readline: show completions only after 2nd TAB

Show `TAB` completion suggestions only after the user has pressed `TAB`
twice in a row, so that the full list of suggestions doesn’t present
a distraction. The first time a `TAB` key is pressed, only partial
longest-common-prefix completion is performed.

This moves the `readline` autocompletion a lot closer to what e.g.
`bash` does.

Fixes: nodejs#7665

lib/readline.js

@@ -41,6 +41,7 @@ function Interface(input, output, completer, terminal) {
 
   this._sawReturn = false;
   this.isCompletionEnabled = true;
+  this._previousKey = null;
 
   EventEmitter.call(this);
   var historySize;
@@ -391,7 +392,7 @@ Interface.prototype._insertString = function(c) {
   }
 };
 
-Interface.prototype._tabComplete = function() {
+Interface.prototype._tabComplete = function(lastKeypressWasTab) {
   var self = this;
 
   self.pause();
@@ -407,9 +408,7 @@ Interface.prototype._tabComplete = function() {
     const completeOn = rv[1];  // the text that was completed
     if (completions && completions.length) {
       // Apply/show completions.
-      if (completions.length === 1) {
-        self._insertString(completions[0].slice(completeOn.length));
-      } else {
+      if (lastKeypressWasTab) {
         self._writeToOutput('\r\n');
         var width = completions.reduce(function(a, b) {
           return a.length > b.length ? a : b;
@@ -429,16 +428,15 @@ Interface.prototype._tabComplete = function() {
           }
         }
         handleGroup(self, group, width, maxColumns);
+      }
 
-        // If there is a common prefix to all matches, then apply that
-        // portion.
-        var f = completions.filter(function(e) { if (e) return e; });
-        var prefix = commonPrefix(f);
-        if (prefix.length > completeOn.length) {
-          self._insertString(prefix.slice(completeOn.length));
-        }
-
+      // If there is a common prefix to all matches, then apply that portion.
+      const f = completions.filter(function(e) { if (e) return e; });
+      const prefix = commonPrefix(f);
+      if (prefix.length > completeOn.length) {
+        self._insertString(prefix.slice(completeOn.length));
       }
+
       self._refreshLine();
     }
   });
@@ -688,7 +686,9 @@ Interface.prototype._moveCursor = function(dx) {
 
 // handle a write from the tty
 Interface.prototype._ttyWrite = function(s, key) {
+  const previousKey = this._previousKey;
   key = key || {};
+  this._previousKey = key;
 
   // Ignore escape key - Fixes #2876
   if (key.name == 'escape') return;
@@ -892,7 +892,8 @@ Interface.prototype._ttyWrite = function(s, key) {
       case 'tab':
         // If tab completion enabled, do that...
         if (typeof this.completer === 'function' && this.isCompletionEnabled) {
-          this._tabComplete();
+          const lastKeypressWasTab = previousKey && previousKey.name === 'tab';
+          this._tabComplete(lastKeypressWasTab);
           break;
         }
         // falls through

test/parallel/test-readline-undefined-columns.js

@@ -1,6 +1,6 @@
 'use strict';
 
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 const PassThrough = require('stream').PassThrough;
 const readline = require('readline');
@@ -26,12 +26,17 @@ oStream.on('data', function(data) {
   output += data;
 });
 
-oStream.on('end', function() {
+oStream.on('end', common.mustCall(() => {
   const expect = 'process.stdout\r\n' +
                  'process.stdin\r\n' +
                  'process.stderr';
   assert(new RegExp(expect).test(output));
-});
+}));
+
+iStream.write('process.s\t');
+
+assert(/process.std\b/.test(output));  // Completion works.
+assert(!/stdout/.test(output));  // Completion doesn’t show all results yet.
 
-iStream.write('process.std\t');
+iStream.write('\t');
 oStream.end();