scheduler.c

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/* -*- C -*-
 * Serene programming language
 * Copyright (C) 2019-2026 Sameer Rahmani <[email protected]>
 *
 * This library is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This library 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 Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this library.  If not, see <https://www.gnu.org/licenses/>.
 */
 
#include <inttypes.h>
#include <stdatomic.h>
#include <stdio.h>
 
#include "serene/rt/context.h"
#include "serene/rt/engine.h"
#include "serene/rt/fiber.h"
#include "serene/rt/fiber/thread.h"
#include "serene/rt/mm/interface.h"
#include "serene/rt/reactor.h"
#include "serene/utils.h"
 
#define SCHED_LOG(FMT, ...) DBG("SCHED", FMT __VA_OPT__(, ) __VA_ARGS__)
 
/// Per-operation deque and queue tracing (push, pop, steal, wake). This fires
/// on the hot path, so it floods a debug build and is off unless
/// SRN_SCHED_TRACE is defined. SCHED_LOG (scheduler lifecycle, stop, shutdown
/// reap) stays on in a debug build. Both are silent in release, since DBG is.
#ifdef SRN_SCHED_TRACE
#  define SCHED_TRACE(FMT, ...) DBG("SCHED", FMT __VA_OPT__(, ) __VA_ARGS__)
#else
#  define SCHED_TRACE(...) \
    do {                   \
    } while (0)
#endif
 
// -----------------------------------------------------------------------------
// Model
// -----------------------------------------------------------------------------
// The scheduler decides what runs next and owns the global queue and the
// registry. It does not run fibers. The os threads do. Each os thread runs the
// worker routine over its own worker, which holds a local run queue. The
// routine finds a fiber -- the worker's local queue first, then the global
// queue, then stealing one from a peer -- switches into it, and handles how it
// gives up control. A fiber yields, suspends, or finishes by switching to its
// worker's loop, which is therefore the resumer. The single scheduler is shared
// by every worker.
//
// Migration hygiene. A fiber may run on any worker and resume on a different
// one than it last ran on, so it carries no thread identity. The only
// thread-bound state is the `current_worker` thread-local, read fresh at each
// use and never stored on a fiber across a switch. Allocating through a fiber's
// context is safe from any worker, since the memory manager locks its own
// blocks, and a context is the shared engine arena rather than a per-thread
// one. The one rule the IO layer must keep is to read `errno` before any
// suspend or yield, since the fiber may resume on another thread where `errno`
// differs.
 
// -----------------------------------------------------------------------------
// Work-stealing
// -----------------------------------------------------------------------------
// Each worker owns a Chase-Lev work-stealing deque (a fixed ring of ready
// fibers, see srn_worker_t), and the scheduler keeps one shared global queue
// besides. Two signed, monotonically increasing counters index a worker's ring.
// `bottom` is the owner's end and `top` is the thieves' end. The live slot for
// an index is `index & (SRN_FIBER_LOCAL_RING_CAP - 1)`.
//
//   top (thieves take the oldest)      bottom (owner pushes/pops the newest)
//    |                                  |
//    [ f3 ][ f4 ][ f5 ][ f6 ][ f7 ]
//
// The owner pushes and pops `bottom`, and since only it touches that end the
// common path is lock free and uncontended. Thieves take from `top` with a
// compare-and-swap, so several can race and exactly one wins. The owner and the
// thieves touch opposite ends, so they collide only over the last remaining
// element, a race `local_pop` and `local_steal` settle with a seq-cst fence and
// the CAS on `top`. The counters are signed so the transient `bottom - 1` on an
// empty deque reads as -1 ("empty") rather than wrapping.
//
// Finding work. A worker drains its own deque, then the global queue, then
// steals one fiber from each peer in turn (`find_work`). Work made while a
// fiber runs (a yield, spawn, or wake) goes onto the running worker's own
// deque, keeping it local. Work from off a worker, or work that overflows a
// full deque, goes to the global queue. So the global lock stays off the hot
// path while every os thread is busy -- it is taken only for the global queue
// and to wake a parked os thread (when one exists).
//
// The deque follows the weak memory correct formulation, so the fences are
// right on ARM and the like, not only on x86:
//   - Chase & Lev, "Dynamic Circular Work-Stealing Deque", SPAA 2005.
//     https://doi.org/10.1145/1073970.1073974
//   - Le, Pop, Cohen & Zappa Nardelli, "Correct and Efficient Work-Stealing for
//     Weak Memory Models", PPoPP 2013. https://doi.org/10.1145/2442516.2442524
//     (PDF: https://fzn.fr/readings/ppopp13.pdf)
 
/// Defined here, not in fiber.h, which only forward declares `srn_scheduler_t`.
/// Consumers hold a `srn_scheduler_t *` and never see the layout. Two reasons:
///
///  1. The layout can evolve without recompiling or disturbing consumers. Going
///     M:N this struct grows per-thread local queues, a worker array, a reactor
///     handle, steal state -- none of which should ripple into every
///     translation unit that includes `fiber.h`.
///
///  2. It makes the decide/execute boundary physical. The scheduler owns the
///     ready queue and the picking policy. The worker routine and fibers must
///     reach it only through enqueue/yield/ready. Keeping the fields private
///     means no
///     caller *can* poke the queue directly -- the encapsulation is the
///     contract, enforced by the compiler rather than by convention.
typedef struct srn_worker_t srn_worker_t;
 
/// The scheduler's lifecycle as one atomic value. `RUNNING` means a run is
/// servicing the queues. `DRAINING` is the graceful wind down set by
/// `srn_sched_drain`, workers keep running every runnable fiber and let
/// in-flight IO finish, but new IO submissions are fenced (see
/// `srn_sched_accepting_submissions`) so fibers unwind instead of parking on
/// fresh ops, and the pool converges to quiescence. `STOPPING` tells the
/// workers to wind down at once, set at natural quiescence or abruptly by
/// `srn_sched_stop`. `IDLE` is the resting state before a run starts. Workers
/// read it without the lock, so it is atomic.
///
/// The order matters, a state at or past `DRAINING` no longer accepts new IO,
/// which `srn_sched_accepting_submissions` relies on.
typedef enum srn_sched_state_t {
  SRN_SCHED_IDLE,
  SRN_SCHED_RUNNING,
  SRN_SCHED_DRAINING,
  SRN_SCHED_STOPPING,
} srn_sched_state_t;
 
struct srn_scheduler_t {
  srn_engine_t *engine;
  /// Global lock. Guards the global/overflow queue, the registry, and the
  /// worker coordination fields below. It does NOT guard the per-worker local
  /// queues, which carry their own locks. Lock order is global-before-local, a
  /// worker may hold this while taking a local lock (only the park scan does),
  /// but a local lock is never held while taking this.
  srn_mutex_t lock;
  /// Global / overflow queue. Holds fibers enqueued with no current worker --
  /// an external waker, or the initial fibers made before the run. A worker
  /// drains its own local queue first, then this. FIFO through the intrusive
  /// `link`.
  srn_fiber_t *ready_head;
  srn_fiber_t *ready_tail;
  /// Registry, head of the doubly-linked list (through `reg_prev`/`reg_next`)
  /// of every live fiber, on a different axis from the run queues. A fiber
  /// joins at srn_fiber_make and leaves when reaped, so the list is every fiber
  /// the scheduler is still responsible for, including SUSPENDED ones that sit
  /// on no run queue. It is how the scheduler accounts for and cleans them up.
  srn_fiber_t *registry;
 
