cell/source/scheduler.c

#include <pthread.h>
#include <sys/time.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include <stdatomic.h>

#include "stb_ds.h"
#include "cell.h"
#include "quickjs-internal.h"
#include "cell_internal.h"

#ifdef _WIN32
#include <windows.h>
#endif

typedef struct actor_node {
    cell_rt *actor;
    struct actor_node *next;
} actor_node;

typedef enum {
    TIMER_JS,
    TIMER_NATIVE_REMOVE,
    TIMER_PAUSE,
    TIMER_KILL
} timer_type;

typedef struct {
    uint64_t execute_at_ns;
    cell_rt *actor;
    uint32_t timer_id;
    timer_type type;
    uint32_t turn_gen;  /* generation at registration time */
} timer_node;

static timer_node *timer_heap = NULL; 

// Priority queue indices
#define PQ_READY     0
#define PQ_REFRESHED 1
#define PQ_EXHAUSTED 2
#define PQ_SLOW      3
#define PQ_COUNT     4

// --- 3. The Global Engine State ---
static struct {
    pthread_mutex_t lock;       // Protects queue, shutdown flag, and timers
    pthread_cond_t  wake_cond;  // Wakes up workers
    pthread_cond_t  timer_cond; // Wakes up the timer thread
    pthread_cond_t  main_cond;  // Wakes up the main thread

    actor_node *q_head[PQ_COUNT], *q_tail[PQ_COUNT];     // worker queues
    actor_node *mq_head[PQ_COUNT], *mq_tail[PQ_COUNT];   // main-thread queues

    int shutting_down;
    int quiescence_enabled;     // set after bootstrap, before actor_loop
    _Atomic int quiescent_count; // actors idle with no messages and no timers

    pthread_t *worker_threads;
    int num_workers;
    pthread_t timer_thread;
} engine;

static int state_to_pq(int state) {
  switch (state) {
  case ACTOR_REFRESHED: return PQ_REFRESHED;
  case ACTOR_EXHAUSTED: return PQ_EXHAUSTED;
  case ACTOR_SLOW:      return PQ_SLOW;
  default:              return PQ_READY;
  }
}

static actor_node *dequeue_priority(actor_node *heads[], actor_node *tails[]) {
  for (int i = 0; i < PQ_COUNT; i++) {
    if (heads[i]) {
      actor_node *n = heads[i];
      heads[i] = n->next;
      if (!heads[i]) tails[i] = NULL;
      return n;
    }
  }
  return NULL;
}

static int has_any_work(actor_node *heads[]) {
  for (int i = 0; i < PQ_COUNT; i++)
    if (heads[i]) return 1;
  return 0;
}

static pthread_mutex_t *actors_mutex;
static struct { char *key; cell_rt *value; } *actors = NULL;

#define lockless_shdel(NAME, KEY) pthread_mutex_lock(NAME##_mutex); shdel(NAME, KEY); pthread_mutex_unlock(NAME##_mutex);
#define lockless_shlen(NAME) ({ \
  pthread_mutex_lock(NAME##_mutex); \
  size_t _len = shlen(NAME); \
  pthread_mutex_unlock(NAME##_mutex); \
  _len; \
})
#define lockless_shgeti(NAME, KEY) ({ \
  pthread_mutex_lock(NAME##_mutex); \
  int _idx = shgeti(NAME, KEY); \
  pthread_mutex_unlock(NAME##_mutex); \
  _idx; \
})
#define lockless_shget(NAME, KEY) ({ \
  pthread_mutex_lock(NAME##_mutex); \
  cell_rt *_actor = shget(NAME, KEY); \
  pthread_mutex_unlock(NAME##_mutex); \
  _actor; \
})
#define lockless_shput_unique(NAME, KEY, VALUE) ({ \
  pthread_mutex_lock(NAME##_mutex); \
  int _exists = shgeti(NAME, KEY) != -1; \
  if (!_exists) shput(NAME, KEY, VALUE); \
  pthread_mutex_unlock(NAME##_mutex); \
  !_exists; \
})

