Add KeyPromiseQueue to Push and Job StatusHandlers (#7267)

* Add KeyPromiseQueue to Push and Job StatusHandlers

* Update CHANGELOG.md

* Update CHANGELOG.md

src/KeyPromiseQueue.js

@@ -0,0 +1,43 @@
+// KeyPromiseQueue is a simple promise queue
+// used to queue operations per key basis.
+// Once the tail promise in the key-queue fulfills,
+// the chain on that key will be cleared.
+export class KeyPromiseQueue {
+  constructor() {
+    this.queue = {};
+  }
+
+  enqueue(key, operation) {
+    const tuple = this.beforeOp(key);
+    const toAwait = tuple[1];
+    const nextOperation = toAwait.then(operation);
+    const wrappedOperation = nextOperation.then(result => {
+      this.afterOp(key);
+      return result;
+    });
+    tuple[1] = wrappedOperation;
+    return wrappedOperation;
+  }
+
+  beforeOp(key) {
+    let tuple = this.queue[key];
+    if (!tuple) {
+      tuple = [0, Promise.resolve()];
+      this.queue[key] = tuple;
+    }
+    tuple[0]++;
+    return tuple;
+  }
+
+  afterOp(key) {
+    const tuple = this.queue[key];
+    if (!tuple) {
+      return;
+    }
+    tuple[0]--;
+    if (tuple[0] <= 0) {
+      delete this.queue[key];
+      return;
+    }
+  }
+}