Writing to ffpmeg stdin freezes program-CodePudding

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.

CodePudding user response：

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