system: add disk usage task

src/asynctask.js

@@ -0,0 +1,50 @@
+'use strict';
+
+const debug = require('debug')('box:asynctask'),
+    EventEmitter = require('events'),
+    safe = require('safetydance');
+
+// this runs in-process
+class AsyncTask extends EventEmitter {
+    #name;
+    #abortController;
+
+    constructor(name) {
+        super();
+        this.#name = name;
+        this.#abortController = new AbortController();
+    }
+
+    async _run(/*signal*/) {
+        // subclasses implement this
+    }
+
+    async start() {
+        debug(`start: ${this.#name} started`);
+        const [error] = await safe(this._run(this.#abortController.signal)); // background
+        debug(`start: ${this.#name} done`, error);
+        this.done(error);
+    }
+
+    stop() {
+        debug(`stop: ${this.#name} stopped`);
+        this.#abortController.abort();
+    }
+
+    done(error) {
+        debug(`done: ${this.#name} finished`);
+        this.emit('done', { errorMessage: error?.message || '' });
+    }
+
+    emitProgress(percent, message) {
+        this.emit('data', 'progress', { percent, message });
+    }
+
+    emitData(obj) {
+        this.emit('data', 'data', obj);
+    }
+}
+
+exports = module.exports = {
+    AsyncTask
+};