  /// Worker coordination. Parked os threads wait on `work`. `idle` counts
  /// parked os threads and `runnable` counts fibers waiting in ANY queue (local
  /// or global). Both are atomic because a push reads `idle`, and the park path
  /// reads `runnable`, without holding the lock the other side updates them
  /// under. The "idle++ then read runnable" park ordering against the
  /// "runnable++ then read idle" push ordering is what makes a lost wakeup
  /// impossible.
  ///
  /// WARNING: that pairing is correct ONLY because all four of those operations
  /// are seq_cst (the default for `atomic_fetch_add` and `atomic_load`).
  /// `announce_work` does its `runnable++` and its `idle` read with NO lock
  /// held, so the single seq_cst total order is the only thing tying it to the
  /// park path. Do NOT weaken these to acq_rel or relaxed for "speed". Weaken
  /// them and a push and a park can each fail to see the other, so an os thread
  /// sleeps on `work` forever while a runnable fiber sits in a queue. That is a
  /// lost wakeup, a hang. If these ever must be relaxed, the whole
  /// `runnable`/`idle` handshake has to move under the lock first, the way
  /// `global_enqueue` already does it, so the lock supplies the ordering the
  /// weaker atomics would not.
  ///
  /// `nworkers` is fixed for a run. `state` drives termination. An os thread
  /// stops once it observes `SRN_SCHED_STOPPING`, set at quiescence (idle ==
  /// nworkers and runnable == 0) or by `srn_sched_stop`.
  srn_cond_t work;
  atomic_size_t idle;
  atomic_size_t runnable;
  size_t nworkers;
 
  _Atomic srn_sched_state_t state;
  /// `srn_sched_run` allocates these two arrays and `srn_sched_shutdown` frees
  /// them. They live until shutdown, so shutdown can join the threads and reap.
  /// The scheduler struct itself is immortal, but these arrays are not.
  ///
  /// `workers` holds all `nworkers` workers in one array, so each worker can
  /// find the others to steal from. `os_threads` holds the OS threads the
  /// scheduler started. There is not one thread per worker. The caller's own
  /// thread runs worker 0, so the scheduler never starts a thread for it. Only
  /// workers 1 through nworkers-1 get a thread, so slot 0 of `os_threads` is
  /// unused and shutdown joins slots 1 through nworkers-1. A thread belongs
  /// here, not in `srn_worker_t`, because it marks a thread the scheduler
  /// started and must join, which is not the same as being a worker.
  srn_worker_t *workers;
  srn_thread_t *os_threads;
  /// True for the duration of an `srn_sched_run` call. `srn_sched_shutdown`
  /// reads it to reject being called while a run is still in flight (it must
  /// run after `run` has returned). Atomic because shutdown may read it from a
  /// different thread than the one inside `run`.
  _Atomic bool run_active;
  /// Set once `srn_sched_shutdown` has torn the scheduler down. The scheduler
  /// is not usable afterwards, a further `run` panics, and a further `shutdown`
  /// is a no-op.
  bool destroyed;
};
 
/// Capacity of each worker's local work-stealing deque. Must be a power of two:
/// the live slot for a deque index is `index & (cap - 1)`. 256 matches the
/// common choice (Go, Tokio). A fiber that does not fit overflows to the global
/// queue. This is the single source of truth for the size, so a configuration
/// layer can later drive it.
#define SRN_FIBER_LOCAL_RING_CAP 256
static_assert(
  (SRN_FIBER_LOCAL_RING_CAP & (SRN_FIBER_LOCAL_RING_CAP - 1)) == 0,
  "SRN_FIBER_LOCAL_RING_CAP must be a power of two"
);
 
/// The state one os thread uses to run fibers. The worker's loop (`loop`)
/// represents the os thread itself and is the resumer for every fiber this
/// worker runs. Each worker owns a lock free Chase-Lev work stealing deque. The
/// owner pushes and pops the `bottom` end, while thieves take from the `top`
/// end. So the common push and pop touch no lock, and only a steal contends.
struct srn_worker_t {
  srn_scheduler_t *sched;
  srn_fiber_t loop;
  srn_fiber_t *current;
  srn_worker_id_t id;
 
  /// Count of fibers this worker has created so far. Feeds the autogenerated
  /// fiber names, worker local so no synchronization is needed.
  uint64_t spawned;
 
  /// Chase-Lev deque. `top` and `bottom` are signed and monotonically
  /// increasing (signed so the transient `bottom - 1` on an empty deque does
  /// not wrap), and the live slot for an index is `index &
  /// (SRN_FIBER_LOCAL_RING_CAP - 1)`.
  /// TODO(lxsameer): Make the ring capacity configurable via CLI args.
  atomic_intptr_t top;
  atomic_intptr_t bottom;
  _Atomic(srn_fiber_t *) ring[SRN_FIBER_LOCAL_RING_CAP];
  // TODO(lxsameer): a per-thread ring of free stacks to recycle could live
  // here.
};
 
/// The worker the calling os thread is running, or null when this os thread is
/// not running the worker routine (srn_sched_run is not active on it). This
/// thread-local is the seam that resolves the resumer, the current fiber, and
/// "are we in a fiber?" -- all per os thread state that cannot live in the
/// single shared scheduler.
static _Thread_local srn_worker_t *current_worker = nullptr;
 
// -----------------------------------------------------------------------------
// Lifecycle
// -----------------------------------------------------------------------------
 
srn_scheduler_t *srn_sched_init(srn_engine_t *engine) {
  PANIC_IF_NULL(engine);
  // The scheduler outlives every context and fiber, so it is allocated from the
  // immortal region rather than a releasable block.
  srn_scheduler_t *sched = srn_mm_immortal_allocate(engine->mm, srn_scheduler_t);
  PANIC_IF_NULL(sched);
 
  sched->engine     = engine;
  sched->ready_head = nullptr;
  sched->ready_tail = nullptr;
  sched->registry   = nullptr;
  sched->idle       = 0;
  sched->runnable   = 0;
  sched->nworkers   = 0;
  sched->state      = SRN_SCHED_IDLE;
  sched->workers    = nullptr;
  sched->os_threads = nullptr;
  sched->destroyed  = false;
  atomic_init(&sched->run_active, false);
 