// Forward declarations
uint32_t actor_remove_cb(cell_rt *actor, uint32_t id, uint32_t interval);
void actor_turn(cell_rt *actor);

void heap_push(uint64_t when, cell_rt *actor, uint32_t timer_id, timer_type type) {
    timer_node node = { .execute_at_ns = when, .actor = actor, .timer_id = timer_id, .type = type, .turn_gen = 0 };
    if (type == TIMER_PAUSE || type == TIMER_KILL)
      node.turn_gen = atomic_load_explicit(&actor->turn_gen, memory_order_relaxed);
    arrput(timer_heap, node);
    
    // Bubble up
    int i = arrlen(timer_heap) - 1;
    while (i > 0) {
        int parent = (i - 1) / 2;
        if (timer_heap[i].execute_at_ns >= timer_heap[parent].execute_at_ns) break;
        
        timer_node tmp = timer_heap[i];
        timer_heap[i] = timer_heap[parent];
        timer_heap[parent] = tmp;
        i = parent;
    }
}

// Helper: Heap Pop
int heap_pop(timer_node *out) {
    if (arrlen(timer_heap) == 0) return 0;
    
    *out = timer_heap[0];
    timer_node last = arrpop(timer_heap);
    
    if (arrlen(timer_heap) > 0) {
        timer_heap[0] = last;
        // Bubble down
        int i = 0;
        int n = arrlen(timer_heap);
        while (1) {
            int left = 2 * i + 1;
            int right = 2 * i + 2;
            int smallest = i;
            
            if (left < n && timer_heap[left].execute_at_ns < timer_heap[smallest].execute_at_ns)
                smallest = left;
            if (right < n && timer_heap[right].execute_at_ns < timer_heap[smallest].execute_at_ns)
                smallest = right;
            
            if (smallest == i) break;
            
            timer_node tmp = timer_heap[i];
            timer_heap[i] = timer_heap[smallest];
            timer_heap[smallest] = tmp;
            i = smallest;
        }
    }
    return 1;
}

void *timer_thread_func(void *arg) {
  while (1) {
    pthread_mutex_lock(&engine.lock);
    
    if (engine.shutting_down) {
      pthread_mutex_unlock(&engine.lock);
      return NULL;
    }

    if (arrlen(timer_heap) == 0) {
      pthread_cond_wait(&engine.timer_cond, &engine.lock);
    } else {
      uint64_t now = cell_ns();
      if (timer_heap[0].execute_at_ns <= now) {
        // --- TIMER FIRED ---
        timer_node t;
        heap_pop(&t);
        pthread_mutex_unlock(&engine.lock);

        if (t.type == TIMER_NATIVE_REMOVE) {
          actor_remove_cb(t.actor, t.timer_id, 0);
        } else if (t.type == TIMER_PAUSE || t.type == TIMER_KILL) {
          /* Only fire if turn_gen still matches (stale timers are ignored) */
          uint32_t cur = atomic_load_explicit(&t.actor->turn_gen, memory_order_relaxed);
          if (cur == t.turn_gen) {
            if (t.type == TIMER_PAUSE) {
              JS_SetPauseFlag(t.actor->context, 1);
            } else {
              t.actor->disrupt = 1;
              JS_SetPauseFlag(t.actor->context, 2);
            }
          }
        } else {
          pthread_mutex_lock(t.actor->msg_mutex);
          int idx = hmgeti(t.actor->timers, t.timer_id);
          if (idx != -1) {
            JSValue cb = t.actor->timers[idx].value;
            hmdel(t.actor->timers, t.timer_id);
            actor_clock(t.actor, cb);
            JS_FreeValue(t.actor->context, cb);
          }
          pthread_mutex_unlock(t.actor->msg_mutex);
        }
        
        continue; 
      } else {
        // --- WAIT FOR DEADLINE ---
        struct timespec ts;
        uint64_t wait_ns = timer_heap[0].execute_at_ns - now;
        
