readline: avoid painting default prompt before prompt()

Track whether prompt() has been invoked and whether the configured prompt
is still the documented default. Until then, skip the default prefix when
redrawing the line or computing cursor position, fixing inconsistent
appearance after keys that refresh the line (see issue #12606).

Set the same flag when tab completion redraws the line so behavior matches
prior releases. Document that callers should call prompt() from a line
listener when they want a prompt on each new line (REPL already does).

Refs: #12606
Assisted-by: Cursor IDE

Author: Tseian

doc/api/readline.md

@@ -321,6 +321,10 @@ location at which to provide input.
 When called, `rl.prompt()` will resume the `input` stream if it has been
 paused.
 
+To show the prompt on each new line of input, call `rl.prompt()` from your
+`'line'` event listener (the built-in REPL does this after evaluating each
+line).
+
 If the `InterfaceConstructor` was created with `output` set to `null` or
 `undefined` the prompt is not written.
 
@@ -710,6 +714,9 @@ added: v17.0.0
     to the history list duplicates an older one, this removes the older line
     from the list. **Default:** `false`.
   * `prompt` {string} The prompt string to use. **Default:** `'> '`.
+    For TTY interfaces, the default prompt is not written when the line is
+    redrawn until `rl.prompt()` has been called at least once (see
+    [`rl.prompt()`][]). A non-default `prompt` is written on redraw as before.
   * `crlfDelay` {number} If the delay between `\r` and `\n` exceeds
     `crlfDelay` milliseconds, both `\r` and `\n` will be treated as separate
     end-of-line input. `crlfDelay` will be coerced to a number no less than
@@ -975,6 +982,9 @@ changes:
     to the history list duplicates an older one, this removes the older line
     from the list. **Default:** `false`.
   * `prompt` {string} The prompt string to use. **Default:** `'> '`.
+    For TTY interfaces, the default prompt is not written when the line is
+    redrawn until `rl.prompt()` has been called at least once (see
+    [`rl.prompt()`][]). A non-default `prompt` is written on redraw as before.
   * `crlfDelay` {number} If the delay between `\r` and `\n` exceeds
     `crlfDelay` milliseconds, both `\r` and `\n` will be treated as separate
     end-of-line input. `crlfDelay` will be coerced to a number no less than
@@ -1492,4 +1502,5 @@ const { createInterface } = require('node:readline');
 [`process.stdin`]: process.md#processstdin
 [`process.stdout`]: process.md#processstdout
 [`rl.close()`]: #rlclose
+[`rl.prompt()`]: #rlpromptpreservecursor
 [reading files]: #example-read-file-stream-line-by-line

lib/internal/readline/interface.js

@@ -127,6 +127,8 @@ const kOnLine = Symbol('_onLine');
 const kSetLine = Symbol('_setLine');
 const kPreviousKey = Symbol('_previousKey');
 const kPrompt = Symbol('_prompt');
+const kPromptInvoked = Symbol('_promptInvoked');
+const kEffectivePrompt = Symbol('_effectivePrompt');
 const kPushToKillRing = Symbol('_pushToKillRing');
 const kPushToUndoStack = Symbol('_pushToUndoStack');
 const kQuestionCallback = Symbol('_questionCallback');
@@ -254,6 +256,7 @@ function InterfaceConstructor(input, output, completer, terminal) {
   this.completer = completer;
 
   this.setPrompt(prompt);
+  this[kPromptInvoked] = false;
 
   this.terminal = !!terminal;
 
@@ -437,6 +440,7 @@ class Interface extends InterfaceConstructor {
    */
   prompt(preserveCursor) {
     if (this.paused) this.resume();
+    this[kPromptInvoked] = true;
     if (this.terminal && process.env.TERM !== 'dumb') {
       if (!preserveCursor) this.cursor = 0;
       this[kRefreshLine]();
@@ -502,9 +506,20 @@ class Interface extends InterfaceConstructor {
     return this.historyManager.addHistory(this[kIsMultiline], this[kLastCommandErrored]);
   }
 
