use indexedDB to cache assets: https://bugzilla.mozilla.org/show_bug.cgi?id=1371255#c11

platform/webext/vapi-cachestorage.js

@@ -0,0 +1,269 @@
+/*******************************************************************************
+
+    uMatrix - a browser extension to block requests.
+    Copyright (C) 2016-2017 The uBlock Origin authors
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see {http://www.gnu.org/licenses/}.
+
+    Home: https://github.com/gorhill/uBlock
+*/
+
+/* global indexedDB, IDBDatabase */
+
+'use strict';
+
+/******************************************************************************/
+
+// The code below has been originally manually imported from:
+// Commit: https://github.com/nikrolls/uBlock-Edge/commit/d1538ea9bea89d507219d3219592382eee306134
+// Commit date: 29 October 2016
+// Commit author: https://github.com/nikrolls
+// Commit message: "Implement cacheStorage using IndexedDB"
+
+// The original imported code has been subsequently modified as it was not
+// compatible with Firefox.
+// (a Promise thing, see https://github.com/dfahlander/Dexie.js/issues/317)
+// Furthermore, code to migrate from browser.storage.local to vAPI.cacheStorage
+// has been added, for seamless migration of cache-related entries into
+// indexedDB.
+
+// Imported from uBlock Origin project.
+
+vAPI.cacheStorage = (function() {
+    const STORAGE_NAME = 'uMatrixCacheStorage';
+    var db;
+    var pending = [];
+
+    // prime the db so that it's ready asap for next access.
+    getDb(noopfn);
+
+    return { get, set, remove, clear, getBytesInUse };
+
+    function get(input, callback) {
+        if ( typeof callback !== 'function' ) { return; }
+        if ( input === null ) {
+            return getAllFromDb(callback);
+        }
+        var toRead, output = {};
+        if ( typeof input === 'string' ) {
+            toRead = [ input ];
+        } else if ( Array.isArray(input) ) {
+            toRead = input;
+        } else /* if ( typeof input === 'object' ) */ {
+            toRead = Object.keys(input);
+            output = input;
+        }
+        return getFromDb(toRead, output, callback);
+    }
+
+    function set(input, callback) {
+        putToDb(input, callback);
+    }
+
+    function remove(key, callback) {
+        deleteFromDb(key, callback);
+    }
+
+    function clear(callback) {
+        clearDb(callback);
+    }
+
+    function getBytesInUse(keys, callback) {
+        // TODO: implement this
+        callback(0);
+    }
+
+    function genericErrorHandler(error) {
+        console.error('[uMatrix cacheStorage]', error);
+    }
+
+    function noopfn() {
+    }
+
+    function processPendings() {
+        var cb;
+        while ( (cb = pending.shift()) ) {
+            cb(db);
+        }
+    }
+
+    function getDb(callback) {
+        if ( pending === undefined ) {
+            return callback();
+        }
+        if ( pending.length !== 0 ) {
+            return pending.push(callback);
+        }
+        if ( db instanceof IDBDatabase ) {
+            return callback(db);
+        }
+        pending.push(callback);
+        if ( pending.length !== 1 ) { return; }
+        // This will fail in private browsing mode.
+        var req = indexedDB.open(STORAGE_NAME, 1);
+        req.onupgradeneeded = function(ev) {
+            db = ev.target.result;
+            db.onerror = genericErrorHandler;
+            var table = db.createObjectStore(STORAGE_NAME, { keyPath: 'key' });
+            table.createIndex('value', 'value', { unique: false });
+        };
+        req.onsuccess = function(ev) {
+            db = ev.target.result;
+            db.onerror = genericErrorHandler;
+            processPendings();
+        };
+        req.onerror = function() {
+            console.log(this.error);
+            processPendings();
+            pending = undefined;
+        };
+    }
+
+    function getFromDb(keys, store, callback) {
+        if ( typeof callback !== 'function' ) { return; }
+        if ( keys.length === 0 ) { return callback(store); }
+        var notfoundKeys = new Set(keys);
+        var gotOne = function() {
+            if ( typeof this.result === 'object' ) {
+                store[this.result.key] = this.result.value;
+                notfoundKeys.delete(this.result.key);
+            }
+        };
+        getDb(function(db) {
+            if ( !db ) { return callback(); }
+            var transaction = db.transaction(STORAGE_NAME);
+            transaction.oncomplete = transaction.onerror = function() {
+                if ( notfoundKeys.size === 0 ) {
+                    return callback(store);
+                }
+                maybeMigrate(Array.from(notfoundKeys), store, callback);
+            };
+            var table = transaction.objectStore(STORAGE_NAME);
+            for ( var key of keys ) {
+                var req = table.get(key);
+                req.onsuccess = gotOne;
+                req.onerror = noopfn;
+            }
+        });
+    }
+
+    // Migrate from storage API
+    // TODO: removes once all users are migrated to the new cacheStorage.
+    function maybeMigrate(keys, store, callback) {
+        var toMigrate = new Set(),
+            i = keys.length;
+        while ( i-- ) {
+            var key = keys[i];
+            toMigrate.add(key);
+            // If migrating a compiled list, also migrate the non-compiled
+            // counterpart.
+            if ( /^cache\/compiled\//.test(key) ) {
+                toMigrate.add(key.replace('/compiled', ''));
+            }
+        }
+        vAPI.storage.get(Array.from(toMigrate), function(bin) {
+            if ( bin instanceof Object === false ) {
+                return callback(store);
+            }
+            var migratedKeys = Object.keys(bin);
+            if ( migratedKeys.length === 0 ) {
+                return callback(store);
+            }
+            var i = migratedKeys.length;
+            while ( i-- ) {
+                var key = migratedKeys[i];
+                store[key] = bin[key];
+            }
+            vAPI.storage.remove(migratedKeys);
+            vAPI.cacheStorage.set(bin);
+            callback(store);
+        });
+    }
+
+    function getAllFromDb(callback) {
+        if ( typeof callback !== 'function' ) {
+            callback = noopfn;
+        }
+        getDb(function(db) {
+            if ( !db ) { return callback(); }
+            var output = {};
+            var transaction = db.transaction(STORAGE_NAME);
+            transaction.oncomplete = transaction.onerror = function() {
+                callback(output);
+            };
+            var table = transaction.objectStore(STORAGE_NAME),
+                req = table.openCursor();
+            req.onsuccess = function(ev) {
+                var cursor = ev.target.result;
+                if ( !cursor ) { return; }
+                output[cursor.key] = cursor.value;
+                cursor.continue();
+            };
+        });
+    }
+
+    function putToDb(input, callback) {
+        if ( typeof callback !== 'function' ) {
+            callback = noopfn;
+        }
+        var keys = Object.keys(input);
+        if ( keys.length === 0 ) { return callback(); }
+        getDb(function(db) {
+            if ( !db ) { return callback(); }
+            var transaction = db.transaction(STORAGE_NAME, 'readwrite');
+            transaction.oncomplete = transaction.onerror = callback;
+            var table = transaction.objectStore(STORAGE_NAME),
+                entry = {};
+            for ( var key of keys ) {
+                entry.key = key;
+                entry.value = input[key];
+                table.put(entry);
+            }
+        });
+    }
+
+    function deleteFromDb(input, callback) {
+        if ( typeof callback !== 'function' ) {
+            callback = noopfn;
+        }
+        var keys = Array.isArray(input) ? input.slice() : [ input ];
+        if ( keys.length === 0 ) { return callback(); }
+        getDb(function(db) {
+            if ( !db ) { return callback(); }
+            var transaction = db.transaction(STORAGE_NAME, 'readwrite');
+            transaction.oncomplete = transaction.onerror = callback;
+            var table = transaction.objectStore(STORAGE_NAME);
+            for ( var key of keys ) {
+                table.delete(key);
+            }
+        });
+        // TODO: removes once all users are migrated to the new cacheStorage.
+        vAPI.storage.remove(keys);
+    }
+
+    function clearDb(callback) {
+        if ( typeof callback !== 'function' ) {
+            callback = noopfn;
+        }
+        getDb(function(db) {
+            if ( !db ) { return callback(); }
+            var req = db.transaction(STORAGE_NAME, 'readwrite')
+                        .objectStore(STORAGE_NAME)
+                        .clear();
+            req.onsuccess = req.onerror = callback;
+        });
+    }
+}());
+
+/******************************************************************************/

