util: improve iterator inspect output

1) So far extra keys on an (Set|Map)Iterator were ignored. Those
   will now be visible.
2) Improve the performance of showing (Set|Map)Iterator by using
   the cloned iterator instead of copying all entries first.
3) So far the output was strictly limited to up to 100 entries.
   The limit will now depend on `maxArrayLength` instead (that
   default is set to 100 as well) and the output indicates that
   more entries exist than visible.

PR-URL: #19259
Reviewed-By: Yosuke Furukawa <yosuke.furukawa@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/bootstrap/node.js

@@ -471,8 +471,8 @@
     // functions lazily (unless --nolazy is set) so we need to do this
     // before we turn off --allow_natives_syntax again.
     const v8 = NativeModule.require('internal/v8');
-    v8.previewMapIterator(new Map().entries(), 1);
-    v8.previewSetIterator(new Set().entries(), 1);
+    v8.previewMapIterator(new Map().entries());
+    v8.previewSetIterator(new Set().entries());
     // Disable --allow_natives_syntax again unless it was explicitly
     // specified on the command line.
     const re = /^--allow[-_]natives[-_]syntax$/;

lib/internal/v8.js

@@ -1,21 +1,23 @@
 'use strict';
 
-function take(it, n) {
-  const result = [];
-  for (const e of it) {
-    if (--n < 0)
-      break;
-    result.push(e);
-  }
-  return result;
-}
+// This file provides access to some of V8's native runtime functions. See
+// https://github.com/v8/v8/wiki/Built-in-functions for further information
+// about their implementation.
+// They have to be loaded before anything else to make sure we deactivate them
+// before executing any other code. Gaining access is achieved by using a
+// specific flag that is used internally in the startup phase.
 
-function previewMapIterator(it, n) {
-  return take(%MapIteratorClone(it), n);
+// Clone the provided Map Iterator.
+function previewMapIterator(it) {
+  return %MapIteratorClone(it);
 }
 
-function previewSetIterator(it, n) {
-  return take(%SetIteratorClone(it), n);
+// Clone the provided Set Iterator.
+function previewSetIterator(it) {
+  return %SetIteratorClone(it);
 }
 
-module.exports = { previewMapIterator, previewSetIterator };
+module.exports = {
+  previewMapIterator,
+  previewSetIterator
+};

lib/util.js

@@ -30,7 +30,10 @@ const {
 const { TextDecoder, TextEncoder } = require('internal/encoding');
 const { isBuffer } = require('buffer').Buffer;
 
-const { previewMapIterator, previewSetIterator } = require('internal/v8');
+const {
+  previewMapIterator,
+  previewSetIterator
+} = require('internal/v8');
 
 const {
   getPromiseDetails,
@@ -836,25 +839,29 @@ function formatMap(ctx, value, recurseTimes, keys) {
   return output;
 }
 
-function formatCollectionIterator(preview, ctx, value, recurseTimes,
-                                  visibleKeys, keys) {
-  const nextRecurseTimes = recurseTimes === null ? null : recurseTimes - 1;
-  const vals = preview(value, 100);
+function formatCollectionIterator(preview, ctx, value, recurseTimes, keys) {
   const output = [];
-  for (const o of vals) {
-    output.push(formatValue(ctx, o, nextRecurseTimes));
+  for (const entry of preview(value)) {
+    if (ctx.maxArrayLength === output.length) {
+      output.push('... more items');
+      break;
+    }
+    output.push(formatValue(ctx, entry, recurseTimes));
+  }
+  for (var n = 0; n < keys.length; n++) {
+    output.push(formatProperty(ctx, value, recurseTimes, keys[n], 0));
   }
   return output;
 }
 
-function formatMapIterator(ctx, value, recurseTimes, visibleKeys, keys) {
+function formatMapIterator(ctx, value, recurseTimes, keys) {
   return formatCollectionIterator(previewMapIterator, ctx, value, recurseTimes,
-                                  visibleKeys, keys);
+                                  keys);
 }
 
-function formatSetIterator(ctx, value, recurseTimes, visibleKeys, keys) {
+function formatSetIterator(ctx, value, recurseTimes, keys) {
   return formatCollectionIterator(previewSetIterator, ctx, value, recurseTimes,
-                                  visibleKeys, keys);
+                                  keys);
 }
 
 function formatPromise(ctx, value, recurseTimes, keys) {

test/parallel/test-util-inspect.js

@@ -448,7 +448,7 @@ assert.strictEqual(util.inspect(-5e-324), '-5e-324');
 {
   const map = new Map();
   map.set(1, 2);
-  const vals = previewMapIterator(map.entries(), 100);
+  const vals = previewMapIterator(map.entries());
   const valsOutput = [];
   for (const o of vals) {
     valsOutput.push(o);
@@ -924,6 +924,10 @@ if (typeof Symbol !== 'undefined') {
   const keys = map.keys();
   assert.strictEqual(util.inspect(keys), '[Map Iterator] { \'foo\' }');
   assert.strictEqual(util.inspect(keys), '[Map Iterator] { \'foo\' }');
+  keys.extra = true;
+  assert.strictEqual(
+    util.inspect(keys, { maxArrayLength: 0 }),
+    '[Map Iterator] { ... more items, extra: true }');
 }
 
 // Test Set iterators.
@@ -937,6 +941,10 @@ if (typeof Symbol !== 'undefined') {
   const keys = aSet.keys();
   assert.strictEqual(util.inspect(keys), '[Set Iterator] { 1, 3 }');
   assert.strictEqual(util.inspect(keys), '[Set Iterator] { 1, 3 }');
+  keys.extra = true;
+  assert.strictEqual(
+    util.inspect(keys, { maxArrayLength: 1 }),
+    '[Set Iterator] { 1, ... more items, extra: true }');
 }
 
 // Test alignment of items in container.