shell: When stdin is not TTY, use a minimal editor.

The old behavior was to source /dev/stdin, meaning that Elvish can behave very
differently when the stdin is not TTY. By using a minimal editor, the
difference is reduced. This also removes the dependency on /dev/stdin, which
does not exist on Windows.

shell/editor.go

@@ -0,0 +1,27 @@
+package shell
+
+import (
+	"bufio"
+	"io"
+	"os"
+)
+
+type editor interface {
+	ReadLine() (string, error)
+	Close()
+}
+
+type minEditor struct {
+	in  *bufio.Reader
+	out io.Writer
+}
+
+func newMinEditor(in, out *os.File) *minEditor {
+	return &minEditor{bufio.NewReader(in), out}
+}
+func (ed *minEditor) ReadLine() (string, error) {
+	return ed.in.ReadString('\n')
+}
+
+func (editor *minEditor) Close() {
+}

shell/shell.go

@@ -53,8 +53,6 @@ func (sh *Shell) Run(args []string) int {
 		} else {
 			script(sh.ev, arg)
 		}
-	} else if !sys.IsATTY(0) {
-		script(sh.ev, "/dev/stdin")
 	} else {
 		interact(sh.ev, sh.daemon)
 	}
@@ -137,7 +135,12 @@ func readFileUTF8(fname string) (string, error) {
 
 func interact(ev *eval.Evaler, daemon *api.Client) {
 	// Build Editor.
-	ed := makeEditor(os.Stdin, os.Stderr, ev, daemon)
+	var ed editor
+	if sys.IsATTY(0) {
+		ed = makeEditor(os.Stdin, os.Stderr, ev, daemon)
+	} else {
+		ed = newMinEditor(os.Stdin, os.Stderr)
+	}
 	defer ed.Close()
 
 	// Source rc.elv.

shell/sys_windows.go

@@ -1,25 +1,11 @@
 package shell
 
 import (
-	"bufio"
-	"io"
 	"os"
 )
 
-type minEditor struct {
-	in  *bufio.Reader
-	out io.Writer
-}
-
 func makeEditor(in, out *os.File, _, _ interface{}) *minEditor {
-	return &minEditor{bufio.NewReader(in), out}
-}
-
-func (ed *minEditor) ReadLine() (string, error) {
-	return ed.in.ReadString('\n')
-}
-
-func (editor *minEditor) Close() {
+	return newMinEditor(in, out)
 }
 
 func handleSignal(_ os.Signal) {