this.withLock {
      runningSyncCalls.add(call)
    }

  /** Used by [AsyncCall.run] to signal completion. */
  internal fun finished(call: AsyncCall) {
    call.callsPerHost.decrementAndGet()
    finished(runningAsyncCalls, call)
  }

  /** Used by [Call.execute] to signal completion. */
  internal fun finished(call: RealCall) {
    finished(runningSyncCalls, call)
  }