veilidchat/packages/mutex

mutex

A library for creating locks to ensure mutual exclusion when running critical sections of code.

Purpose

Mutexes can be used to protect critical sections of code to prevent race conditions.

Although Dart uses a single thread of execution, race conditions can still occur when asynchronous operations are used inside critical sections. For example,

x = 42;
synchronousOperations(); // this does not modify x
assert(x == 42); // x will NOT have changed

y = 42; // a variable that other asynchronous code can modify
await asynchronousOperations(); // this does NOT modify y, but...
// There is NO GUARANTEE other async code didn't run and change it!
assert(y == 42 || y != 42); // WARNING: y might have changed

An example is when Dart is used to implement a server-side Web server that updates a database (assuming database transactions are not being used). The update involves querying the database, performing calculations on those retrieved values, and then updating the database with the result. You don't want the database to be changed by "something else" while performing the calculations, since the results you would write will not incorporate those other changes. That "something else" could be the same Web server handling another request in parallel.

This package provides a normal mutex and a read-write mutex.

Mutex

A mutex guarantees at most only one lock can exist at any one time.

If the lock has already been acquired, attempts to acquire another lock will be blocked until the lock has been released.

import 'package:mutex/mutex.dart';

...

final m = Mutex();

Acquiring the lock before running the critical section of code, and then releasing the lock.

await m.acquire();
// No other lock can be acquired until the lock is released

try {
  // critical section with asynchronous code
  await ...
} finally {
  m.release();
}

protect

The following code uses the protect convenience method to do the same thing as the above code. Use the convenence method whenever possible, since it ensures the lock will always be released.

await m.protect(() async {
  // critical section
});

If the critial section returns a Future to a value, the protect convenience method will return a Future to that value.

final result = await m.protect<int>(() async {
  // critical section
  return valueFromCriticalSection;
});
// result contains the valueFromCriticalSection

Read-write mutex

A read-write mutex allows multiple reads locks to be exist simultaneously, but at most only one write lock can exist at any one time. A write lock and any read locks cannot both exist together at the same time.

If there is one or more read locks, attempts to acquire a write lock will be blocked until all the read locks have been released. But attempts to acquire more read locks will not be blocked. If there is a write lock, attempts to acquire any lock (read or write) will be blocked until that write lock is released.

A read-write mutex can also be described as a single-writer mutex, multiple-reader mutex, or a reentrant lock.

import 'package:mutex/mutex.dart';

...

final m = ReadWriteMutex();

Acquiring a write lock:

await m.acquireWrite();
// No other locks (read or write) can be acquired until released

try {
  // critical write section with asynchronous code
  await ...
} finally {
  m.release();
}

Acquiring a read lock:

await m.acquireRead();
// No write lock can be acquired until all read locks are released,
// but additional read locks can be acquired.

try {
  // critical read section with asynchronous code
  await ...
} finally {
  m.release();
}

protectWrite and protectRead

The following code uses the protectWrite and protectRead convenience methods to do the same thing as the above code. Use the convenence method whenever possible, since it ensures the lock will always be released.

await m.protectWrite(() async {
  // critical write section
});

await m.protectRead(() async {
  // critical read section
});

If the critial section returns a Future to a value, these convenience methods will return a Future to that value.

final result1 await m.protectWrite<String>(() async {
  // critical write section
  return valueFromCritialSection1;
});
// result1 contains the valueFromCriticalSection1

final result2 = await m.protectRead(() async {
  // critical read section
  return valueFromCritialSection2;
});
// result2 contains the valueFromCriticalSection2

When mutual exclusion is not needed

The critical section should always contain some asynchronous code. If the critical section only contains synchronous code, there is no need to put it in a critical section. In Dart, synchronous code cannot be interrupted, so there is no need to protect it using mutual exclusion.

Also, if the critical section does not involve data or shared resources that can be accessed by other asynchronous code, it also does not need to be protected. For example, if it only uses local variables that other asynchronous code won't have access to: while the other asynchronous code could run, it won't be able to make unexpected changes to the local variables it can't access.

Features and bugs

Please file feature requests and bugs at the issue tracker.