Implement maxQueuedJobs pool option (#188)

docs/usage-pool.md

@@ -36,6 +36,7 @@ Note that `pool.queue()` will schedule a task to be run in a deferred way. It mi
 ```ts
 interface PoolOptions {
   concurrency?: number
+  maxQueuedJobs?: number
   name?: string
   size?: number
 }
@@ -49,6 +50,7 @@ The first argument passed to the `Pool()` factory must be a function that spawns
 The second argument is optional and can either be the number of workers to spawn as a `number` or an options object (see `PoolOptions`):
 
 - `options.concurrency`: number of tasks to run simultaneously per worker, defaults to one
+- `options.maxQueuedJobs`: maximum number of tasks to queue before throwing on `.queue()`, defaults to unlimited
 - `options.name`: give the pool a custom name to use in the debug log, so you can tell multiple pools apart when debugging
 - `options.size`: number of workers to spawn, defaults to the number of CPU cores
 

src/master/pool.ts

@@ -91,6 +91,9 @@ export interface PoolOptions {
   /** Maximum no. of tasks to run on one worker thread at a time. Defaults to one. */
   concurrency?: number
 
+  /** Maximum no. of jobs to be queued for execution before throwing an error. */
+  maxQueuedJobs?: number
+
   /** Gives that pool a name to be used for debug logging, letting you distinguish between log output of different pools. */
   name?: string
 
@@ -267,6 +270,8 @@ class WorkerPool<ThreadType extends Thread> implements Pool<ThreadType> {
   }
 
   public queue(taskFunction: TaskRunFunction<ThreadType, any>) {
+    const { maxQueuedJobs = Infinity } = this.options
+
     if (this.isClosing) {
       throw Error(`Cannot schedule pool tasks after terminate() has been called.`)
     }
@@ -297,6 +302,14 @@ class WorkerPool<ThreadType extends Thread> implements Pool<ThreadType> {
       }
     }
 
+    if (this.taskQueue.length >= maxQueuedJobs) {
+      throw Error(
+        "Maximum number of pool tasks queued. Refusing to queue another one.\n" +
+        "This usually happens for one of two reasons: We are either at peak " +
+        "workload right now or some tasks just won't finish, thus blocking the pool."
+      )
+    }
+
     this.debug(`Queueing task #${task.id}...`)
     this.taskQueue.push(task)