1 files changed, 108 insertions, 0 deletions
diff --git a/browser/components/newtab/lib/PersistentCache.jsm b/browser/components/newtab/lib/PersistentCache.jsm
new file mode 100644
index 0000000000..e8024e27bc
--- /dev/null
+++ b/browser/components/newtab/lib/PersistentCache.jsm
@@ -0,0 +1,108 @@
+/* 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/. */
+"use strict";
+
+const { XPCOMUtils } = ChromeUtils.import(
+  "resource://gre/modules/XPCOMUtils.jsm"
+);
+
+ChromeUtils.defineModuleGetter(this, "OS", "resource://gre/modules/osfile.jsm");
+XPCOMUtils.defineLazyGlobalGetters(this, ["fetch"]);
+
+/**
+ * A file (disk) based persistent cache of a JSON serializable object.
+ */
+this.PersistentCache = 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.${name}.json`;
+    if (preload) {
+      this._load();
+    }
+  }
+
+  /**
+   * Set a value to be cached with the specified key.
+   *
+   * @param {string} key The cache key.
+   * @param {object} value The data to be cached.
+   */
+  async set(key, value) {
+    const data = await this._load();
+    data[key] = value;
+    await this._persist(data);
+  }
+
+  /**
+   * Get a value from the cache.
+   *
+   * @param {string} key (optional) The cache key. If not provided, we return the full cache.
+   * @returns {object} The cached data.
+   */
+  async get(key) {
+    const data = await this._load();
+    return key ? data[key] : data;
+  }
+
+  /**
+   * Load the cache into memory if it isn't already.
+   */
+  _load() {
+    return (
+      this._cache ||
+      // eslint-disable-next-line no-async-promise-executor
+      (this._cache = new Promise(async (resolve, reject) => {
+        let filepath;
+        try {
+          filepath = OS.Path.join(
+            OS.Constants.Path.localProfileDir,
+            this._filename
+          );
+        } catch (error) {
+          reject(error);
+          return;
+        }
+
+        let file;
+        try {
+          file = await fetch(`file://${filepath}`);
+        } catch (error) {} // Cache file doesn't exist yet.
+
+        let data = {};
+        if (file) {
+          try {
+            data = await file.json();
+          } catch (error) {
+            Cu.reportError(
+              `Failed to parse ${this._filename}: ${error.message}`
+            );
+          }
+        }
+
+        resolve(data);
+      }))
+    );
+  }
+
+  /**
+   * Persist the cache to file.
+   */
+  _persist(data) {
+    const filepath = OS.Path.join(
+      OS.Constants.Path.localProfileDir,
+      this._filename
+    );
+    return OS.File.writeAtomic(filepath, JSON.stringify(data), {
+      tmpPath: `${filepath}.tmp`,
+    });
+  }
+};
+
+const EXPORTED_SYMBOLS = ["PersistentCache"];