+  [kEffectivePrompt]() {
+    if (this[kPromptInvoked]) {
+      return this[kPrompt];
+    }
+    // Issue #12606: default prompt is not painted until `prompt()`, but
+    // non-default prompts (including multi-line prompts) participate in
+    // layout/cursor math the same as before.
+    return this[kPrompt] === kDefaultPrompt ? '' : this[kPrompt];
+  }
+
   [kRefreshLine]() {
     // line length
-    const line = this[kPrompt] + this.line;
+    const promptPrefix = this[kEffectivePrompt]();
+    const line = promptPrefix + this.line;
     const dispPos = this[kGetDisplayPos](line);
     const lineCols = dispPos.cols;
     const lineRows = dispPos.rows;
@@ -526,7 +541,7 @@ class Interface extends InterfaceConstructor {
     if (this[kIsMultiline]) {
       const lines = StringPrototypeSplit(this.line, '\n');
       // Write first line with normal prompt
-      this[kWriteToOutput](this[kPrompt] + lines[0]);
+      this[kWriteToOutput](promptPrefix + lines[0]);
 
       // For continuation lines, add the "|" prefix
       for (let i = 1; i < lines.length; i++) {
@@ -732,6 +747,10 @@ class Interface extends InterfaceConstructor {
       return;
     }
 
+    // Tab completion redraws the whole line; treat it like `prompt()` for the
+    // purpose of painting the default prompt (see #12606).
+    this[kPromptInvoked] = true;
+
     // If there is a common prefix to all matches, then apply that portion.
     const prefix = commonPrefix(
       ArrayPrototypeFilter(completions, (e) => e !== ''),
@@ -1032,7 +1051,9 @@ class Interface extends InterfaceConstructor {
     }
 
     if (needsRewriteFirstLine) {
-      this[kWriteToOutput](`${this[kPrompt]}${beforeCursor}\n${kMultilinePrompt.description}`);
+      this[kWriteToOutput](
+        `${this[kEffectivePrompt]()}${beforeCursor}\n${kMultilinePrompt.description}`,
+      );
     } else {
       this[kWriteToOutput](kMultilinePrompt.description);
     }
@@ -1233,7 +1254,8 @@ class Interface extends InterfaceConstructor {
    *   }}
    */
   getCursorPos() {
-    const strBeforeCursor = this[kPrompt] + StringPrototypeSlice(this.line, 0, this.cursor);
+    const strBeforeCursor = this[kEffectivePrompt]() +
+      StringPrototypeSlice(this.line, 0, this.cursor);
 
     return this[kGetDisplayPos](strBeforeCursor);
   }

test/parallel/test-readline-interface.js

@@ -1271,6 +1271,58 @@ for (let i = 0; i < 12; i++) {
     }
   }
 
+  // Do not paint the configured prompt on refresh until prompt() is called.
+  // https://github.com/nodejs/node/issues/12606
+  if (terminal) {
+    const fi = new FakeInput();
+    fi.isTTY = true;
+    fi.columns = 80;
+    const output = [];
+    fi.write = (chunk) => {
+      output.push(chunk.toString());
+      return true;
+    };
+
+    const rli = readline.createInterface({
+      input: fi,
+      output: fi,
+      terminal: true,
+    });
+
+    rli.write('a');
+    output.length = 0;
+    rli.write(undefined, { name: 'backspace' });
+    assert.strictEqual(output.join('').includes('> '), false);
+    rli.close();
+  }
+
+  // gh-12606: redraw each new line by calling `prompt()` from `'line'` (REPL pattern).
+  if (terminal) {
+    const fi = new FakeInput();
+    fi.isTTY = true;
+    fi.columns = 80;
+    const output = [];
+    fi.write = (chunk) => {
+      output.push(chunk.toString());
+      return true;
+    };
+
+    const rli = readline.createInterface({
+      input: fi,
+      output: fi,
+      terminal: true,
+    });
+
+    rli.prompt(false);
+    rli.on('line', () => {
+      rli.prompt(false);
+    });
+    output.length = 0;
+    fi.emit('data', 'x\n');
+    assert.strictEqual(output.join('').includes('> '), true);
+    rli.close();
+  }
+
   {
     const expected = terminal ?
       ['\u001b[1G', '\u001b[0J', '$ ', '\u001b[3G'] :