refactor: use io.Pipe and ffmpeg's stdin (#1148)

This removes the usage of `syscall.Mkfifo` which was previously used and
won't work on Windows systems and opens the door for other processes on
the computer to interfere in the rtmp stream (dumping bad content in the
fifo, removing the file, blocking the file in offline status).
Instead, this patch introduces an `io.Pipe` which pipes the RTMP stream
to the ffmpeg command while staying in Owncast.

Further links:
* ffmpeg on using `pipe:0` as an input: https://ffmpeg.org/ffmpeg-protocols.html#pipe

core/transcoder/transcoder.go

@@ -3,6 +3,7 @@ package transcoder
 import (
 	"bufio"
 	"fmt"
+	"io"
 	"os/exec"
 	"strconv"
 	"strings"
@@ -22,6 +23,7 @@ var _commandExec *exec.Cmd
 // Transcoder is a single instance of a video transcoder.
 type Transcoder struct {
 	input                string
+	stdin                *io.PipeReader
 	segmentOutputPath    string
 	playlistOutputPath   string
 	variants             []HLSVariant
@@ -95,6 +97,11 @@ func (t *Transcoder) Start() {
 	}
 
 	_commandExec = exec.Command("sh", "-c", command)
+
+	if t.stdin != nil {
+	  _commandExec.Stdin = t.stdin
+	}
+
 	stdout, err := _commandExec.StderrPipe()
 	if err != nil {
 		panic(err)
@@ -240,7 +247,7 @@ func NewTranscoder() *Transcoder {
 	// Playlists are available via the local HTTP server
 	transcoder.playlistOutputPath = config.PublicHLSStoragePath
 
-	transcoder.input = utils.GetTemporaryPipePath(fmt.Sprint(data.GetRTMPPortNumber()))
+	transcoder.input = "pipe:0" // stdin
 
 	for index, quality := range transcoder.currentStreamOutputSettings {
 		variant := getVariantFromConfigQuality(quality, index)
@@ -389,6 +396,11 @@ func (t *Transcoder) SetInput(input string) {
 	t.input = input
 }
 
+// SetStdin sets the Stdin of the ffmpeg command.
+func (t *Transcoder) SetStdin(rtmp *io.PipeReader) {
+	t.stdin = rtmp
+}
+
 // SetOutputPath sets the root directory that should include playlists and video segments.
 func (t *Transcoder) SetOutputPath(output string) {
 	t.segmentOutputPath = output