        // Convert relative wait time to absolute CLOCK_REALTIME
        struct timespec now_real;
        clock_gettime(CLOCK_REALTIME, &now_real);
        
        uint64_t deadline_real_ns = (uint64_t)now_real.tv_sec * 1000000000ULL + 
                                     (uint64_t)now_real.tv_nsec + wait_ns;
        ts.tv_sec = deadline_real_ns / 1000000000ULL;
        ts.tv_nsec = deadline_real_ns % 1000000000ULL;
        
        pthread_cond_timedwait(&engine.timer_cond, &engine.lock, &ts);
      }
    }
    pthread_mutex_unlock(&engine.lock);
  }
  return NULL;
}

void enqueue_actor_priority(cell_rt *actor) {
  actor_node *n = malloc(sizeof(actor_node));
  n->actor = actor;
  n->next = NULL;
  int pq = state_to_pq(actor->state);
  pthread_mutex_lock(&engine.lock);
  if (actor->main_thread_only) {
    if (engine.mq_tail[pq]) engine.mq_tail[pq]->next = n;
    else engine.mq_head[pq] = n;
    engine.mq_tail[pq] = n;
    pthread_cond_signal(&engine.main_cond);
  } else {
    if (engine.q_tail[pq]) engine.q_tail[pq]->next = n;
    else engine.q_head[pq] = n;
    engine.q_tail[pq] = n;
    pthread_cond_signal(&engine.wake_cond);
  }
  pthread_mutex_unlock(&engine.lock);
}

void *actor_runner(void *arg) {
  while (1) {
    pthread_mutex_lock(&engine.lock);

    while (!has_any_work(engine.q_head) && !engine.shutting_down) {
      pthread_cond_wait(&engine.wake_cond, &engine.lock);
    }

    if (engine.shutting_down && !has_any_work(engine.q_head)) {
      pthread_mutex_unlock(&engine.lock);
      break;
    }

    actor_node *node = dequeue_priority(engine.q_head, engine.q_tail);
    pthread_mutex_unlock(&engine.lock);

    if (node) {
      actor_turn(node->actor);
      free(node);
    }
  }
  return NULL;
}

void actor_initialize(void) {
    pthread_mutex_init(&engine.lock, NULL);
    pthread_cond_init(&engine.wake_cond, NULL);
    pthread_cond_init(&engine.timer_cond, NULL);
    pthread_cond_init(&engine.main_cond, NULL);

    engine.shutting_down = 0;
    for (int i = 0; i < PQ_COUNT; i++) {
      engine.q_head[i] = NULL;
      engine.q_tail[i] = NULL;
      engine.mq_head[i] = NULL;
      engine.mq_tail[i] = NULL;
    }
    
    actors_mutex = malloc(sizeof(pthread_mutex_t));
    pthread_mutex_init(actors_mutex, NULL);
    
    // Start Timer Thread
    pthread_create(&engine.timer_thread, NULL, timer_thread_func, NULL);

    // Start Workers
#ifdef _WIN32
    SYSTEM_INFO sysinfo;
    GetSystemInfo(&sysinfo);
    long n = sysinfo.dwNumberOfProcessors;
#else
    long n = sysconf(_SC_NPROCESSORS_ONLN);
#endif
    engine.num_workers = (int)n;
    engine.worker_threads = malloc(sizeof(pthread_t) * n);
    for (int i=0; i < n; i++) {
        pthread_create(&engine.worker_threads[i], NULL, actor_runner, NULL);
    }
}

void actor_free(cell_rt *actor)
{
  if (actor->is_quiescent) {
    actor->is_quiescent = 0;
    atomic_fetch_sub(&engine.quiescent_count, 1);
  }
  lockless_shdel(actors, actor->id);

  // Purge any pending timers referencing this actor from the timer heap
  // to prevent the timer thread from accessing freed memory.
  {
    pthread_mutex_lock(&engine.lock);
    int n = arrlen(timer_heap);
    int w = 0;
    for (int r = 0; r < n; r++) {
      if (timer_heap[r].actor != actor)
        timer_heap[w++] = timer_heap[r];
    }
    arrsetlen(timer_heap, w);
    for (int i = w / 2 - 1; i >= 0; i--) {
      int j = i;
      while (1) {
        int left = 2 * j + 1, right = 2 * j + 2, smallest = j;
        if (left < w && timer_heap[left].execute_at_ns < timer_heap[smallest].execute_at_ns)
          smallest = left;
        if (right < w && timer_heap[right].execute_at_ns < timer_heap[smallest].execute_at_ns)
          smallest = right;
        if (smallest == j) break;
        timer_node tmp = timer_heap[j];
        timer_heap[j] = timer_heap[smallest];
        timer_heap[smallest] = tmp;
        j = smallest;
      }
    }
    pthread_mutex_unlock(&engine.lock);
  }

