add: scheduler config option for cluster

scheduler let's use a custom scheduler function for scheduling workers.

lib/internal/cluster/primary.js

@@ -42,6 +42,7 @@ cluster.SCHED_RR = SCHED_RR;      // Primary distributes connections.
 let ids = 0;
 let debugPortOffset = 1;
 let initialized = false;
+let scheduler = RoundRobinHandle.scheduler;
 
 // XXX(bnoordhuis) Fold cluster.schedulingPolicy into cluster.settings?
 let schedulingPolicy = process.env.NODE_CLUSTER_SCHED_POLICY;
@@ -58,6 +59,19 @@ else if (process.platform === 'win32') {
 
 cluster.schedulingPolicy = schedulingPolicy;
 
+function validateAndReturnScheduler(scheduler, schedulingPolicy) {
+  if (scheduler !== undefined) {
+    if (typeof scheduler !== 'function') {
+      throw new TypeError('scheduler must be a function');
+    }
+    return scheduler;
+  } else if (schedulingPolicy === SCHED_RR) {
+    return RoundRobinHandle.scheduler;
+  } else if (schedulingPolicy !== SCHED_NONE) {
+    assert(false, `Bad cluster.schedulingPolicy: ${schedulingPolicy}`);
+  }
+}
+
 cluster.setupPrimary = function(options) {
   const settings = {
     args: ArrayPrototypeSlice(process.argv, 2),
@@ -89,6 +103,7 @@ cluster.setupPrimary = function(options) {
   assert(schedulingPolicy === SCHED_NONE || schedulingPolicy === SCHED_RR,
          `Bad cluster.schedulingPolicy: ${schedulingPolicy}`);
 
+  scheduler = validateAndReturnScheduler(cluster.settings.scheduler, schedulingPolicy);
   process.nextTick(setupSettingsNT, settings);
 
   process.on('internalMessage', (message) => {
@@ -310,6 +325,8 @@ function queryServer(worker, message) {
         message.addressType === 'udp6') {
       handle = new SharedHandle(key, address, message);
     } else {
+      // FIXME: Better structure to code
+      message.scheduler = scheduler;
       handle = new RoundRobinHandle(key, address, message);
     }
 

lib/internal/cluster/round_robin_handle.js

@@ -15,12 +15,13 @@ const { constants } = internalBinding('tcp_wrap');
 
 module.exports = RoundRobinHandle;
 
-function RoundRobinHandle(key, address, { port, fd, flags, backlog }) {
+function RoundRobinHandle(key, address, { port, fd, flags, backlog, scheduler }) {
   this.key = key;
   this.all = new SafeMap();
-  this.free = new SafeMap();
+  this.workers = new SafeMap();
   this.handles = init(ObjectCreate(null));
   this.handle = null;
+  this.scheduler = scheduler;
   this.server = net.createServer(assert.fail);
 
   if (fd >= 0)
@@ -47,6 +48,7 @@ function RoundRobinHandle(key, address, { port, fd, flags, backlog }) {
 RoundRobinHandle.prototype.add = function(worker, send) {
   assert(this.all.has(worker.id) === false);
   this.all.set(worker.id, worker);
+  this.workers.set(worker.id, worker);
 
   const done = () => {
     if (this.handle.getsockname) {
@@ -58,7 +60,7 @@ RoundRobinHandle.prototype.add = function(worker, send) {
       send(null, null, null);  // UNIX socket.
     }
 
-    this.handoff(worker);  // In case there are connections pending.
+    this.handoff();  // In case there are connections pending.
   };
 
   if (this.server === null)
@@ -77,7 +79,7 @@ RoundRobinHandle.prototype.remove = function(worker) {
   if (!existed)
     return false;
 
-  this.free.delete(worker.id);
+  this.workers.delete(worker.id);
 
   if (this.all.size !== 0)
     return false;
@@ -95,28 +97,32 @@ RoundRobinHandle.prototype.remove = function(worker) {
 
 RoundRobinHandle.prototype.distribute = function(err, handle) {
   append(this.handles, handle);
-  // eslint-disable-next-line node-core/no-array-destructuring
-  const [ workerEntry ] = this.free; // this.free is a SafeMap
+  this.handoff();
+};
+
+RoundRobinHandle.scheduler = function(workers) {
+  const [ workerEntry ] = workers;
 
   if (ArrayIsArray(workerEntry)) {
     const { 0: workerId, 1: worker } = workerEntry;
-    this.free.delete(workerId);
-    this.handoff(worker);
+    workers.delete(workerId);
+    workers.set(workerId, worker)
+    return worker;
   }
 };
 
-RoundRobinHandle.prototype.handoff = function(worker) {
-  if (!this.all.has(worker.id)) {
-    return;  // Worker is closing (or has closed) the server.
-  }
-
+RoundRobinHandle.prototype.handoff = function () {
   const handle = peek(this.handles);
 
   if (handle === null) {
-    this.free.set(worker.id, worker);  // Add to ready queue again.
     return;
   }
 
+  const worker = this.scheduler(this.workers);
+  if (typeof worker === 'undefined' || !this.all.has(worker.id)) {
+    return;  // Worker is closing (or has closed) the server.
+  }
+
   remove(handle);
 
   const message = { act: 'newconn', key: this.key };
@@ -127,6 +133,6 @@ RoundRobinHandle.prototype.handoff = function(worker) {
     else
       this.distribute(0, handle);  // Worker is shutting down. Send to another.
 
-    this.handoff(worker);
+    this.handoff();
   });
 };