readline: avoid painting default prompt before prompt()

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

@@ -99,6 +99,8 @@
 
 const kMultilinePrompt = Symbol('| ');
 
+const kDefaultPrompt = '> ';
+
 const kAddHistory = Symbol('_addHistory');
 const kBeforeEdit = Symbol('_beforeEdit');
 const kDecoder = Symbol('_decoder');
@@ -125,6 +127,8 @@
 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');
@@ -170,7 +174,7 @@
   FunctionPrototypeCall(EventEmitter, this);
 
   let crlfDelay;
-  let prompt = '> ';
+  let prompt = kDefaultPrompt;
   let signal;
 
   if (input?.input) {
@@ -252,6 +256,7 @@
   this.completer = completer;
 
   this.setPrompt(prompt);
+  this[kPromptInvoked] = false;
 
   this.terminal = !!terminal;
 
@@ -353,6 +358,13 @@
   this[kSetLine]('');
 
   input.resume();
+
+  // If the default prompt prompt is used and the terminal is active, the prompt is automatically displayed.
+  if (prompt === kDefaultPrompt && this.terminal && output !== null && output !== undefined) {    
+    process.nextTick(() => {
+      this.prompt();
+    });
+  }
 }
 
 ObjectSetPrototypeOf(InterfaceConstructor.prototype, EventEmitter.prototype);
@@ -428,6 +440,7 @@
    */
   prompt(preserveCursor) {
     if (this.paused) this.resume();
+    this[kPromptInvoked] = true;
     if (this.terminal && process.env.TERM !== 'dumb') {
       if (!preserveCursor) this.cursor = 0;
       this[kRefreshLine]();
@@ -463,6 +476,9 @@
       cb(line);
     } else {
       this.emit('line', line);
+      if (this[kPrompt] === kDefaultPrompt && this.terminal && this.output !== null && this.output !== undefined) {  
+        this.prompt();  
+      }  
     }
   }
 
@@ -490,9 +506,20 @@
     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;
@@ -514,7 +541,7 @@
     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++) {
@@ -720,6 +747,10 @@
       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 !== ''),
@@ -1020,7 +1051,9 @@
     }
 
     if (needsRewriteFirstLine) {
-      this[kWriteToOutput](`${this[kPrompt]}${beforeCursor}\n${kMultilinePrompt.description}`);
+      this[kWriteToOutput](
+        `${this[kEffectivePrompt]()}${beforeCursor}\n${kMultilinePrompt.description}`,
+      );
     } else {
       this[kWriteToOutput](kMultilinePrompt.description);
     }
@@ -1221,7 +1254,8 @@
    *   }}
    */
   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

@@ -1,3 +1,3 @@
 // Copyright Joyent, Inc. and other Node contributors.
 //
 // Permission is hereby granted, free of charge, to any person obtaining a
@@ -1271,6 +1271,58 @@
     }
   }
 
+  // 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'] :