x/term: handle delete key

Fixes golang/go#76826

Change-Id: I80dc9468c2cc716dab10a8e165ca5f72817ef4b9
Reviewed-on: https://go-review.googlesource.com/c/term/+/730440
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

terminal.go

@@ -160,6 +160,7 @@ const (
 	keyEnd
 	keyDeleteWord
 	keyDeleteLine
+	keyDelete
 	keyClearScreen
 	keyPasteStart
 	keyPasteEnd
@@ -228,6 +229,10 @@ func bytesToKey(b []byte, pasteActive bool) (rune, []byte) {
 		}
 	}
 
+	if !pasteActive && len(b) >= 4 && b[0] == keyEscape && b[1] == '[' && b[2] == '3' && b[3] == '~' {
+		return keyDelete, b[4:]
+	}
+
 	if !pasteActive && len(b) >= 6 && b[0] == keyEscape && b[1] == '[' && b[2] == '1' && b[3] == ';' && b[4] == '3' {
 		switch b[5] {
 		case 'C':
@@ -590,7 +595,7 @@ func (t *Terminal) handleKey(key rune) (line string, ok bool) {
 		}
 		t.line = t.line[:t.pos]
 		t.moveCursorToPos(t.pos)
-	case keyCtrlD:
+	case keyCtrlD, keyDelete:
 		// Erase the character under the current position.
 		// The EOF case when the line is empty is handled in
 		// readLine().

terminal_test.go

@@ -238,6 +238,31 @@ var keyPressTests = []struct {
 		in:  "a\003\r",
 		err: io.EOF,
 	},
+	{
+		// Delete at EOL: nothing
+		in:   "abc\x1b[3~\r",
+		line: "abc",
+	},
+	{
+		// Delete in empty string: nothing
+		in:   "\x1b[3~\r",
+		line: "",
+	},
+	{
+		// Move left, delete: delete 'c'
+		in:   "abc\x1b[D\x1b[3~\r",
+		line: "ab",
+	},
+	{
+		// Home, delete: delete 'a'
+		in:   "abc\x1b[H\x1b[3~\r",
+		line: "bc",
+	},
+	{
+		// Home, delete twice: delete 'a' and 'b'
+		in:   "abc\x1b[H\x1b[3~\x1b[3~\r",
+		line: "c",
+	},
 }
 
 func TestKeyPresses(t *testing.T) {
@@ -387,7 +412,7 @@ func TestReadPasswordLineEnd(t *testing.T) {
 		input string
 		want  string
 	}
-	var tests = []testType{
+	tests := []testType{
 		{"\r\n", ""},
 		{"test\r\n", "test"},
 		{"test\r", "test"},