Allow package users to set the child process priority (coder#17)

localexec.go

@@ -9,6 +9,8 @@ import (
 
 // LocalExecer executes command on the local system.
 type LocalExecer struct {
+	// ChildProcessPriority overrides the default niceness of all child processes launch by LocalExecer.
+	ChildProcessPriority *int
 }
 
 func (l *localProcess) Stdin() io.WriteCloser {

localexec_unix.go

@@ -91,6 +91,15 @@ func (l LocalExecer) Start(ctx context.Context, c Command) (Process, error) {
 		if err != nil {
 			return nil, xerrors.Errorf("start command: %w", err)
 		}
+
+		if l.ChildProcessPriority != nil {
+			pid := process.cmd.Process.Pid
+			niceness := *l.ChildProcessPriority
+			err := syscall.Setpriority(syscall.PRIO_PROCESS, pid, niceness)
+			if err != nil {
+				return nil, xerrors.Errorf("set process (pid: %d) priority to (niceness: %d): %w", pid, niceness, err)
+			}
+		}
 	}
 
 	return &process, nil