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lib: expose FixedQueue internally and fix nextTick bug #20468

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lib: expose FixedQueue internally and fix nextTick bug #20468

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lib: expose FixedQueue internally and fix nextTick bug
apapirovski May 2, 2018
a6a26a1
fixup: improve perf slightly and add tests
apapirovski May 3, 2018
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113 changes: 113 additions & 0 deletions lib/internal/fixed_queue.js
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Original file line number Diff line number Diff line change
@@ -0,0 +1,113 @@
'use strict';

// Currently optimal queue size, tested on V8 6.0 - 6.6. Must be power of two.
const kSize = 2048;
const kMask = kSize - 1;

// The FixedQueue is implemented as a singly-linked list of fixed-size
// circular buffers. It looks something like this:
//
// head tail
// | |
// v v
// +-----------+ <-----\ +-----------+ <------\ +-----------+
// | [null] | \----- | next | \------- | next |
// +-----------+ +-----------+ +-----------+
// | item | <-- bottom | item | <-- bottom | [empty] |
// | item | | item | | [empty] |
// | item | | item | | [empty] |
// | item | | item | | [empty] |
// | item | | item | bottom --> | item |
// | item | | item | | item |
// | ... | | ... | | ... |
// | item | | item | | item |
// | item | | item | | item |
// | [empty] | <-- top | item | | item |
// | [empty] | | item | | item |
// | [empty] | | [empty] | <-- top top --> | [empty] |
// +-----------+ +-----------+ +-----------+
//
// Or, if there is only one circular buffer, it looks something
// like either of these:
//
// head tail head tail
// | | | |
// v v v v
// +-----------+ +-----------+
// | [null] | | [null] |
// +-----------+ +-----------+
// | [empty] | | item |
// | [empty] | | item |
// | item | <-- bottom top --> | [empty] |
// | item | | [empty] |
// | [empty] | <-- top bottom --> | item |
// | [empty] | | item |
// +-----------+ +-----------+
//
// Adding a value means moving `top` forward by one, removing means
// moving `bottom` forward by one. After reaching the end, the queue
// wraps around.
//
// When `top === bottom` the current queue is empty and when
// `top + 1 === bottom` it's full. This wastes a single space of storage
// but allows much quicker checks.

const FixedCircularBuffer = class FixedCircularBuffer {
constructor() {
this.bottom = 0;
this.top = 0;
this.list = new Array(kSize);
this.next = null;
}

isEmpty() {
return this.top === this.bottom;
}

isFull() {
return ((this.top + 1) & kMask) === this.bottom;
}

push(data) {
this.list[this.top] = data;
this.top = (this.top + 1) & kMask;
}

shift() {
const nextItem = this.list[this.bottom];
if (nextItem === undefined)
return null;
this.list[this.bottom] = undefined;
this.bottom = (this.bottom + 1) & kMask;
return nextItem;
}
};

module.exports = class FixedQueue {
constructor() {
this.head = this.tail = new FixedCircularBuffer();
}

isEmpty() {
return this.head.isEmpty();
}

push(data) {
if (this.head.isFull()) {
// Head is full: Creates a new queue, sets the old queue's `.next` to it,
// and sets it as the new main queue.
this.head = this.head.next = new FixedCircularBuffer();
}
this.head.push(data);
}

shift() {
const { tail } = this;
const next = tail.shift();
if (tail.isEmpty() && tail.next !== null) {
// If there is another queue, it forms the new tail.
this.tail = tail.next;
}
return next;
}
};
124 changes: 8 additions & 116 deletions lib/internal/process/next_tick.js
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@ function setupNextTick() {
} = require('internal/async_hooks');
const promises = require('internal/process/promises');
const { ERR_INVALID_CALLBACK } = require('internal/errors').codes;
const FixedQueue = require('internal/fixed_queue');
const { emitPromiseRejectionWarnings } = promises;

// tickInfo is used so that the C++ code in src/node.cc can
Expand All @@ -31,119 +32,7 @@ function setupNextTick() {
const kHasScheduled = 0;
const kHasPromiseRejections = 1;

// Queue size for each tick array. Must be a power of two.
const kQueueSize = 2048;
const kQueueMask = kQueueSize - 1;

// The next tick queue is implemented as a singly-linked list of fixed-size
// circular buffers. It looks something like this:
//
// head tail
// | |
// v v
// +-----------+ <-----\ +-----------+ <------\ +-----------+
// | [null] | \----- | next | \------- | next |
// +-----------+ +-----------+ +-----------+
// | tick | <-- bottom | tick | <-- bottom | [empty] |
// | tick | | tick | | [empty] |
// | tick | | tick | | [empty] |
// | tick | | tick | | [empty] |
// | tick | | tick | bottom --> | tick |
// | tick | | tick | | tick |
// | ... | | ... | | ... |
// | tick | | tick | | tick |
// | tick | | tick | | tick |
// | [empty] | <-- top | tick | | tick |
// | [empty] | | tick | | tick |
// | [empty] | | tick | | tick |
// +-----------+ +-----------+ <-- top top --> +-----------+
//
// Or, if there is only one fixed-size queue, it looks something
// like either of these:
//
// head tail head tail
// | | | |
// v v v v
// +-----------+ +-----------+
// | [null] | | [null] |
// +-----------+ +-----------+
// | [empty] | | tick |
// | [empty] | | tick |
// | tick | <-- bottom top --> | [empty] |
// | tick | | [empty] |
// | [empty] | <-- top bottom --> | tick |
// | [empty] | | tick |
// +-----------+ +-----------+
//
// Adding a value means moving `top` forward by one, removing means
// moving `bottom` forward by one.
//
// We let `bottom` and `top` wrap around, so when `top` is conceptually
// pointing to the end of the list, that means that the actual value is `0`.
//
// In particular, when `top === bottom`, this can mean *either* that the
// current queue is empty or that it is full. We can differentiate by
// checking whether an entry in the queue is empty (a.k.a. `=== undefined`).