  // Do not go forward with actor destruction until the actor is completely free
  pthread_mutex_lock(actor->msg_mutex);
  pthread_mutex_lock(actor->mutex);

  JSContext *js = actor->context;
  
  JS_DeleteGCRef(js, &actor->idx_buffer_ref);
  JS_DeleteGCRef(js, &actor->on_exception_ref);
  JS_DeleteGCRef(js, &actor->message_handle_ref);
  JS_DeleteGCRef(js, &actor->unneeded_ref);
  JS_DeleteGCRef(js, &actor->actor_sym_ref);
  
  for (int i = 0; i < hmlen(actor->timers); i++) {
    JS_FreeValue(js, actor->timers[i].value);
  }
  
  hmfree(actor->timers);
  
  /* Free all letters in the queue */
  for (int i = 0; i < arrlen(actor->letters); i++) {
    if (actor->letters[i].type == LETTER_BLOB) {
      blob_destroy(actor->letters[i].blob_data);
    } else if (actor->letters[i].type == LETTER_CALLBACK) {
      JS_FreeValue(js, actor->letters[i].callback);
    }
  }
  
  arrfree(actor->letters);
  
  JS_FreeContext(js);
  free(actor->id);
  
  pthread_mutex_unlock(actor->mutex);
  pthread_mutex_destroy(actor->mutex);
  free(actor->mutex);
  pthread_mutex_unlock(actor->msg_mutex);
  pthread_mutex_destroy(actor->msg_mutex);
  free(actor->msg_mutex);
  
  free(actor);

  int actor_count = lockless_shlen(actors);
  if (actor_count == 0) {
    pthread_mutex_lock(&engine.lock);
    engine.shutting_down = 1;
    pthread_cond_broadcast(&engine.wake_cond);
    pthread_cond_broadcast(&engine.timer_cond);
    pthread_cond_broadcast(&engine.main_cond);
    pthread_mutex_unlock(&engine.lock);
  }
}

int scheduler_actor_count(void) {
  return (int)lockless_shlen(actors);
}

void scheduler_enable_quiescence(void) {
  engine.quiescence_enabled = 1;
  // Check if all actors are already quiescent
  int qc = atomic_load(&engine.quiescent_count);
  int total = (int)lockless_shlen(actors);
  if (qc >= total && total > 0) {
    pthread_mutex_lock(&engine.lock);
    engine.shutting_down = 1;
    pthread_cond_broadcast(&engine.wake_cond);
    pthread_cond_broadcast(&engine.timer_cond);
    pthread_cond_broadcast(&engine.main_cond);
    pthread_mutex_unlock(&engine.lock);
  }
}

void exit_handler(void) {
    static int already_exiting = 0;
    if (already_exiting) return;
    already_exiting = 1;

    pthread_mutex_lock(&engine.lock);
    engine.shutting_down = 1;
    pthread_cond_broadcast(&engine.wake_cond);
    pthread_cond_broadcast(&engine.timer_cond);
    pthread_cond_broadcast(&engine.main_cond);
    pthread_mutex_unlock(&engine.lock);

    pthread_join(engine.timer_thread, NULL);

    for (int i=0; i < engine.num_workers; i++) {
        pthread_join(engine.worker_threads[i], NULL);
    }

    free(engine.worker_threads);
    pthread_mutex_destroy(&engine.lock);
    pthread_cond_destroy(&engine.wake_cond);
    pthread_cond_destroy(&engine.timer_cond);
    pthread_cond_destroy(&engine.main_cond);

    pthread_mutex_destroy(actors_mutex);
    free(actors_mutex);

    arrfree(timer_heap);
}

int actor_exists(const char *id)
{
  int idx = lockless_shgeti(actors, id);
  return idx != -1;
}

void set_actor_state(cell_rt *actor)
{
  if (actor->disrupt) {
#ifdef SCHEDULER_DEBUG
    fprintf(stderr, "set_actor_state: %s disrupted, freeing\n", actor->name ? actor->name : actor->id);
#endif
    actor_free(actor);
    return;
  }
  pthread_mutex_lock(actor->msg_mutex);