  PANIC_IF(
    srn_mutex_init(&sched->lock) != SRN_THREAD_OK, "failed to initialise the scheduler lock"
  );
 
  PANIC_IF(
    srn_cond_init(&sched->work) != SRN_THREAD_OK, "failed to initialise the scheduler condition"
  );
 
  // srn_engine_make will store this scheduler in the engine
  return sched;
}
 
/// Insert at the head of the registry. Caller must hold `sched->lock`.
static void registry_add(srn_scheduler_t *sched, srn_fiber_t *fiber) {
  fiber->reg_prev = nullptr;
  fiber->reg_next = sched->registry;
 
  if (sched->registry != nullptr) {
    sched->registry->reg_prev = fiber;
  }
 
  sched->registry = fiber;
}
 
/// Unlink from the registry. O(1), thanks to the back pointer. Caller must hold
/// `sched->lock`.
static void registry_remove(srn_scheduler_t *sched, srn_fiber_t *fiber) {
  if (fiber->reg_prev != nullptr) {
    fiber->reg_prev->reg_next = fiber->reg_next;
  } else {
    sched->registry = fiber->reg_next;
  }
 
  if (fiber->reg_next != nullptr) {
    fiber->reg_next->reg_prev = fiber->reg_prev;
  }
 
  fiber->reg_prev = nullptr;
  fiber->reg_next = nullptr;
}
 
void srn_sched_register(srn_scheduler_t *sched, srn_fiber_t *fiber) {
  PANIC_IF_NULL(sched);
  PANIC_IF_NULL(fiber);
 
  srn_mutex_lock(&sched->lock);
  registry_add(sched, fiber);
  srn_mutex_unlock(&sched->lock);
}
 
void srn_sched_shutdown(srn_scheduler_t *sched) {
  PANIC_IF_NULL(sched);
 
  if (sched->destroyed) {
    return;
  }
 
  // It must run on a thread outside the pool, a worker, or a fiber (which runs
  // on a worker), would be tearing down the scheduler it is itself running on.
  // `current_worker` is null only off a worker, so it is the test for that.
  PANIC_IF(
    current_worker != nullptr, "srn_sched_shutdown must be called from outside the worker pool"
  );
 
  // And it must run after `srn_sched_run` has returned, not while a run is in
  // flight. Stop a running pool with `srn_sched_stop` and let `srn_sched_run`
  // return first.
  PANIC_IF(
    atomic_load(&sched->run_active),
    "srn_sched_shutdown called while srn_sched_run is active; call "
    "srn_sched_stop and let the run return first"
  );
 
  // The run has returned, so worker 0 has stopped. The spawned os threads may
  // still be winding down (`srn_sched_run` does not join them), so join them
  // now. `os_threads` slot 0 is the inline worker 0, never spawned, so the
  // spawned os threads to join are 1..nworkers-1.
  for (size_t i = 1; i < sched->nworkers; i++) {
    (void)srn_thread_join(&sched->os_threads[i]);
  }
 
  // Every worker is gone, so this runs single threaded now.
  //
  // Any fiber still in the registry never finished, it was left parked in
  // SRN_FIBER_SUSPENDED with no party able to wake it (a deadlock), or was left
  // queued when the run stopped early. Release its stack so it does not leak.
  // The fiber structs themselves live in context blocks and are reclaimed with
  // those blocks, not here. The scheduler is immortal-allocated, so it is not
  // freed either.
  //
  // Unlike the reap path, this unmaps for good, shutdown runs after the workers
  // are gone, so the per-thread stack ring from the fiber.h TODO no longer
  // exists and there is nothing to recycle into. (Draining that ring, when it
  // exists, also belongs here.)
  srn_fiber_t *fiber = sched->registry;
  while (fiber != nullptr) {
    srn_fiber_t *next = fiber->reg_next;
    SCHED_LOG(
      "shutdown reaping unfinished fiber '%s' (never scheduled, or suspended with no waker?)",
      fiber->name
    );
    // TODO(lxsameer): Free up the ring here as well
    FIBER_TRACEPOINT(fiber_stack_free, (void *)fiber, fiber->name);
    srn_fiber_stack_free(fiber->stack);
    srn_fiber_on_reap(fiber);
    fiber->reg_prev = nullptr;
    fiber->reg_next = nullptr;
    fiber           = next;
  }
  sched->registry = nullptr;
 
  // Release the run-scoped storage (srn_mm_free tolerates null, so a scheduler
  // that never ran is fine).
  srn_mm_free(sched->engine->mm, sched->os_threads);
  srn_mm_free(sched->engine->mm, sched->workers);
  sched->os_threads = nullptr;
  sched->workers    = nullptr;
  sched->nworkers   = 0;
 
  // Destroy the synchronisation primitives. The scheduler is not usable after
  // this, so they are not re-initialised. No worker holds or waits on them now,
  // the join above made sure of that.
  (void)srn_cond_destroy(&sched->work);
  (void)srn_mutex_destroy(&sched->lock);
 
  sched->destroyed = true;
}
 
/// Wake the os thread of one parked worker after a fiber has joined a queue.
/// "Parked" means that os thread is asleep in `srn_cond_wait` because it found
/// no runnable fiber anywhere. This is not a fiber suspending. It is the whole
/// os thread blocked, and the notify wakes it so it looks again.
///
/// `runnable` is bumped first, then `idle` is read. Paired against the park
/// path, which bumps `idle` then reads `runnable`, this ordering means the two
/// sides can never both miss, so a wakeup is never lost. The notify takes the
/// global lock (the condition's lock) but only when an os thread is actually
/// parked, so the common busy case never touches it.
///
/// WARNING: this runs with NO lock around the `runnable++` and the `idle` read,
/// so its only tie to the park path is the seq_cst total order. Both must stay
/// seq_cst. RELAX EITHER AND THE WAKEUP CAN BE LOST (an os thread parked with a
/// runnable fiber queued). See the `srn_scheduler_t` coordination comment for
/// the full reasoning.
static void announce_work(srn_scheduler_t *sched) {
  PANIC_IF_NULL(sched);
 
  atomic_fetch_add(&sched->runnable, 1);
 
  if (atomic_load(&sched->idle) > 0) {
    // We have idle os threads. Wake them up
    srn_mutex_lock(&sched->lock);
    SCHED_TRACE("waking a parked os thread (runnable=%ld)", (long)atomic_load(&sched->runnable));
    srn_cond_notify_one(&sched->work);
    srn_mutex_unlock(&sched->lock);
  }
}
 
// The local deque is a Chase-Lev work-stealing deque. The fence placement
// follows Le, Pop, Cohen and Zappa Nardelli, "Correct and Efficient
// Work-Stealing for Weak Memory Models" (PPoPP 2013), so it is correct on
// weakly-ordered CPUs, not just on x86's strong model. The buffer is fixed, so
// there is no resize and no buffer reclamation. The `runnable` count is
// adjusted by the enqueue path (push) and by find_work (the only taker).
 
