object SynchronizedFuture extends WartTraverser
When a synchronized block returns a scala.concurrent.Future, then the synchronization
typically does not extend to the computations performed inside the future. This often hides a
concurrency bug that is hard to spot during review because the computation's code is lexically
inside the synchronized block and it may not be obvious that scala.concurrent.Futures are in
play.
For example, synchronized blocks are often used to read and update the state of an object
atomically. When the update operation involves a scala.concurrent.Future like below, then
the write to the state is actually not guarded by the synchronized call and the update is not
atomic.
synchronized {
val x = this.atomicRef.get()
futureComputation(x).map { y =>
this.atomicRef.set(y)
y
}
}The proper approach in the above example is to use a semaphore instead of a synchronized
block.
blocking { this.semaphore.acquire() }
val x = this.atomicRef.get()
futureComputation(x).map { y =>
this.atomicRef.set(y)
y
}.thereafter { _ => this.semaphore.release() }If the synchronization intentionally does not have to extend over the scala.concurrent.Future computation, it usually helps with readability to move the future out of the synchronized block. For example,
blocking(synchronized {
val x = criticalSection()
Future { nonCriticalSection(x) }
})should be written as follows:
val x = blocking(synchronized { criticalSection() })
Future { nonCriticalSection(x) }There are cases where the synchronized block is supposed to return a
scala.concurrent.Future, for example when dealing with the promise of a future. In such a
case, the warning should simply be suppressed locally.
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