#ifdef SCHEDULER_DEBUG
  fprintf(stderr, "set_actor_state: %s state=%d letters=%ld\n", actor->name ? actor->name : actor->id, actor->state, (long)arrlen(actor->letters));
#endif
  switch(actor->state) {
    case ACTOR_RUNNING:
    case ACTOR_READY:
    if (actor->ar)
      actor->ar = 0;
    break;

    case ACTOR_IDLE:
      if (arrlen(actor->letters)) {
        if (actor->is_quiescent) {
          actor->is_quiescent = 0;
          atomic_fetch_sub(&engine.quiescent_count, 1);
        }
#ifdef SCHEDULER_DEBUG
        fprintf(stderr, "set_actor_state: %s IDLE->READY, enqueueing (main=%d)\n", actor->name ? actor->name : actor->id, actor->main_thread_only);
#endif
        actor->state = ACTOR_READY;
        actor->ar = 0;
        enqueue_actor_priority(actor);

      } else if (!hmlen(actor->timers)) {
          // No messages AND no timers
          // Only count as quiescent if no $unneeded callback registered
          int has_unneeded = !JS_IsNull(actor->unneeded_ref.val);
          if (!actor->is_quiescent && actor->id && !has_unneeded) {
            actor->is_quiescent = 1;
            int qc = atomic_fetch_add(&engine.quiescent_count, 1) + 1;
            int total = (int)lockless_shlen(actors);
            if (qc >= total && total > 0 && engine.quiescence_enabled) {
              pthread_mutex_lock(&engine.lock);
              engine.shutting_down = 1;
              pthread_cond_broadcast(&engine.wake_cond);
              pthread_cond_broadcast(&engine.timer_cond);
              pthread_cond_broadcast(&engine.main_cond);
              pthread_mutex_unlock(&engine.lock);
            }
          }

          if (!engine.shutting_down) {
            // Schedule remove timer
            static uint32_t global_timer_id = 1;
            uint32_t id = global_timer_id++;
            actor->ar = id;

            uint64_t now = cell_ns();
            uint64_t execute_at = now + (uint64_t)(actor->ar_secs * 1e9);

            pthread_mutex_lock(&engine.lock);
            heap_push(execute_at, actor, id, TIMER_NATIVE_REMOVE);
            if (timer_heap[0].timer_id == id) {
                pthread_cond_signal(&engine.timer_cond);
            }
            pthread_mutex_unlock(&engine.lock);
          }
      } else {
          // Has timers but no letters — waiting, not quiescent
          if (actor->is_quiescent) {
            actor->is_quiescent = 0;
            atomic_fetch_sub(&engine.quiescent_count, 1);
          }
      }
      break;
  }

  pthread_mutex_unlock(actor->msg_mutex);
}

uint32_t actor_remove_cb(cell_rt *actor, uint32_t id, uint32_t interval)
{
  pthread_mutex_lock(actor->mutex);
  // Check if this timer is still valid (match actor->ar)
  if (actor->ar != id && id != 0) { // id 0 means force (optional)
      pthread_mutex_unlock(actor->mutex);
      return 0;
  }
  
  actor->disrupt = 1;
  
  if (!JS_IsNull(actor->unneeded_ref.val)) {
    JSValue ret = JS_Call(actor->context, actor->unneeded_ref.val, JS_NULL, 0, NULL);
    uncaught_exception(actor->context, ret);
  }

  int should_free = (actor->state == ACTOR_IDLE);
  pthread_mutex_unlock(actor->mutex);
  
  if (should_free) actor_free(actor);
  return 0;
}

void actor_unneeded(cell_rt *actor, JSValue fn, double seconds)
{
  if (actor->disrupt) return;

  if (!JS_IsFunction(fn)) {
    actor->unneeded_ref.val = JS_NULL;
    goto END;
  }

  actor->unneeded_ref.val = fn;
  actor->ar_secs = seconds;
  
