timers: use custom inspection for linked lists

lib/internal/timers.js

@@ -17,6 +17,8 @@ const {
 } = require('internal/errors').codes;
 const { validateNumber } = require('internal/validators');
 
+const { inspect } = require('util');
+
 // Timeout values > TIMEOUT_MAX are set to 1.
 const TIMEOUT_MAX = 2 ** 31 - 1;
 
@@ -82,6 +84,17 @@ function Timeout(callback, after, args, isRepeat) {
   initAsyncResource(this, 'Timeout');
 }
 
+// Make sure the linked list only shows the minimal necessary information.
+Timeout.prototype[inspect.custom] = function(_, options) {
+  return inspect(this, {
+    ...options,
+    // Only inspect one level.
+    depth: 0,
+    // It should not recurse.
+    customInspect: false
+  });
+};
+
 Timeout.prototype.refresh = function() {
   if (this[kRefed])
     getTimers().active(this);

lib/timers.js

@@ -239,6 +239,17 @@ function TimersList(expiry, msecs) {
   this.priorityQueuePosition = null;
 }
 
+// Make sure the linked list only shows the minimal necessary information.
+TimersList.prototype[util.inspect.custom] = function(_, options) {
+  return util.inspect(this, {
+    ...options,
+    // Only inspect one level.
+    depth: 0,
+    // It should not recurse.
+    customInspect: false
+  });
+};
+
 const { _tickCallback: runNextTicks } = process;
 function processTimers(now) {
   debug('process timer lists %d', now);

test/parallel/test-http2-socket-proxy.js

@@ -8,6 +8,7 @@ if (!common.hasCrypto)
 const assert = require('assert');
 const h2 = require('http2');
 const net = require('net');
+const util = require('util');
 
 const { kTimeout } = require('internal/timers');
 
@@ -35,6 +36,22 @@ server.on('stream', common.mustCall(function(stream, headers) {
   socket.setTimeout(987);
   assert.strictEqual(session[kTimeout]._idleTimeout, 987);
 
+  // The indentation is corrected depending on the depth.
+  let inspectedTimeout = util.inspect(session[kTimeout]);
+  assert(inspectedTimeout.includes('  _idlePrev: [TimersList]'));
+  assert(inspectedTimeout.includes('  _idleNext: [TimersList]'));
+  assert(!inspectedTimeout.includes('   _idleNext: [TimersList]'));
+
+  inspectedTimeout = util.inspect([ session[kTimeout] ]);
+  assert(inspectedTimeout.includes('    _idlePrev: [TimersList]'));
+  assert(inspectedTimeout.includes('    _idleNext: [TimersList]'));
+  assert(!inspectedTimeout.includes('     _idleNext: [TimersList]'));
+
+  const inspectedTimersList = util.inspect([[ session[kTimeout]._idlePrev ]]);
+  assert(inspectedTimersList.includes('      _idlePrev: [Timeout]'));
+  assert(inspectedTimersList.includes('      _idleNext: [Timeout]'));
+  assert(!inspectedTimersList.includes('       _idleNext: [Timeout]'));
+
   common.expectsError(() => socket.destroy, errMsg);
   common.expectsError(() => socket.emit, errMsg);
   common.expectsError(() => socket.end, errMsg);