readline: avoid painting default prompt before prompt()

lib/internal/readline/interface.js

@@ -126,6 +126,9 @@ 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 kDefaultPrompt = '> ';
 const kPushToKillRing = Symbol('_pushToKillRing');
 const kPushToUndoStack = Symbol('_pushToUndoStack');
 const kQuestionCallback = Symbol('_questionCallback');
@@ -173,7 +176,7 @@ function InterfaceConstructor(input, output, completer, terminal) {
   FunctionPrototypeCall(EventEmitter, this);
 
   let crlfDelay;
-  let prompt = '> ';
+  let prompt = kDefaultPrompt;
   let signal;
   let historyOptions;
 
@@ -260,6 +263,7 @@ function InterfaceConstructor(input, output, completer, terminal) {
   this.completer = completer;
 
   this.setPrompt(prompt);
+  this[kPromptInvoked] = false;
 
   this.terminal = !!terminal;
 
@@ -440,6 +444,7 @@ 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]();
@@ -448,6 +453,15 @@ class Interface extends InterfaceConstructor {
     }
   }
 
+  // Issue #12606: default prompt is not painted until `prompt()`, but
+  // non-default prompts participate in layout/cursor math as before.
+  [kEffectivePrompt]() {
+    if (this[kPromptInvoked]) {
+      return this[kPrompt];
+    }
+    return this[kPrompt] === kDefaultPrompt ? '' : this[kPrompt];
+  }
+
   [kQuestion](query, cb) {
     if (this.closed) {
       throw new ERR_USE_AFTER_CLOSE('readline');
@@ -504,7 +518,8 @@ class Interface extends InterfaceConstructor {
 
   [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] +
+      this[kWriteToOutput](promptPrefix +
                            (this[kColorize]?.(lines[0]) ?? lines[0]));
 
       // For continuation lines, add the "|" prefix
@@ -536,7 +551,7 @@ class Interface extends InterfaceConstructor {
       }
     } else {
       // Write the prompt and the current buffer content.
-      this[kWriteToOutput](this[kPrompt] +
+      this[kWriteToOutput](promptPrefix +
                            (this[kColorize]?.(this.line) ?? this.line));
     }
 
@@ -772,6 +787,9 @@ class Interface extends InterfaceConstructor {
     }
 
     this[kBeforeEdit](this.line, this.cursor);
+    // Tab completion redraws the whole line; treat it like `prompt()` for the
+    // purpose of painting the default prompt (see #12606).
+    this[kPromptInvoked] = true;
 
     // Apply/show completions.
     const completionsWidth = ArrayPrototypeMap(completions, (e) =>
@@ -1045,8 +1063,10 @@ class Interface extends InterfaceConstructor {
       }
     }
 
-    if (needsRewriteFirstLine) {
-      this[kWriteToOutput](`${this[kPrompt]}${beforeCursor}\n${kMultilinePrompt.description}`);
+      if (needsRewriteFirstLine) {
+        this[kWriteToOutput](
+          `${this[kEffectivePrompt]()}${beforeCursor}\n${kMultilinePrompt.description}`,
+        );
     } else {
       this[kWriteToOutput](kMultilinePrompt.description);
     }
@@ -1247,7 +1267,8 @@ 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);
   }