  END:
  if (actor->ar)
    actor->ar = 0;
  set_actor_state(actor);
}

cell_rt *get_actor(char *id)
{
  int idx = lockless_shgeti(actors, id);
  if (idx == -1) {
    return NULL;
  }

  return lockless_shget(actors, id);
}

void actor_loop()
{
  while (!engine.shutting_down) {
    pthread_mutex_lock(&engine.lock);
    while (!has_any_work(engine.mq_head) && !engine.shutting_down) {
      pthread_cond_wait(&engine.main_cond, &engine.lock);
    }

    if (engine.shutting_down && !has_any_work(engine.mq_head)) {
      pthread_mutex_unlock(&engine.lock);
      break;
    }

    actor_node *node = dequeue_priority(engine.mq_head, engine.mq_tail);
    pthread_mutex_unlock(&engine.lock);

    if (node) {
      actor_turn(node->actor);
      free(node);
    }
  }
}

cell_rt *create_actor(void *wota)
{
  cell_rt *actor = calloc(sizeof(*actor), 1);
#ifdef HAVE_MIMALLOC
  actor->heap = mi_heap_new();
#endif
  actor->init_wota = wota;
  /* GCRef fields are registered after JSContext creation in script_startup.
     For now, zero-init from calloc is sufficient (val = 0 = JS_MKVAL(JS_TAG_INT,0),
     which is not a pointer so GC-safe). The actual JS_NULL assignment and
     JS_AddGCRef happen in script_startup. */

  arrsetcap(actor->letters, 5);

  actor->mutex = malloc(sizeof(pthread_mutex_t));
  pthread_mutexattr_t attr;
  pthread_mutexattr_init(&attr);
  pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
  pthread_mutex_init(actor->mutex, &attr);
  actor->msg_mutex = malloc(sizeof(pthread_mutex_t));
  pthread_mutex_init(actor->msg_mutex, &attr); // msg_mutex can be recursive too to be safe
  pthread_mutexattr_destroy(&attr);
  
  /* Lock actor->mutex while initializing JS runtime. */
  pthread_mutex_lock(actor->mutex);
  script_startup(actor);
  set_actor_state(actor);
  pthread_mutex_unlock(actor->mutex);  

  return actor;
}

const char *register_actor(const char *id, cell_rt *actor, int mainthread, double ar)
{
  actor->main_thread_only = mainthread;
  actor->id = strdup(id);
  actor->ar_secs = ar;
  int added = lockless_shput_unique(actors, actor->id, actor);
  if (!added) {
    free(actor->id);
    return "Actor with given ID already exists.";
  }
  // Now that actor is in the registry, track its quiescent state
  if (actor->state == ACTOR_IDLE && !arrlen(actor->letters)
      && !hmlen(actor->timers) && JS_IsNull(actor->unneeded_ref.val)) {
    actor->is_quiescent = 1;
    atomic_fetch_add(&engine.quiescent_count, 1);
  }
  return NULL;
}


const char *send_message(const char *id, void *msg)
{
  cell_rt *target = get_actor(id);
  if (!target) {
    blob_destroy((blob *)msg);
    return "Could not get actor from id.";
  }
  
  letter l;
  l.type = LETTER_BLOB;
  l.blob_data = (blob *)msg;
  
  pthread_mutex_lock(target->msg_mutex);
  arrput(target->letters, l);
  pthread_mutex_unlock(target->msg_mutex);

  if (target->ar)
    target->ar = 0;
  
  set_actor_state(target);
  
  return NULL;
}

void actor_turn(cell_rt *actor)
{
  pthread_mutex_lock(actor->mutex);
#ifdef ACTOR_TRACE
  int prev_state = actor->state;
#endif
  actor->state = ACTOR_RUNNING;
#ifdef ACTOR_TRACE
  fprintf(stderr, "[ACTOR_TRACE] %s: %d -> RUNNING\n",
          actor->name ? actor->name : actor->id, prev_state);
#endif

  if (!actor->trace_hook) {
    cell_hook gh = cell_trace_gethook();
    if (gh) actor->trace_hook = gh;
  }

  if (actor->trace_hook)
    actor->trace_hook(actor, CELL_HOOK_ENTER);

  JSValue result;

  if (actor->vm_suspended) {
    /* RESUME path: continue suspended turn under kill timer only */
    g_crash_ctx = actor->context;
    atomic_fetch_add_explicit(&actor->turn_gen, 1, memory_order_relaxed);
    JS_SetPauseFlag(actor->context, 0);
    actor->turn_start_ns = cell_ns();