/// This operation is only for the owner of the ring. Push a fiber on the
/// bottom. Returns false when the deque is full, so the caller can overflow it
/// to the global queue. The caller has set `state`.
static bool local_push(srn_worker_t *w, srn_fiber_t *fiber) {
  PANIC_IF_NULL(w);
  PANIC_IF_NULL(fiber);
 
  intptr_t b = atomic_load_explicit(&w->bottom, memory_order_relaxed);
  intptr_t t = atomic_load_explicit(&w->top, memory_order_acquire);
  if (b - t >= (intptr_t)SRN_FIBER_LOCAL_RING_CAP) {
    return false; // full
  }
 
  atomic_store_explicit(&w->ring[b & (SRN_FIBER_LOCAL_RING_CAP - 1)], fiber, memory_order_relaxed);
  // After ^^^, the slot isn't published to thieves yet, because they decide
  // what's live by reading bottom, which we haven't bumped
 
  // Publish the slot. write before the bottom store that exposes it to a thief.
  // This is the key barrier. It orders the slot write before the bottom bump
  // that follows. Paired with a thief's `acquire-load` of bottom in
  // `local_steal`, it guarantees, if a thief sees the new bottom, it also sees
  // the fiber we just wrote, never a stale/garbage slot.
  atomic_thread_fence(memory_order_release);
  atomic_store_explicit(&w->bottom, b + 1, memory_order_relaxed);
 
  SCHED_TRACE("worker %zu local-push fiber %p", w->id, (void *)fiber);
  return true;
}
 
/// Owner only. Pop a fiber from the bottom, or null when empty. The seq_cst
/// fence and the compare-and-swap settle the race with a thief over the last
/// element.
static srn_fiber_t *local_pop(srn_worker_t *w) {
  PANIC_IF_NULL(w);
 
  // Since bottom is local to the owner, there is no other writer competing to
  // write to it. So a load/store is enough here no need for `atomic_fetch_sub`.
  intptr_t b = atomic_load_explicit(&w->bottom, memory_order_relaxed) - 1;
  atomic_store_explicit(&w->bottom, b, memory_order_relaxed);
 
  atomic_thread_fence(memory_order_seq_cst);
  intptr_t t = atomic_load_explicit(&w->top, memory_order_relaxed);
 
  srn_fiber_t *fiber = nullptr;
  if (t <= b) {
    // Non-empty.
    fiber =
      atomic_load_explicit(&w->ring[b & (SRN_FIBER_LOCAL_RING_CAP - 1)], memory_order_relaxed);
    if (t == b) {
      // Last element. The owner and a thief can race for it, so settle it with
      // the CAS on `top`. Exactly one of them wins.
      if (
        atomic_compare_exchange_strong_explicit(
          &w->top, &t, t + 1, memory_order_seq_cst, memory_order_relaxed
        )
      ) {
        SCHED_TRACE("worker %zu popped the last fiber %p", w->id, (void *)fiber);
 
      } else {
        SCHED_TRACE("worker %zu lost the last fiber %p to a thief", w->id, (void *)fiber);
        fiber = nullptr; // the thief won
      }
      atomic_store_explicit(&w->bottom, b + 1, memory_order_relaxed);
    }
  } else {
    // Empty. Restore bottom.
    atomic_store_explicit(&w->bottom, b + 1, memory_order_relaxed);
  }
  return fiber;
}
 
/// Thief side. Take a fiber from `victim`'s top, or null when the deque is
/// empty or a concurrent take won the race -- the caller then just moves to the
/// next victim.
static srn_fiber_t *local_steal(srn_worker_t *victim) {
  PANIC_IF_NULL(victim);
 
  intptr_t t = atomic_load_explicit(&victim->top, memory_order_acquire);
  // Pairs with the `seq_cst` fence in `local_pop`. The two fences force a
  // single total order in which the owner (lowering bottom, fence, reading top)
  // and this thief (reading top, fence, reading bottom) cannot both decide they
  // got the last element.
  // Basically this fence handles the steal race against local_pop.
  // Note: Don't mixup `memory_order_seq_cst` with `memory_order_acquire` that
  // we use for loading victim's bottom the next line.
  atomic_thread_fence(memory_order_seq_cst);
  // This acquire on bottom handles slot visibility against `local_push`
  intptr_t b = atomic_load_explicit(&victim->bottom, memory_order_acquire);
 
  srn_fiber_t *fiber = nullptr;
  if (t < b) {
    fiber =
      atomic_load_explicit(&victim->ring[t & (SRN_FIBER_LOCAL_RING_CAP - 1)], memory_order_relaxed);
    if (!atomic_compare_exchange_strong_explicit(
          &victim->top, &t, t + 1, memory_order_seq_cst, memory_order_relaxed
        )) {
      fiber = nullptr; // lost the race
    }
  }
  return fiber;
}
 
/// Append a fiber to the global/overflow queue. The caller has set its
/// `state`. The push, the `runnable` bump, and the wake all run under the
/// global lock, so this path is trivially serialized against the park path and
/// needs no separate ordering argument.
///
/// Put a fiber on the global queue and wake a parked os thread if any. Unlike
/// `announce_work`, the `runnable++`, the `idle` read, and the notify all
/// happen under the lock, so this path is safe by mutual exclusion and does not
/// lean on the seq_cst ordering the lockless path does.
static void global_enqueue(srn_scheduler_t *sched, srn_fiber_t *fiber) {
  srn_mutex_lock(&sched->lock);
 
  fiber->link = nullptr;
 
  if (sched->ready_tail == nullptr) {
    sched->ready_head = fiber;
  } else {
    sched->ready_tail->link = fiber;
  }
  sched->ready_tail = fiber;
 
  atomic_fetch_add(&sched->runnable, 1);
 
  SCHED_TRACE(
    "global-push fiber %p (runnable=%ld)", (void *)fiber, (long)atomic_load(&sched->runnable)
  );
 
  if (atomic_load(&sched->idle) > 0) {
    srn_cond_notify_one(&sched->work);
  }
  srn_mutex_unlock(&sched->lock);
}
 