src/background.html

@@ -10,6 +10,7 @@
 <script src="lib/publicsuffixlist.js"></script>
 <script src="js/vapi-common.js"></script>
 <script src="js/vapi-background.js"></script>
+<script src="js/vapi-cachestorage.js"></script><!-- Optional -->
 <script src="js/background.js"></script>
 <script src="js/xal.js"></script>
 <script src="js/usersettings.js"></script>

tools/make-webext-hybrid.sh

@@ -18,6 +18,7 @@ cp    LICENSE.txt                        $DES/webextension/
 
 cp    platform/webext/background.html      $DES/webextension/
 cp    platform/webext/polyfill.js          $DES/webextension/js/
+cp    platform/webext/vapi-cachestorage.js $DES/webextension/js/
 cp    platform/webext/from-legacy.js       $DES/webextension/js/
 cp    platform/webext/manifest.json        $DES/webextension/
 cp    platform/webext/bootstrap.js         $DES/

tools/make-webext.sh

@@ -19,6 +19,7 @@ cp    LICENSE.txt                        $DES/
 cp    platform/webext/options_ui.html      $DES/
 rm    $DES/js/options_ui.js
 cp    platform/webext/polyfill.js          $DES/js/
+cp    platform/webext/vapi-cachestorage.js $DES/js/
 cp    platform/webext/manifest.json        $DES/
 
 echo "*** uMatrix.webext: Generating meta..."