    /* Register kill timer only for resume */
    pthread_mutex_lock(&engine.lock);
    heap_push(actor->turn_start_ns + ACTOR_SLOW_TIMER_NS,
              actor, 0, TIMER_KILL);
    pthread_cond_signal(&engine.timer_cond);
    pthread_mutex_unlock(&engine.lock);

    result = JS_ResumeRegisterVM(actor->context);
    actor->vm_suspended = 0;

    if (JS_IsSuspended(result)) {
      /* Still suspended after kill timer — shouldn't happen, kill it */
      actor->disrupt = 1;
      goto ENDTURN;
    }
    if (JS_IsException(result)) {
      if (!uncaught_exception(actor->context, result))
        actor->disrupt = 1;
    }
    actor->slow_strikes++;
#ifdef ACTOR_TRACE
    fprintf(stderr, "[ACTOR_TRACE] %s: slow strike %d/%d\n",
            actor->name ? actor->name : actor->id,
            actor->slow_strikes, ACTOR_SLOW_STRIKES_MAX);
#endif
    if (actor->slow_strikes >= ACTOR_SLOW_STRIKES_MAX) {
#ifdef ACTOR_TRACE
      fprintf(stderr, "[ACTOR_TRACE] %s: %d slow strikes, killing\n",
              actor->name ? actor->name : actor->id, actor->slow_strikes);
#endif
      actor->disrupt = 1;
    }
    goto ENDTURN;
  }

  /* NORMAL path: pop a message and execute */
  pthread_mutex_lock(actor->msg_mutex);
  int pending = arrlen(actor->letters);
  if (!pending) {
    pthread_mutex_unlock(actor->msg_mutex);
    goto ENDTURN;
  }
#ifdef SCHEDULER_DEBUG
  fprintf(stderr, "actor_turn: %s has %d letters, type=%d\n",
          actor->name ? actor->name : actor->id, pending, actor->letters[0].type);
#endif
  letter l = actor->letters[0];
  arrdel(actor->letters, 0);
  pthread_mutex_unlock(actor->msg_mutex);

  atomic_fetch_add_explicit(&actor->turn_gen, 1, memory_order_relaxed);
  JS_SetPauseFlag(actor->context, 0);
  actor->turn_start_ns = cell_ns();

  /* Register both pause and kill timers */
  pthread_mutex_lock(&engine.lock);
  heap_push(actor->turn_start_ns + ACTOR_FAST_TIMER_NS,
            actor, 0, TIMER_PAUSE);
  heap_push(actor->turn_start_ns + ACTOR_SLOW_TIMER_NS + ACTOR_FAST_TIMER_NS,
            actor, 0, TIMER_KILL);
  pthread_cond_signal(&engine.timer_cond);
  pthread_mutex_unlock(&engine.lock);

  g_crash_ctx = actor->context;

  if (l.type == LETTER_BLOB) {
    size_t size = blob_length(l.blob_data) / 8;
    JSValue arg = js_new_blob_stoned_copy(actor->context,
                    (void *)blob_data(l.blob_data), size);
    blob_destroy(l.blob_data);
    result = JS_Call(actor->context, actor->message_handle_ref.val,
                     JS_NULL, 1, &arg);
    if (JS_IsSuspended(result)) {
      actor->vm_suspended = 1;
      actor->state = ACTOR_SLOW;
      JS_FreeValue(actor->context, arg);
      goto ENDTURN_SLOW;
    }
    if (!uncaught_exception(actor->context, result))
      actor->disrupt = 1;
    JS_FreeValue(actor->context, arg);
  } else if (l.type == LETTER_CALLBACK) {
    result = JS_Call(actor->context, l.callback, JS_NULL, 0, NULL);
    if (JS_IsSuspended(result)) {
      actor->vm_suspended = 1;
      actor->state = ACTOR_SLOW;
      JS_FreeValue(actor->context, l.callback);
      goto ENDTURN_SLOW;
    }
    if (!uncaught_exception(actor->context, result))
      actor->disrupt = 1;
    JS_FreeValue(actor->context, l.callback);
  }

  if (actor->disrupt) goto ENDTURN;
  actor->slow_strikes = 0; /* completed within fast timer */

