readline: bind keystroke ctrl+6 to redo

1. Any keystroke emits `0x1E` will do redo action.
2. Fix bug of undo/redo.
3. More detailed document.
4. Unit tests.

PR-URL: nodejs#41662
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/readline.md

@@ -1354,7 +1354,18 @@ const { createInterface } = require('readline');
   <tr>
     <td><kbd>Ctrl</kbd>+<kbd>-</kbd></td>
     <td>Undo previous change</td>
-    <td>Any keystroke emits key code <code>0x1F</code> would do this action.</td>
+    <td>Any keystroke that emits key code <code>0x1F</code> will do this action.
+    In many terminals, for example <code>xterm</code>,
+    this is bound to <kbd>Ctrl</kbd>+<kbd>-</kbd>.</td>
+    <td></td>
+  </tr>
+  <tr>
+    <td><kbd>Ctrl</kbd>+<kbd>6</kbd></td>
+    <td>Redo previous change</td>
+    <td>Many terminals don't have a default redo keystroke.
+    We choose key code <code>0x1E</code> to perform redo.
+    In <code>xterm</code>, it is bound to <kbd>Ctrl</kbd>+<kbd>6</kbd>
+    by default.</td>
     <td></td>
   </tr>
   <tr>

lib/internal/readline/interface.js

@@ -889,24 +889,30 @@ class Interface extends InterfaceConstructor {
   [kUndo]() {
     if (this[kUndoStack].length <= 0) return;
 
-    const entry = this[kUndoStack].pop();
+    ArrayPrototypePush(
+      this[kRedoStack],
+      { text: this.line, cursor: this.cursor },
+    );
 
+    const entry = ArrayPrototypePop(this[kUndoStack]);
     this.line = entry.text;
     this.cursor = entry.cursor;
 
-    ArrayPrototypePush(this[kRedoStack], entry);
     this[kRefreshLine]();
   }
 
   [kRedo]() {
     if (this[kRedoStack].length <= 0) return;
 
-    const entry = this[kRedoStack].pop();
+    ArrayPrototypePush(
+      this[kUndoStack],
+      { text: this.line, cursor: this.cursor },
+    );
 
+    const entry = ArrayPrototypePop(this[kRedoStack]);
     this.line = entry.text;
     this.cursor = entry.cursor;
 
-    ArrayPrototypePush(this[kUndoStack], entry);
     this[kRefreshLine]();
   }
 
@@ -1071,11 +1077,18 @@ class Interface extends InterfaceConstructor {
       }
     }
 
-    // Undo
-    if (typeof key.sequence === 'string' &&
-        StringPrototypeCodePointAt(key.sequence, 0) === 0x1f) {
-      this[kUndo]();
-      return;
+    // Undo & Redo
+    if (typeof key.sequence === 'string') {
+      switch (StringPrototypeCodePointAt(key.sequence, 0)) {
+        case 0x1f:
+          this[kUndo]();
+          return;
+        case 0x1e:
+          this[kRedo]();
+          return;
+        default:
+          break;
+      }
     }
 
     // Ignore escape key, fixes

test/parallel/test-readline-interface.js

@@ -792,24 +792,36 @@ function assertCursorRowsAndCols(rli, rows, cols) {
   rli.close();
 }
 
-// Undo
+// Undo & Redo
 {
   const [rli, fi] = getInterface({ terminal: true, prompt: '' });
   fi.emit('data', 'the quick brown fox');
   assertCursorRowsAndCols(rli, 0, 19);
 
-  // Delete right line from the 5th char
+  // Delete the last eight chars
   fi.emit('keypress', '.', { ctrl: true, shift: false, name: 'b' });
   fi.emit('keypress', '.', { ctrl: true, shift: false, name: 'b' });
   fi.emit('keypress', '.', { ctrl: true, shift: false, name: 'b' });
   fi.emit('keypress', '.', { ctrl: true, shift: false, name: 'b' });
   fi.emit('keypress', ',', { ctrl: true, shift: false, name: 'k' });
-  fi.emit('keypress', ',', { ctrl: true, shift: false, name: 'u' });
-  assertCursorRowsAndCols(rli, 0, 0);
+
+  fi.emit('keypress', '.', { ctrl: true, shift: false, name: 'b' });
+  fi.emit('keypress', '.', { ctrl: true, shift: false, name: 'b' });
+  fi.emit('keypress', '.', { ctrl: true, shift: false, name: 'b' });
+  fi.emit('keypress', '.', { ctrl: true, shift: false, name: 'b' });
+  fi.emit('keypress', ',', { ctrl: true, shift: false, name: 'k' });
+
+  assertCursorRowsAndCols(rli, 0, 11);
+  // Perform undo twice
   fi.emit('keypress', ',', { sequence: '\x1F' });
   assert.strictEqual(rli.line, 'the quick brown');
   fi.emit('keypress', ',', { sequence: '\x1F' });
   assert.strictEqual(rli.line, 'the quick brown fox');
+  // Perform redo twice
+  fi.emit('keypress', ',', { sequence: '\x1E' });
+  assert.strictEqual(rli.line, 'the quick brown');
+  fi.emit('keypress', ',', { sequence: '\x1E' });
+  assert.strictEqual(rli.line, 'the quick b');
   fi.emit('data', '\n');
   rli.close();
 }