class FixedQueue {
constructor() {
this.bottom = 0;
this.top = 0;
this.list = new Array(kQueueSize);
this.next = null;
}

push(data) {
this.list[this.top] = data;
this.top = (this.top + 1) & kQueueMask;
}

shift() {
const nextItem = this.list[this.bottom];
if (nextItem === undefined)
return null;
this.list[this.bottom] = undefined;
this.bottom = (this.bottom + 1) & kQueueMask;
return nextItem;
}
}

var head = new FixedQueue();
var tail = head;

function push(data) {
if (head.bottom === head.top) {
// Either empty or full:
if (head.list[head.top] !== undefined) {
// It's full: Creates a new queue, sets the old queue's `.next` to it,
// and sets it as the new main queue.
head = head.next = new FixedQueue();
} else {
// If the head is empty, that means that it was the only fixed-sized
// queue in existence.
DCHECK_EQ(head.next, null);
// This is the first tick object in existence, so we need to inform
// the C++ side that we do want to run `_tickCallback()`.
tickInfo[kHasScheduled] = 1;
}
}
head.push(data);
}

function shift() {
const next = tail.shift();
if (tail.top === tail.bottom) { // -> .shift() emptied the current queue.
if (tail.next !== null) {
// If there is another queue, it forms the new tail.
tail = tail.next;
} else {
// We've just run out of items. Let the native side know that it
// doesn't need to bother calling into JS to run the queue.
tickInfo[kHasScheduled] = 0;
}
}
return next;
}
const queue = new FixedQueue();

process.nextTick = nextTick;
// Needs to be accessible from beyond this scope.
Expand All @@ -152,7 +41,7 @@ function setupNextTick() {
function _tickCallback() {
let tock;
do {
while (tock = shift()) {
while (tock = queue.shift()) {
const asyncId = tock[async_id_symbol];
emitBefore(asyncId, tock[trigger_async_id_symbol]);
// emitDestroy() places the async_id_symbol into an asynchronous queue
Expand All @@ -175,8 +64,9 @@ function setupNextTick() {

emitAfter(asyncId);
}
tickInfo[kHasScheduled] = 0;
runMicrotasks();
} while (head.top !== head.bottom || emitPromiseRejectionWarnings());
} while (!queue.isEmpty() || emitPromiseRejectionWarnings());
tickInfo[kHasPromiseRejections] = 0;
}

Expand Down Expand Up @@ -222,6 +112,8 @@ function setupNextTick() {
args[i - 1] = arguments[i];
}

push(new TickObject(callback, args, getDefaultTriggerAsyncId()));
if (queue.isEmpty())
tickInfo[kHasScheduled] = 1;
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Nice find!

queue.push(new TickObject(callback, args, getDefaultTriggerAsyncId()));
}
}
1 change: 1 addition & 0 deletions node.gyp
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Expand Up @@ -101,6 +101,7 @@
'lib/internal/constants.js',
'lib/internal/encoding.js',
'lib/internal/errors.js',
'lib/internal/fixed_queue.js',
'lib/internal/freelist.js',
'lib/internal/fs.js',
'lib/internal/http.js',
Expand Down
34 changes: 34 additions & 0 deletions test/parallel/test-fixed-queue.js
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// Flags: --expose-internals
'use strict';

require('../common');

const assert = require('assert');
const FixedQueue = require('internal/fixed_queue');

{
const queue = new FixedQueue();
assert.strictEqual(queue.head, queue.tail);
assert(queue.isEmpty());
queue.push('a');
assert(!queue.isEmpty());
assert.strictEqual(queue.shift(), 'a');
assert.strictEqual(queue.shift(), null);
}

{
const queue = new FixedQueue();
for (let i = 0; i < 2047; i++)
queue.push('a');
assert(queue.head.isFull());
queue.push('a');
assert(!queue.head.isFull());

assert.notStrictEqual(queue.head, queue.tail);
for (let i = 0; i < 2047; i++)
assert.strictEqual(queue.shift(), 'a');
assert.strictEqual(queue.head, queue.tail);
assert(!queue.isEmpty());
assert.strictEqual(queue.shift(), 'a');
assert(queue.isEmpty());
}
18 changes: 18 additions & 0 deletions test/parallel/test-next-tick-fixed-queue-regression.js
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@@ -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)));
});
});