ENDTURN:
  g_crash_ctx = NULL;
  /* Invalidate any outstanding pause/kill timers for this turn */
  atomic_fetch_add_explicit(&actor->turn_gen, 1, memory_order_relaxed);
  actor->state = ACTOR_IDLE;

  if (actor->trace_hook)
    actor->trace_hook(actor, CELL_HOOK_EXIT);

  if (actor->disrupt) {
#ifdef SCHEDULER_DEBUG
    fprintf(stderr, "actor_turn ENDTURN: %s disrupted, freeing\n",
            actor->name ? actor->name : actor->id);
#endif
    pthread_mutex_unlock(actor->mutex);
    actor_free(actor);
    return;
  }

#ifdef SCHEDULER_DEBUG
  fprintf(stderr, "actor_turn ENDTURN: %s has %ld letters, calling set_actor_state\n",
          actor->name ? actor->name : actor->id, (long)arrlen(actor->letters));
#endif
  set_actor_state(actor);
  pthread_mutex_unlock(actor->mutex);
  return;

ENDTURN_SLOW:
  g_crash_ctx = NULL;
#ifdef ACTOR_TRACE
  fprintf(stderr, "[ACTOR_TRACE] %s: suspended mid-turn -> SLOW\n",
          actor->name ? actor->name : actor->id);
#endif
  if (actor->trace_hook)
    actor->trace_hook(actor, CELL_HOOK_EXIT);
  enqueue_actor_priority(actor);
  pthread_mutex_unlock(actor->mutex);
}

/* GC callback: scan actor's letters and timers so the copying GC
   can relocate JSValues stored in C-side data structures. */
void actor_gc_scan(JSContext *ctx,
  uint8_t *from_base, uint8_t *from_end,
  uint8_t *to_base, uint8_t **to_free, uint8_t *to_end)
{
  cell_rt *actor = JS_GetContextOpaque(ctx);
  if (!actor) return;

  /* Lock msg_mutex to synchronize with the timer thread, which reads
     timers and writes letters under the same lock. */
  pthread_mutex_lock(actor->msg_mutex);

  for (int i = 0; i < arrlen(actor->letters); i++) {
    if (actor->letters[i].type == LETTER_CALLBACK) {
      actor->letters[i].callback = gc_copy_value(ctx,
        actor->letters[i].callback,
        from_base, from_end, to_base, to_free, to_end);
    }
  }

  for (int i = 0; i < hmlen(actor->timers); i++) {
    actor->timers[i].value = gc_copy_value(ctx,
      actor->timers[i].value,
      from_base, from_end, to_base, to_free, to_end);
  }

  pthread_mutex_unlock(actor->msg_mutex);
}

void actor_clock(cell_rt *actor, JSValue fn)
{
  letter l;
  l.type = LETTER_CALLBACK;
  l.callback = JS_DupValue(actor->context, fn);
  pthread_mutex_lock(actor->msg_mutex);
  arrput(actor->letters, l);
  pthread_mutex_unlock(actor->msg_mutex);
  set_actor_state(actor);
}

uint32_t actor_delay(cell_rt *actor, JSValue fn, double seconds)
{
  pthread_mutex_lock(actor->msg_mutex); 
  
  static uint32_t global_timer_id = 1;
  uint32_t id = global_timer_id++;
  
  JSValue cb = JS_DupValue(actor->context, fn);
  hmput(actor->timers, id, cb);
  
  uint64_t now = cell_ns();
  uint64_t execute_at = now + (uint64_t)(seconds * 1e9);

  pthread_mutex_lock(&engine.lock);
  heap_push(execute_at, actor, id, TIMER_JS);
  if (timer_heap[0].timer_id == id) {
      pthread_cond_signal(&engine.timer_cond);
  }
  pthread_mutex_unlock(&engine.lock);

  pthread_mutex_unlock(actor->msg_mutex);
  return id;
}

JSValue actor_remove_timer(cell_rt *actor, uint32_t timer_id)
{
  JSValue cb = JS_NULL;
  pthread_mutex_lock(actor->msg_mutex);
  int id = hmgeti(actor->timers, timer_id);
  if (id != -1) {
    cb = actor->timers[id].value;
    hmdel(actor->timers, timer_id);
  }
  pthread_mutex_unlock(actor->msg_mutex);
  // Note: We don't remove from heap, it will misfire safely
  return cb;
}