
fmt.Println("reading")
outBytes, err = io.ReadAll(out)
if err != nil {
	panic(err)
}
fmt.Println("waiting")
err = cmd.Wait()
if err != nil {
	panic(err)
}

return outBytes, nil