lib: document nextTick queue internals

Make this code (a bit more) comprehensible by adding some
internals docs.

With diagrams and everything! 🎉

PR-URL: #19469
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gus Caplan <me@gus.host>

lib/internal/process/next_tick.js

@@ -32,10 +32,60 @@ function setupNextTick() {
   const kHasScheduled = 0;
   const kHasPromiseRejections = 1;
 
-  // Queue size for each tick array. Must be a factor of two.
+  // 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;
@@ -50,11 +100,12 @@ function setupNextTick() {
     }
 
     shift() {
-      const next = this.list[this.bottom];
-      if (next === undefined) return null;
+      const nextItem = this.list[this.bottom];
+      if (nextItem === undefined)
+        return null;
       this.list[this.bottom] = undefined;
       this.bottom = (this.bottom + 1) & kQueueMask;
-      return next;
+      return nextItem;
     }
   }
 
@@ -63,21 +114,34 @@ function setupNextTick() {
 
   function push(data) {
     if (head.bottom === head.top) {
-      if (head.list[head.top] !== undefined)
+      // 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
+      } 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) {
-      if (tail.next)
+    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
+      } 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;
   }