/// Pop the head of the global queue, or null when empty. The `runnable`
/// adjustment is left to `find_work`, the only taker.
static srn_fiber_t *global_take(srn_scheduler_t *sched) {
  srn_mutex_lock(&sched->lock);
  srn_fiber_t *fiber = sched->ready_head;
 
  if (fiber != nullptr) {
    sched->ready_head = fiber->link;
    if (sched->ready_head == nullptr) {
      sched->ready_tail = nullptr;
    }
    fiber->link = nullptr;
  }
 
  srn_mutex_unlock(&sched->lock);
  return fiber;
}
 
/// Put a runnable fiber on a queue, with its `state` already set to `READY`. A
/// fiber enqueued while running on a worker goes onto that worker's local
/// deque, keeping its work local. One enqueued from off a worker (the initial
/// fibers made before the run, or an external waker), or one that does not fit
/// a full local deque, goes to the global queue.
static void push_ready(srn_scheduler_t *sched, srn_fiber_t *fiber) {
  srn_worker_t *w = current_worker;
 
  // The publish (local_push) happens before announce_work bumps `runnable`,
  // so a thief that takes the fiber first transiently drives `runnable` to
  // SIZE_MAX. Every check of the counter is `== 0`, which the wrap cannot
  // satisfy, so this is benign; do not add `> 0` style or signed comparisons
  // on `runnable` without fixing the ordering here.
 
  // On a worker, try its own deque first, falling through on a full deque.
  if (w != nullptr) {
    if (local_push(w, fiber)) {
      announce_work(sched);
      return;
    }
 
    FIBER_TRACEPOINT(sched_overflow, w->id, (void *)fiber);
    SCHED_TRACE("worker %zu local deque full, overflow fiber %p to global", w->id, (void *)fiber);
  }
 
  // Off a worker, or the deque was full, the global queue takes it.
  global_enqueue(sched, fiber);
}
 
void srn_sched_enqueue(srn_scheduler_t *sched, srn_fiber_t *fiber) {
  PANIC_IF_NULL(sched);
  PANIC_IF_NULL(fiber);
 
  fiber->state = SRN_FIBER_READY;
  push_ready(sched, fiber);
}
 
void srn_fiber_schedule(srn_fiber_t *fiber) {
  PANIC_IF_NULL(fiber);
 
  // The NEW to READY flip admits exactly one scheduler of this fiber, the
  // same guard ready_fiber uses for SUSPENDED, so a double schedule panics
  // at the losing call site instead of double enqueuing.
  srn_fiber_state_t expected = SRN_FIBER_NEW;
  PANIC_IF(
    !atomic_compare_exchange_strong(&fiber->state, &expected, SRN_FIBER_READY),
    "srn_fiber_schedule needs a NEW fiber. A fiber is scheduled exactly once, "
    "and a suspended one is woken with srn_fiber_ready"
  );
  push_ready(srn_fiber_get_scheduler_m(fiber), fiber);
}
 
/// Wake a parked fiber by flipping `SUSPENDED` to `READY` and enqueuing it.
/// Only the flip's winner enqueues, so racing wakers cannot double-enqueue it,
/// and a fiber that is not parked is left untouched. The scheduler does not
/// check the awaited condition. A fiber woken early resumes, re-checks, and
/// parks again.
static void ready_fiber(srn_scheduler_t *sched, srn_fiber_t *fiber) {
  srn_fiber_state_t expected = SRN_FIBER_SUSPENDED;
  if (atomic_compare_exchange_strong(&fiber->state, &expected, SRN_FIBER_READY)) {
    push_ready(sched, fiber);
  }
}
 
/// Find a fiber to run, the worker's own deque first, then the global queue,
/// then a steal of one fiber from each peer in turn. Null when nothing is
/// runnable anywhere this worker can reach. Decrements `runnable` for whatever
/// it takes.
static srn_fiber_t *find_work(srn_worker_t *w) {
  srn_scheduler_t *sched = w->sched;
 
  srn_fiber_t *fiber = local_pop(w);
  if (fiber == nullptr) {
    fiber = global_take(sched);
  }
 
  if (fiber == nullptr) {
    for (size_t i = 1; i < sched->nworkers; i++) {
      // We start form the right side neighbour and with `i` growing we will
      // eventually loop back to the left side neighbour in the workers array.
      size_t index         = (w->id + i) % sched->nworkers;
      srn_worker_t *victim = &sched->workers[index];
      fiber                = local_steal(victim);
 
      if (fiber != nullptr) {
        FIBER_TRACEPOINT(sched_steal, w->id, victim->id, (void *)fiber);
        SCHED_TRACE("worker %zu stole fiber %p from worker %zu", w->id, (void *)fiber, victim->id);
        break;
      }
    }
  }
 
  if (fiber != nullptr) {
    atomic_fetch_sub(&sched->runnable, 1);
  }
 
  return fiber;
}
 
/// Run the worker routine over `worker` on the calling os thread. Find a fiber,
/// run it, handle how it gave up control, and park when nothing is runnable,
/// until the pool is quiescent. Owns the `current_worker` thread-local for its
/// duration. The hot path (`find_work` hitting the local deque, run) touches
/// only this worker's own lock free deque. The global lock is reached to park,
/// for the global queue, and on every yield, a yielded fiber goes to the
/// global tail so its peers get a turn, which is the point of yielding.
static void worker_run(srn_worker_t *worker) {
  srn_scheduler_t *sched = worker->sched;
  current_worker         = worker;
 
  for (;;) {
    // We check for termination here, between fibers, so an os thread stops at a
    // clean boundary even while its worker's deque still holds work. Whatever
    // is left unrun is reclaimed by `srn_sched_shutdown` through the registry.
    if (atomic_load(&sched->state) == SRN_SCHED_STOPPING) {
      break;
    }
 
    // Drain this worker's reactor completions before looking for fibers. Runs
    // on the worker, so srn_fiber_ready pushes the woken fibers onto its own
    // deque, keeping them local.
    srn_reactor_consume(sched->engine->reactor, worker->id);
 
    srn_fiber_t *fiber = find_work(worker);
    if (fiber == nullptr) {
      // Nothing runnable here or in any peer to steal from, so park this os
      // thread. Parking while every other os thread is already parked and
      // nothing is queued means the pool is quiescent. Nothing running can
      // produce more work, so the run ends. Move to STOPPING and wake every os
      // thread to exit. The `runnable` re-check in the loop closes the race
      // with a fiber enqueued between find_work and taking the lock, and
      // absorbs spurious wakeups.
      srn_mutex_lock(&sched->lock);
      atomic_fetch_add(&sched->idle, 1);
 
      if (
        atomic_load(&sched->idle) == sched->nworkers && atomic_load(&sched->runnable) == 0 &&
        (int)srn_reactor_idle(sched->engine->reactor)
      ) {
 
