Readme

synchronized

Basic lock mechanism to prevent concurrent access to asynchronous code.

pub package

Goal

You were missing hard to debug deadlocks, here it is!

The goal is to propose a solution similar to critical sections and offer a simple synchronized API à la Java style. It provides a basic Lock/Mutex solution to allow features like transactions.

The name is biased as we are single threaded in Dart. However since we write asychronous code (await) like we would write synchronous code, it makes the overall API feel the same.

The goal is to ensure for a single process (single isolate) that some asynchronous operations can run without conflict. It won't solve cross-process (or cross-isolate) synchronization.

For single process (single isolate) accessing some resources (database..), it can help to

• Provide transaction on database system that don't have transaction mechanism (mongodb, file system)
• In html application make sure some asynchronous UI operation are not conflicting (login, transition)

Feature

• By default a lock is not reentrant
• Timeout support
• Support for reentrant lock (using Zone)
• Consistent behavior (i.e. if it is unlocked calling synchronized grab the lock)
• Values and Errors are properly reported to the caller
• Work on Browser, DartVM and Flutter
• No dependencies (other than the sdk itself)

It differs from the pool package used with a resource count of 1 by supporting a reentrant option

Usage

A simple usage example:

import 'package:synchronized/synchronized.dart';

Future<void> main() async {
  // Use this object to prevent concurrent access to data
  var lock = Lock();
  // ...
  await lock.synchronized(() async {
    // Only this block can run (once) until done
    // ...
  });
}

If you need a re-entrant lock you can use

var lock = Lock(reentrant: true);
// ...
await lock.synchronized(() async {
  // do some stuff
  // ...

  await lock.synchronized(() async {
    // other stuff
  });
});

A basic lock is not reentrant by default and does not use Zone. It behaves like an async executor with a pool capacity of 1

var lock = Lock();
// ...
lock.synchronized(() async {
  // do some stuff
  // ...
});

The return value is preserved

int value = await lock.synchronized(() {
  return 1;
});

Using the package:synchronized/extension.dart import, you can turn any object into a lock. synchronized() can then be called on any object

import 'package:synchronized/extension.dart';

class MyClass {

  /// Perform a long action that won't be called more than one at a time.
  Future<void> performAction() {
    // Lock at the instance level
    return synchronized(() async {
      // ...uninterrupted action
    });
  }
}

How it works

The next tasks is executed once the previous one is done

Re-entrant locks uses Zone to know in which context a block is running in order to be reentrant. It maintains a list of inner tasks to be awaited for.

Example

Consider the following dummy code

Future<void> writeSlow(int value) async {
  await Future<void>.delayed(const Duration(milliseconds: 1));
  stdout.write(value);
}

Future<void> write(List<int> values) async {
  for (var value in values) {
    await writeSlow(value);
  }
}

Future<void> write1234() async {
  await write([1, 2, 3, 4]);
}

Doing

write1234();
write1234();

would print

11223344

while doing

lock.synchronized(write1234);
lock.synchronized(write1234);

would print

12341234

The Lock instance

Have in mind that the Lock instance must be shared between calls in order to effectively prevent concurrent execution. For instance, in the example below the lock instance is the same between all myMethod() calls.

class MyClass {
  final _lock = Lock();

  Future<void> myMethod() async {
    await _lock.synchronized(() async {
      step1();
      step2();
      step3();
    });
  }
}

Typically you would create a global or static instance Lock to prevent concurrent access to a global resource or a class instance Lock to prevent concurrent modifications of class instance data and resources.

Timeout

synchronized() accepts an optional timeout:

try {
  await lock.synchronized(() async {
    // ...
  }, timeout: const Duration(seconds: 1));
} on TimeoutException catch (_) {
  // The lock could not be acquired in time.
}

The timeout bounds acquiring the lock, not running the computation. If it expires, the computation is never called and a TimeoutException is thrown. Once the computation has started it runs to completion: there is no cancellation, so a computation may well outlive the timeout that let it start.

For a MultiLock the timeout is the budget for acquiring the whole set, not for each member lock in turn.

Locking on any object

With package:synchronized/extension.dart, the lock associated with an object is found by equality (==/hashCode), not by identity, and the cache is global to the process. Two objects that compare equal therefore share one lock, which makes value types such as String and int a namespace shared with every other library in the application:

// These contend with each other, and with any other library
// that happens to lock on the same text.
await 'my-key'.synchronized(() async { /* ... */ });

When you want isolation, lock on a private instance instead:

class MyClass {
  final _lock = Object();

  Future<void> myMethod() => _lock.synchronized(() async {
        // ...
      });
}

locked/inLock/canLock status

locked

For basic and reentrant lock, locked returns whether the lock is currently locked.

For multi lock, it returns true if all inner locks locked values are true.

inLock

For reentrant locks, inLock returns whether the current zone is locked by the lock. i.e. it is true if the current block is running inside a synchronized block of the lock.

For basic lock, it matches the locked property and since it does not mean anything, it should not be used as behavior may change in the future.

For multi lock, it returns true if all inner locks inLock values are true.

canLock

canLock returns whether the lock can be locked immediately.

For basic lock, it is true if the lock is not locked.

For reentrant lock, it is true if the current zone lock level is free: outside of the lock, it can be false right after a block completed while other calls are still queued, even if locked is already false.

For multi lock, it returns true if all inner locks canLock values are true.

MultiLock

As of version 3.3.0, a MultiLock is available. It is a lock that can be used to synchronize multiple locks at once.

var lock1 = Lock();
var lock2 = Lock();
var multiLock = MultiLock(locks: [lock1, lock2]);

multiLock.synchronized(() async {
  // lock1 and lock2 are locked at this point
  ...
});

Features and bugs

Please feel free to:

• file feature requests and bugs at the issue tracker
• or contact me
• or visit tekartik.com
• How to guide