Writing to ffpmeg stdin freezes program - go

Question

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.

Answer 1

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