        // All the os threads are idle. Time to stop
        atomic_store(&sched->state, SRN_SCHED_STOPPING);
        FIBER_TRACEPOINT(sched_quiesce, worker->id);
        srn_cond_notify_all(&sched->work);
      }
 
      FIBER_TRACEPOINT(sched_worker_park, worker->id);
      while (atomic_load(&sched->runnable) == 0 &&
             atomic_load(&sched->state) != SRN_SCHED_STOPPING &&
             !srn_reactor_channel_has_completions(sched->engine->reactor, worker->id)) {
        // No runnable fiber around and we are not stopping. A draining pool
        // parks here too, waiting for its in-flight ops to complete and ready
        // their fibers, so they can unwind. Only STOPPING ends the park.
        srn_cond_wait(&sched->work, &sched->lock);
      }
 
      // it has woken. This os thread is no longer parked. Snapshot whether
      // we're stopping, then drop the lock.
      atomic_fetch_sub(&sched->idle, 1);
      FIBER_TRACEPOINT(sched_worker_unpark, worker->id);
      bool stop = atomic_load(&sched->state) == SRN_SCHED_STOPPING;
      srn_mutex_unlock(&sched->lock);
 
      if (stop) {
        break;
      }
 
      continue;
    }
 
    worker->current = fiber;
    // Read here because the switch may reap the fiber, after which only the address value survives,
    // not the name.
    FIBER_TRACEPOINT(sched_fiber_running, worker->id, (void *)fiber, fiber->name);
 
    // The worker routine is the single owner of the RUNNING transition, for a
    // fiber's first run and every resume after a yield.
    fiber->state = SRN_FIBER_RUNNING;
 
    srn_fiber_switch(&worker->loop, fiber);
 
    const char *slice_reason;
 
    // `fiber` has switched back, and is not on any queue. How it gave up the
    // CPU is read from the fiber. A parked fiber left a commit on itself
    // (`park_commit` set), a finished one is `DONE`, and a yielded one is still
    // `RUNNING`.
    if (fiber->park_commit != nullptr) {
      slice_reason = "suspend";
      // Parked. It is fully off the CPU now, with its context saved, so this is
      // the first moment it is safe to wake (by others). Stamp `SUSPENDED`
      // here, not in `srn_fiber_suspend` before the switch, so the label only
      // ever marks a fiber that is parked and safe to resume. A waker flipping
      // `SUSPENDED` to `READY` can therefore never catch a fiber still parking.
      fiber->state = SRN_FIBER_SUSPENDED;
      FIBER_TRACEPOINT(sched_fiber_suspended, worker->id, (void *)fiber, fiber->name);
      // `park_commit`/`park_arg` are one-shot, carrying the commit across the
      // switch. Clearing them now loses nothing, since the next suspend sets
      // them again and the fiber never reads them on resume.
      srn_fiber_park_fn commit = fiber->park_commit;
      void *park_arg           = fiber->park_arg;
      fiber->park_commit       = nullptr;
      fiber->park_arg          = nullptr;
 
      // Run the commit now that the fiber is parked. It hands the fiber to its
      // waker (a waiter list, the reactor, and so on), which reschedules it
      // later. A true return means stay parked. A false return means the
      // condition already held, so wake it back up.
      if (!commit(fiber, park_arg)) {
        ready_fiber(sched, fiber);
      }
    } else if (fiber->state == SRN_FIBER_DONE) {
      slice_reason = "done";
      FIBER_TRACEPOINT(sched_fiber_done, worker->id, (void *)fiber, fiber->name);
      // Detach the waiter list (fibers blocked in `srn_fiber_wait_for`) and
      // drop the fiber from the registry under the global lock, which also
      // guards the waiter list against `wait_for_park`. Then wake the waiters
      // and free the stack outside the lock. Each waiter reads this fiber's
      // result, which outlives the reap since only the stack is freed, not the
      // struct.
      srn_mutex_lock(&sched->lock);
      srn_fiber_t *waiters = fiber->waiters;
      fiber->waiters       = nullptr;
      registry_remove(sched, fiber);
      srn_mutex_unlock(&sched->lock);
 
      while (waiters != nullptr) {
        srn_fiber_t *waiter = waiters;
        // Advance before the wake reuses `link`
        waiters = waiter->link;
        ready_fiber(sched, waiter);
      }
 
      // TODO(lxsameer): Instead of freeing the stack, return it to the ring
      // pool
      FIBER_TRACEPOINT(fiber_stack_free, (void *)fiber, fiber->name);
      srn_fiber_stack_free(fiber->stack);
      srn_fiber_on_reap(fiber);
    } else {
      slice_reason = "yield";
      // Yielded. It is fully off the CPU now, with its context saved, so this
      // is the first moment it is safe to put back on a queue, where another
      // worker may take it at once. `srn_fiber_yield` does not enqueue before
      // switching, which would expose a context still being saved to a resuming
      // worker. The fiber goes to the global queue tail, not the local deque:
      // the worker pops its deque LIFO, so a local push would run the same
      // fiber again immediately and starve its peers, making yield a no-op.
      fiber->state = SRN_FIBER_READY;
      FIBER_TRACEPOINT(sched_fiber_ready, worker->id, (void *)fiber, fiber->name);
      global_enqueue(sched, fiber);
    }
    // Slice end, once the reason is known. Pairs with worke_run_end by the
    // fiber address. Passes only the pointer value, never a dereference, so it
    // is safe even after the done branch reaped the fiber's stack.
    FIBER_TRACEPOINT(sched_fiber_finished, worker->id, (void *)fiber, slice_reason);
 
    worker->current = nullptr;
  }
 
  current_worker = nullptr;
}
 
/// The entry an os thread starts in. It sets up its worker's loop -- on its own
/// os thread, so the sanitizer captures the right stack bounds -- then runs the
/// worker routine until the pool is quiescent. `arg` is the worker.
static void worker_main(void *arg) {
  srn_worker_t *worker = arg;
  srn_fiber_init_thread(&worker->loop);
 
  FIBER_TRACEPOINT(sched_thread_init, worker->id);
 
  worker_run(worker);
}
 
void srn_sched_run(srn_scheduler_t *sched, size_t nworkers) {
  PANIC_IF_NULL(sched);
  PANIC_IF(sched->destroyed, "srn_sched_run called on a scheduler that was already shut down");
 
