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Merge pull request #98 from powersync-ja/release-tab-lock-on-close
Shared worker: Release mutex when tab closes
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‎packages/sqlite_async/build.yaml

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targets:
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$default:
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builders:
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build_web_compilers:entrypoint:
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options:
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# Workers can't be compiled with dartdevc, so use dart2js for the example
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compiler: dart2js

‎packages/sqlite_async/example/web/index.html

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<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<title>sqlite_async web demo</title>
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<script defer src="main.dart.js"></script>
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</head>
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<body>
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<h1>sqlite_async demo</h1>
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<main>
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This page is used to test the sqlite_async package on the web.
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Use the console to open and interact with databases.
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<pre>
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<code>
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const db = await open('test.db');
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const lock = await write_lock(db);
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release_lock(lock);
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</code>
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</pre>
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</main>
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</body>
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</html>

‎packages/sqlite_async/example/web/main.dart

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import 'dart:async';
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import 'dart:js_interop';
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import 'dart:js_interop_unsafe';
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import 'package:sqlite_async/sqlite_async.dart';
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void main() {
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globalContext['open'] = (String path) {
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return Future(() async {
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final db = SqliteDatabase(
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path: path,
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options: SqliteOptions(
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webSqliteOptions: WebSqliteOptions(
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wasmUri:
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'https://cdn.jsdelivr.net/npm/@powersync/dart-wasm-bundles@latest/dist/sqlite3.wasm',
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workerUri: 'worker.dart.js',
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),
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),
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);
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await db.initialize();
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return db.toJSBox;
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}).toJS;
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}.toJS;
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globalContext['write_lock'] = (JSBoxedDartObject db) {
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final hasLock = Completer<void>();
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final completer = Completer<void>();
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(db.toDart as SqliteDatabase).writeLock((_) async {
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print('has write lock!');
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hasLock.complete();
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await completer.future;
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});
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return hasLock.future.then((_) => completer.toJSBox).toJS;
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}.toJS;
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globalContext['release_lock'] = (JSBoxedDartObject db) {
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(db.toDart as Completer<void>).complete();
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}.toJS;
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}

‎packages/sqlite_async/example/web/worker.dart

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import 'package:sqlite_async/sqlite3_web.dart';
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import 'package:sqlite_async/sqlite3_web_worker.dart';
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void main() {
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WebSqlite.workerEntrypoint(controller: AsyncSqliteController());
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}

‎packages/sqlite_async/lib/src/web/worker/worker_utils.dart

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// these requests for shared workers, so we can assume each database is only
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// opened once and we don't need web locks here.
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final mutex = ReadWriteMutex();
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final Map<ClientConnection, _ConnectionState> _state = {};
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AsyncSqliteDatabase({required this.database});
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_ConnectionState _findState(ClientConnection connection) {
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return _state.putIfAbsent(connection, _ConnectionState.new);
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}
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void _markHoldsMutex(ClientConnection connection) {
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final state = _findState(connection);
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state.holdsMutex = true;
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if (!state.hasOnCloseListener) {
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state.hasOnCloseListener = true;
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connection.closed.then((_) {
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if (state.holdsMutex) {
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mutex.release();
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}
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});
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}
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}
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@override
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Future<JSAny?> handleCustomRequest(
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ClientConnection connection, JSAny? request) async {
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switch (message.kind) {
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case CustomDatabaseMessageKind.requestSharedLock:
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await mutex.acquireRead();
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_markHoldsMutex(connection);
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case CustomDatabaseMessageKind.requestExclusiveLock:
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await mutex.acquireWrite();
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_markHoldsMutex(connection);
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case CustomDatabaseMessageKind.releaseLock:
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_findState(connection).holdsMutex = false;
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mutex.release();
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case CustomDatabaseMessageKind.lockObtained:
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throw UnsupportedError('This is a response, not a request');
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return resultSetMap;
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}
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}
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final class _ConnectionState {
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bool hasOnCloseListener = false;
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bool holdsMutex = false;
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}

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