1 files changed, 90 insertions, 0 deletions
diff --git a/browser/components/newtab/lib/PersistentCache.sys.mjs b/browser/components/newtab/lib/PersistentCache.sys.mjs
new file mode 100644
index 0000000000..1db9ca102e
--- /dev/null
+++ b/browser/components/newtab/lib/PersistentCache.sys.mjs
@@ -0,0 +1,90 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * A file (disk) based persistent cache of a JSON serializable object.
+ */
+export class PersistentCache {
+  /**
+   * Create a cache object based on a name.
+   *
+   * @param {string} name Name of the cache. It will be used to create the filename.
+   * @param {boolean} preload (optional). Whether the cache should be preloaded from file. Defaults to false.
+   */
+  constructor(name, preload = false) {
+    this.name = name;
+    this._filename = `activity-stream.${name}.json`;
+    if (preload) {
+      this._load();
+    }
+  }
+
+  /**
+   * Set a value to be cached with the specified key.
+   *
+   * @param {string} key The cache key.
+   * @param {object} value The data to be cached.
+   */
+  async set(key, value) {
+    const data = await this._load();
+    data[key] = value;
+    await this._persist(data);
+  }
+
+  /**
+   * Get a value from the cache.
+   *
+   * @param {string} key (optional) The cache key. If not provided, we return the full cache.
+   * @returns {object} The cached data.
+   */
+  async get(key) {
+    const data = await this._load();
+    return key ? data[key] : data;
+  }
+
+  /**
+   * Load the cache into memory if it isn't already.
+   */
+  _load() {
+    return (
+      this._cache ||
+      // eslint-disable-next-line no-async-promise-executor
+      (this._cache = new Promise(async (resolve, reject) => {
+        let filepath;
+        try {
+          filepath = PathUtils.join(PathUtils.localProfileDir, this._filename);
+        } catch (error) {
+          reject(error);
+          return;
+        }
+
+        let data = {};
+        try {
+          data = await IOUtils.readJSON(filepath);
+        } catch (error) {
+          if (
+            // isInstance() is not available in node unit test. It should be safe to use instanceof as it's directly from IOUtils.
+            // eslint-disable-next-line mozilla/use-isInstance
+            !(error instanceof DOMException) ||
+            error.name !== "NotFoundError"
+          ) {
+            console.error(`Failed to parse ${this._filename}:`, error.message);
+          }
+        }
+
+        resolve(data);
+      }))
+    );
+  }
+
+  /**
+   * Persist the cache to file.
+   */
+  async _persist(data) {
+    const filepath = PathUtils.join(PathUtils.localProfileDir, this._filename);
+    await IOUtils.writeJSON(filepath, data, {
+      tmpPath: `${filepath}.tmp`,
+    });
+  }
+}