lib: expose FixedQueue internally and fix nextTick bug

A bug was introduced together with the FixedQueue implementation for
process.nextTick which meant that the queue wouldn't necessarily
fully clear on each run through. Fix it and abstract the data
structure into an internal module that can later be used elsewhere.

PR-URL: https://github.com/nodejs/node/pull/20468
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/test/parallel/test-next-tick-fixed-queue-regression.js b/test/parallel/test-next-tick-fixed-queue-regression.js
new file mode 100644
index 0000000000..1fe82d02b1
--- /dev/null
+++ b/test/parallel/test-next-tick-fixed-queue-regression.js
@@ -0,0 +1,18 @@
+'use strict';
+
+const common = require('../common');
+
+// This tests a highly specific regression tied to the FixedQueue size, which
+// was introduced in Node.js 9.7.0: https://github.com/nodejs/node/pull/18617
+// More specifically, a nextTick list could potentially end up not fully
+// clearing in one run through if exactly 2048 ticks were added after
+// microtasks were executed within the nextTick loop.
+
+process.nextTick(() => {
+  Promise.resolve(1).then(() => {
+    for (let i = 0; i < 2047; i++)
+      process.nextTick(common.mustCall());
+    const immediate = setImmediate(common.mustNotCall());
+    process.nextTick(common.mustCall(() => clearImmediate(immediate)));
+  });
+});