A FIFO semaphore must reserve a released permit for the oldest waiter. Merely waking the oldest caller is insufficient if a newcomer can consume the permit first. Keep an explicit FIFO queue of private wait records, and transfer the permit directly to the oldest record. Each record has its own monitor, so releasing a permit wakes exactly its chosen recipient.
class FifoBinarySemaphore(initiallyAvailable: Boolean) extends Semaphore {
private final class Waiter { var granted = false }
private val queue = scala.collection.mutable.Queue[Waiter]()
private var available = initiallyAvailable
def down: Unit = {
val mine: Option[Waiter] = synchronized {
if (available) {
assert(queue.isEmpty)
available = false
None
} else {
val w = new Waiter
queue.enqueue(w)
Some(w)
}
}
mine.foreach { w =>
var interrupted = false
w.synchronized {
while (!w.granted) {
try { w.wait() }
catch { case _: InterruptedException => interrupted = true }
}
}
if (interrupted) Thread.currentThread().interrupt()
}
}
def up: Unit = synchronized {
if (queue.nonEmpty) {
val w = queue.dequeue()
// available stays false: this permit is reserved for w.
w.synchronized {
w.granted = true
w.notify()
}
} else {
require(!available, "unmatched binary-semaphore release")
available = true
}
}
}The wait loop is deliberately uninterruptible with interruption status restored after acquisition: abandoning a queued wait record would otherwise strand its reserved permit. A caller that acquires must still complete the buffer operation.
granted is read and written under the wait record's own monitor. The persistent flag prevents a lost wake-up if up runs after registration but before the caller begins waiting. Spurious wakeups simply repeat the loop.The outer monitor serializes registration and the FIFO queue. A caller either consumes a genuinely free permit when nobody is queued, or appends one record.
up removes exactly the front record and leaves available false, even if its owner has not yet resumed. A newcomer must queue behind existing records instead of stealing the handoff. The outer-to-inner lock ordering in up is safe because waiting callers release the outer monitor before acquiring their private monitors; no caller holds a private monitor while trying to reacquire the outer one.private val empty: Semaphore = new FifoBinarySemaphore(true)
private val full: Semaphore = new FifoBinarySemaphore(false)This is a complete implementation when inserted in
SemaphoreMemory: its write and read methods remain exactly the displayed methods. Pending writers obtain empty-slot permissions in registration order on empty; pending readers obtain full-slot permissions in registration order on full. Each handoff permits just one caller to access the slot, soFirst-come-first-served means the order in which concurrent calls register under the monitor; two overlapping calls have no externally determined order. The guarantee is separate for the two kinds of caller. A global order combining reads and writes cannot be required: an initial read must wait while a later write supplies its value. Subject to eventual scheduling and the arrival of matching operations of the opposite kind, no registered caller is overtaken by a later caller of its own kind. The design avoids broadcast wakeups as well as barging. Articles by others on the same topic
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