  // Claiming run_active up front turns an overlapping run into a clean panic
  // instead of two runs clobbering the worker arrays under each other.
  PANIC_IF(
    atomic_exchange(&sched->run_active, true),
    "srn_sched_run called while another run is active on this scheduler"
  );
 
  // A finished run leaves its worker arrays behind for shutdown to join and
  // free. A second run would replace them while stragglers from the first may
  // still be winding down, reviving those stragglers against the new run's
  // state, and the reactor cannot be activated twice either. A scheduler
  // therefore runs once; re-run support requires reactor reactivation.
  PANIC_IF(
    sched->workers != nullptr,
    "srn_sched_run called on a scheduler that has already run; re-run is not "
    "supported (the reactor cannot be reactivated)"
  );
 
  // A caller that does not pick a count gets the configured one, and every
  // request is clamped to the configured ceiling. SRN_MAX_WORKERS stays the
  // absolute ceiling above whatever the configuration asks for.
  const srn_configuration_t *config = &sched->engine->config;
  nworkers                          = nworkers == 0 ? config->fiber.workers : nworkers;
 
  if (nworkers == 0) {
    // A configured count of zero delegates to the machine, one worker per
    // CPU the process may run on.
    nworkers = srn_thread_cpu_count();
  }
 
  if (nworkers > config->fiber.max_workers) {
    nworkers = config->fiber.max_workers;
  }
 
  if (nworkers > SRN_MAX_WORKERS) {
    nworkers = SRN_MAX_WORKERS;
  }
 
  // Allocate `workers` and `os_threads` on the scheduler so
  // `srn_sched_shutdown` can join the threads and free them later. `workers`
  // has one entry per worker. `os_threads` has one per spawned thread, with
  // slot 0 left empty because the caller runs worker 0 inline (see the struct
  // comment). `runnable` is left alone, it already counts the fibers queued
  // before the run.
  sched->workers = srn_mm_malloc(sched->engine->mm, nworkers * sizeof(srn_worker_t));
  PANIC_IF_NULL(sched->workers);
 
  sched->os_threads = srn_mm_malloc(sched->engine->mm, nworkers * sizeof(srn_thread_t));
  PANIC_IF_NULL(sched->os_threads);
 
  for (size_t i = 0; i < nworkers; i++) {
    srn_worker_t *w = &sched->workers[i];
    w->sched        = sched;
    w->id           = i;
    w->current      = nullptr;
    w->spawned      = 0;
    // The deque indices start empty. Its ring slots are written before they are
    // read, and the worker's loop is set up by worker_main on its own os
    // thread.
    atomic_init(&w->top, 0);
    atomic_init(&w->bottom, 0);
  }
 
  // Publish the coordination state before any os thread starts. `nworkers` must
  // be set first so the quiescence check counts the right total, and the state
  // must be RUNNING before an os thread can observe it. `run_active` was
  // claimed at the top of this call; shutdown reads it to see a run in flight.
  sched->idle     = 0;
  sched->nworkers = nworkers;
  atomic_store(&sched->state, SRN_SCHED_RUNNING);
 
  // Bring the reactor up with one channel per worker before any worker starts,
  // so a fiber's first IO has a channel to submit on. The notify seam wakes the
  // worker that owns the channel a completion lands on.
  srn_reactor_activate(sched->engine->reactor, nworkers, srn_sched_wake_worker);
 
  FIBER_TRACEPOINT(sched_run, nworkers);
 
  // Spawn nworkers - 1 os threads. The calling os thread runs worker 0 inline.
  // A spawn failure at startup is fatal, a partial pool would never reach `idle
  // == nworkers` and so never quiesce.
  for (size_t i = 1; i < nworkers; i++) {
    if (srn_thread_spawn(&sched->os_threads[i], worker_main, &sched->workers[i]) != SRN_THREAD_OK) {
      PANIC("failed to spawn an os thread");
    }
    FIBER_TRACEPOINT(sched_thread_spawn, i);
  }
 
  worker_main(&sched->workers[0]);
 
  // Worker 0 has stopped, so the run is over from the caller's point of view.
  // The spawned os threads may still be winding down, so they are NOT joined
  // here. `srn_sched_shutdown` joins them (the `os_threads` live on the
  // scheduler) as part of tearing the subsystem down. Clearing `run_active`
  // lets shutdown proceed. The state stays STOPPING, which keeps any os thread
  // still looping on its way out.
  atomic_store(&sched->run_active, false);
}
 
void srn_sched_stop(srn_scheduler_t *sched) {
  PANIC_IF_NULL(sched);
  // Flip RUNNING or DRAINING to STOPPING once. A drain stalled on an op that
  // never completes must remain abortable, so stop escalates a drain rather
  // than deferring to it. If the scheduler is not running, or is already
  // stopping, there is nothing to do.
  srn_sched_state_t expected = SRN_SCHED_RUNNING;
 
  if (!atomic_compare_exchange_strong(&sched->state, &expected, SRN_SCHED_STOPPING)) {
    expected = SRN_SCHED_DRAINING;
    if (!atomic_compare_exchange_strong(&sched->state, &expected, SRN_SCHED_STOPPING)) {
      return;
    }
  }
 
  // Running os threads see STOPPING at the top of their next turn. Parked os
  // threads are roused to observe it. The notify is under the lock, paired with
  // the park path, so no wakeup is lost.
  srn_mutex_lock(&sched->lock);
  srn_cond_notify_all(&sched->work);
  srn_mutex_unlock(&sched->lock);
  SCHED_LOG("stop requested");
}
 
void srn_sched_drain(srn_scheduler_t *sched) {
  PANIC_IF_NULL(sched);
  // Begin a graceful winddown, only a RUNNING scheduler can enter DRAINING.
  // Already draining or stopping, or not running at all, leaves the state as
  // is.
  srn_sched_state_t expected = SRN_SCHED_RUNNING;
 
  if (!atomic_compare_exchange_strong(&sched->state, &expected, SRN_SCHED_DRAINING)) {
    return;
  }
 
  // From here `srn_sched_accepting_submissions` returns false, so the next IO a
  // fiber attempts is fenced into a cancelled completion and the fiber unwinds
  // rather than parking on a fresh op. Workers do NOT break on DRAINING, so
  // every runnable fiber still runs and every in-flight op still completes; the
  // pool converges to the same quiescence as a natural finish, which then moves
  // the state to STOPPING. A never-completing in-flight op (an idle recv, a
  // long sleep) stalls this until it finishes -- bounding that needs op CANCEL
  // (E1) and is out of scope here.
  //
  // The notify wakes any os thread already parked so it re-checks state. A
  // worker parked on outstanding IO simply re-parks (DRAINING keeps it
  // parking), which is harmless.
  srn_mutex_lock(&sched->lock);
  srn_cond_notify_all(&sched->work);
  srn_mutex_unlock(&sched->lock);
  SCHED_LOG("drain requested");
}
 
