Tracking Fibers

FiberSet and FiberMap track groups of running fibers and clean them up with an owning scope. Use a set for an unkeyed group, or a map when each running operation has an identity such as a client ID.

Managing groups of running fibers

function runGroup() {
  using workers = FiberSet.make<number>()
  const first = FiberSet.run(workers) {
    sleep("30 millis")
    return 1
  }
  const second = FiberSet.run(workers) {
    sleep("50 millis")
    return 2
  }
  logInfo("tracked workers", FiberSet.size(workers))
  return [first.join(), second.join()]
}

FiberSet.run forks a program and registers the resulting fiber. FiberSet.add can register an existing fiber, while size reports how many are currently tracked. Joining individual handles collects their results; tracking itself does not produce an ordered result list.

Tracking fibers by key

function replacePreview() {
  using previews = FiberMap.make<string, string>()
  FiberMap.run(previews, "editor-1") {
    sleep("1 second")
    return "old preview"
  }
  const current = FiberMap.run(previews, "editor-1") {
    sleep("20 millis")
    return "new preview"
  }
  return current.join()
}

Starting another fiber under an existing key interrupts the previous fiber for that key. This supports latest-request behavior without retaining an obsolete handle elsewhere. FiberMap.has and get inspect tracked keys, remove interrupts and removes a keyed fiber, and onlyIfMissing: true avoids replacing existing work when passed to run.

Removing completed work

function waitForAllTracked() {
  using workers = FiberSet.make<void>()
  FiberSet.run(workers, sleep("10 millis"))
  FiberSet.run(workers, sleep("20 millis"))
  FiberSet.awaitEmpty(workers)
  return FiberSet.size(workers) // 0
}

Completed fibers are removed automatically, so a long-lived owner does not accumulate finished handles. awaitEmpty waits for the current group to drain, whereas FiberSet.join waits on the set's failure signal and propagates a tracked failure; it is not a successful drain operation. Maps also remove completed fibers automatically, including protection against an older completion deleting a newer fiber under the same key.

Interrupting tracked work on scope closure

function serveBriefly() {
  {
    using workers = FiberSet.make<void>()
    FiberSet.run(workers) {
      try {
        sleep("1 minute")
      } finally {
        logInfo("tracked worker cleaned up")
      }
    }
    sleep("20 millis")
  }
  logInfo("worker scope has closed")
}

make acquires a scope-owned collection. Here using ties that scope to the block, whose exit interrupts remaining work and waits for cleanup. Without using, the collection belongs to the caller's runtime scope and Scope remains in the function's requirements; use that form when a service layer should own the group.