10_drain.c

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/* -*- C -*-
 * Serene programming language
 * Copyright (C) 2019-2026 Sameer Rahmani <[email protected]>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */
 
// Example 10 -- graceful drain.
//
// srn_sched_stop (example 06) is abrupt, queued fibers never run.
// srn_sched_drain is the graceful counterpart. Workers keep running every
// runnable fiber and let in-flight IO finish, but new IO submissions are
// fenced, a fiber's next IO call fails with the CANCELED error tag so it
// unwinds instead of parking on fresh work. The pool then reaches the same
// quiescence as a natural finish and the run returns.
//
// A worker fiber loops doing short sleeps. A controller fiber lets it run a few
// rounds, then drains the scheduler. The worker's in-flight sleep still
// completes, but its next sleep fails with CANCELED, so it stops its loop and
// returns. No slice is ever cut mid-step.
 
#include <stdint.h>
#include <stdio.h>
 
#include <serene/rt/fiber/io.h>
#include <serene/runtime.h>
 
static srn_fiber_result_t worker_loop(srn_context_t *ctx, void *arg) {
  (void)ctx;
  (void)arg;
  for (int i = 0;; i++) {
    srn_error_t *err = srn_fiber_sleep_ms(20);
    if (err != nullptr && err->tag == CANCELED) {
      printf("worker: sleep cancelled after %d rounds, winding down\n", i);
      break;
    }
    printf("worker: round %d\n", i);
  }
  return nullptr;
}
 
static srn_fiber_result_t controller(srn_context_t *ctx, void *arg) {
  (void)ctx;
  srn_scheduler_t *sched = arg;
  // Let the worker fiber run a few rounds before winding the pool down. This
  // sleep is submitted before the drain, so it completes normally.
  (void)srn_fiber_sleep_ms(70);
  printf("controller: draining\n");
  srn_sched_drain(sched);
  return nullptr;
}
 
int main(void) {
  SERENE_RUNTIME_INIT_WITH_SCHED(engine, sched, nullptr);
  srn_context_t *ctx = srn_context_make(engine);
 
  (void)srn_fiber_spawn(ctx, worker_loop, nullptr);
  (void)srn_fiber_spawn(ctx, controller, sched);
 
  srn_sched_run(sched, 1);
  printf("run returned; drain converged\n");
 
  srn_context_release(ctx);
  SERENE_RUNTIME_SHUTDOWN(engine);
  return 0;
}