// -----------------------------------------------------------------------------
// Fiber-facing operations
// -----------------------------------------------------------------------------
// yield  = switch to the loop, which re-enqueues self once the context is saved
// suspend = switch to the loop, which then runs the commit to publish the
//           parked fiber to its waker
// ready  = enqueue(a named fiber), without switching
 
void srn_fiber_yield(void) {
  srn_worker_t *worker = current_worker;
  PANIC_IF_NULL(worker);
 
  // Switch to the worker's loop without enqueuing first. The worker routine
  // puts this fiber back on the ready queue once the switch has saved its
  // context. Enqueuing here, before the switch, would let another os thread
  // dequeue and resume the fiber while this os thread is still saving its
  // context -- two os threads on one fiber stack, which corrupts the switch.
  srn_fiber_t *self = worker->current;
  PANIC_IF_NULL(self);
  srn_fiber_switch(self, &worker->loop);
}
 
/// A suspended fiber is on no scheduler queue, and the scheduler does not track
/// what it waits on -- whoever wakes it does. The `commit` callback runs on the
/// worker's loop side once the fiber has switched out. It hands the fiber's
/// pointer to the event source it blocks on (a peer fiber, a lock's waiter
/// list, the IO reactor's fd table), so that party can call srn_fiber_ready
/// when the awaited event occurs. Running commit only after the suspend
/// completes is what makes the hand-off race free, a waker can never observe a
/// half-suspended fiber. If commit registers the fiber nowhere, it is genuinely
/// lost -- a deadlock, like an os thread blocking on a condition nobody
/// signals.
void srn_fiber_suspend(srn_fiber_park_fn commit, void *arg) {
  PANIC_IF_NULL(commit);
 
  srn_worker_t *worker = current_worker;
  PANIC_IF_NULL(worker);
 
  srn_fiber_t *self = worker->current;
  PANIC_IF_NULL(self);
 
  // The fiber carries its own commit. The worker routine runs it after we
  // switch out -- the one safe point to publish a fully suspended fiber to its
  // waker. The routine also stamps the `SUSPENDED` state once the switch
  // completes, so the `state` never marks a fiber that is still suspending.
  // This call leaves the `state` as `RUNNING` and lets the switch carry the
  // fiber off the os thread.
  self->park_commit = commit;
  self->park_arg    = arg;
  srn_fiber_switch(self, &worker->loop);
}
 
void srn_fiber_ready(srn_fiber_t *fiber) {
  PANIC_IF_NULL(fiber);
 
  // Wake a suspended fiber. The flip in `ready_fiber` lets exactly one of
  // several racing wakers enqueue it (an IO completion and a timeout firing on
  // it, say), while the rest find it no longer `SUSPENDED` and do nothing. The
  // scheduler is resolved from the fiber, not the calling os thread, so the
  // reactor -- the one legitimate waker outside the worker pool, since
  // quiescence accounts for its in-flight ops -- can wake it too. An
  // unrelated os thread must not, its pending wake is invisible to
  // quiescence, so the run can end before the wake arrives.
  ready_fiber(srn_fiber_get_scheduler_m(fiber), fiber);
}
 
srn_fiber_t *srn_fiber_current(void) {
  return current_worker != nullptr ? current_worker->current : nullptr;
}
 
srn_fiber_t *srn_fiber_worker_loop(void) {
  PANIC_IF_NULL(current_worker);
  return &current_worker->loop;
}
 
/// Add the calling fiber to the target's waiter list and stay parked, unless
/// the target has already finished, in which case decline to park so the caller
/// resumes at once. The DONE check and the list insert run together under the
/// global lock, which also guards the list against the DONE handler that drains
/// it. So this either sees the target finished and declines, or joins the list
/// before the drain and is woken by it, never lost in between.
static bool wait_for_park(srn_fiber_t *self, void *arg) {
  srn_fiber_t *target    = arg;
  srn_scheduler_t *sched = srn_fiber_get_scheduler_m(self);
 
  srn_mutex_lock(&sched->lock);
 
  if (target->state == SRN_FIBER_DONE) {
    srn_mutex_unlock(&sched->lock);
    return false;
  }
 
  self->link      = target->waiters;
  target->waiters = self;
 
  srn_mutex_unlock(&sched->lock);
  return true;
}
 
srn_fiber_result_t srn_fiber_wait_for(srn_fiber_t *target) {
  PANIC_IF_NULL(target);
  PANIC_IF(target == srn_fiber_current(), "srn_fiber_wait_for: a fiber cannot wait for itself");
 
  // Suspend until the target finishes (wait_for_park registers us on its waiter
  // list). The target's DONE handling in the worker routine wakes us. The
  // result is read from the struct, which survives the target's reap.
  srn_fiber_suspend(wait_for_park, target);
  return target->result;
}
 
void srn_fiber_autoname(srn_engine_t *engine, char *dst, size_t size) {
  PANIC_IF_NULL(engine);
  PANIC_IF_NULL(dst);
 
  srn_worker_t *w = current_worker;
  if (w != nullptr) {
    // The tag names the creating worker and its spawn count. Provenance, not
    // placement, the fiber may be stolen and run anywhere.
    (void)snprintf(dst, size, "f#%zu:%" PRIu64, (size_t)w->id, ++w->spawned);
    return;
  }
  // Off the pool there is no worker id. The tag is `m` and the engine wide
  // object id keeps names unique across every creating thread.
  (void)snprintf(dst, size, "f#m:%" PRIu64, srn_allocate_object_id(engine));
}
 
srn_worker_id_t srn_sched_current_worker_id() {
  // We use the SIZE_MAX as an idicator that there is no current
  // worker for the running os thread. (size_t)-1 == SIZE_MAX
  return current_worker == nullptr ? (srn_worker_id_t)-1 : current_worker->id;
}
 
bool srn_sched_accepting_submissions(srn_scheduler_t *sched) {
  PANIC_IF_NULL(sched);
  // Only a RUNNING scheduler takes new IO. Once DRAINING or STOPPING, the IO
  // bridge fences submissions so fibers unwind instead of parking on ops the
  // wind-down would have to wait out.
  return atomic_load(&sched->state) == SRN_SCHED_RUNNING;
}
 
void srn_sched_wake_worker(srn_scheduler_t *sched, size_t channel) {
  // TODO(lxsameer): Wake up the worker in charge of the given channel. instead
  // of waking all.
  UNUSED(channel);
  srn_mutex_lock(&sched->lock);
  srn_cond_notify_all(&sched->work);
  srn_mutex_unlock(&sched->lock);
}