assert: fix infinite loop

In rare cirumstances it is possible to get a identical error diff.
In such a case the advances diffing runs into a infinite loop.
This fixes it by properly checking for extra entries.

Backport-PR-URL: #19230
PR-URL: #18611
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/internal/errors.js

@@ -110,6 +110,8 @@ function createErrDiff(actual, expected, operator) {
     }
     actualLines.pop();
     expectedLines.pop();
+    if (actualLines.length === 0 || expectedLines.length === 0)
+      break;
     a = actualLines[actualLines.length - 1];
     b = expectedLines[expectedLines.length - 1];
   }

test/parallel/test-assert.js

@@ -25,6 +25,7 @@
 
 const common = require('../common');
 const assert = require('assert');
+const { inspect } = require('util');
 const a = assert;
 
 function makeBlock(f) {
@@ -890,6 +891,17 @@ common.expectsError(
     () => assert.deepEqual(Array(12).fill(1), Array(12).fill(2)),
     { message });
 
+  const obj1 = {};
+  const obj2 = { loop: 'forever' };
+  obj2[inspect.custom] = () => '{}';
+  // No infinite loop and no custom inspect.
+  assert.throws(() => assert.deepEqual(obj1, obj2), {
+    message: `${start}\n` +
+    `${actExp}\n` +
+    '\n' +
+    '  {}'
+  });
+
   // notDeepEqual tests
   message = 'Identical input passed to notDeepStrictEqual:\n[\n  1\n]';
   assert.throws(