Add support for compilation options

Add options to the Compile function, and in turn, the Run function
in order to modify the compiled command instance before executing
it. This is helpful to, for example, specify that the command
should be run in a separate process group in order for ffmpeg not
to react on SIGINTs sent to the parent proces.

run.go

@@ -9,6 +9,7 @@ import (
 	"os/exec"
 	"sort"
 	"strings"
+	"syscall"
 	"time"
 )
 
@@ -91,7 +92,7 @@ func _allocateFilterStreamNames(nodes []*Node, outOutingEdgeMaps map[int]map[Lab
 		// todo sort
 		for _, l := range _getAllLabelsSorted(om) {
 			if len(om[l]) > 1 {
-				panic(fmt.Sprintf(`encountered %s with multiple outgoing edges 
+				panic(fmt.Sprintf(`encountered %s with multiple outgoing edges
 with same upstream label %s; a 'split'' filter is probably required`, n.name, l))
 			}
 			streamNameMap[fmt.Sprintf("%d%s", n.Hash(), l)] = fmt.Sprintf("s%d", sc)
@@ -236,8 +237,19 @@ func (s *Stream) ErrorToStdOut() *Stream {
 	return s.WithErrorOutput(os.Stdout)
 }
 
+type CompilationOption func(s *Stream, cmd *exec.Cmd)
+
+// SeparateProcessGroup ensures that the command is run in a separate process
+// group. This is useful to enable handling of signals such as SIGINT without
+// propagating them to the ffmpeg process.
+func SeparateProcessGroup() CompilationOption {
+	return func(s *Stream, cmd *exec.Cmd) {
+		cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true, Pgid: 0}
+	}
+}
+
 // for test
-func (s *Stream) Compile() *exec.Cmd {
+func (s *Stream) Compile(options ...CompilationOption) *exec.Cmd {
 	args := s.GetArgs()
 	cmd := exec.CommandContext(s.Context, "ffmpeg", args...)
 	if a, ok := s.Context.Value("Stdin").(io.Reader); ok {
@@ -249,11 +261,14 @@ func (s *Stream) Compile() *exec.Cmd {
 	if a, ok := s.Context.Value("Stderr").(io.Writer); ok {
 		cmd.Stderr = a
 	}
+	for _, option := range options {
+		option(s, cmd)
+	}
 	log.Printf("compiled command: ffmpeg %s\n", strings.Join(args, " "))
 	return cmd
 }
 
-func (s *Stream) Run() error {
+func (s *Stream) Run(options ...CompilationOption) error {
 	if s.Context.Value("run_hook") != nil {
 		hook := s.Context.Value("run_hook").(*RunHook)
 		go hook.f()
@@ -264,5 +279,5 @@ func (s *Stream) Run() error {
 			<-hook.done
 		}()
 	}
-	return s.Compile().Run()
+	return s.Compile(options...).Run()
 }