Take safepoint lock before going to sleep in the scheduler.

This avoids a deadlock during exit. Between a thread going to sleep and the thread exiting

src/julia_internal.h

@@ -1129,7 +1129,7 @@ void jl_safepoint_end_gc(void);
 // The caller should set it **BEFORE** calling this function.
 void jl_safepoint_wait_gc(void) JL_NOTSAFEPOINT;
 void jl_safepoint_wait_thread_resume(void) JL_NOTSAFEPOINT;
-
+int8_t jl_safepoint_take_sleep_lock(jl_ptls_t ptls) JL_NOTSAFEPOINT_ENTER;
 // Set pending sigint and enable the mechanisms to deliver the sigint.
 void jl_safepoint_enable_sigint(void);
 // If the safepoint is enabled to deliver sigint, disable it

src/safepoint.c

@@ -276,6 +276,21 @@ void jl_safepoint_wait_thread_resume(void)
     jl_atomic_store_release(&ct->ptls->gc_state, state);
     uv_mutex_unlock(&ct->ptls->sleep_lock);
 }
+// This takes the sleep lock and puts the thread in GC_SAFE
+int8_t jl_safepoint_take_sleep_lock(jl_ptls_t ptls)
+{
+    int8_t gc_state = jl_gc_safe_enter(ptls);
+    uv_mutex_lock(&ptls->sleep_lock);
+    if (jl_atomic_load_relaxed(&ptls->suspend_count)) {
+        // defer this broadcast until we determine whether uv_cond_wait is really going to be needed
+        uv_mutex_unlock(&ptls->sleep_lock);
+        uv_mutex_lock(&safepoint_lock);
+        uv_cond_broadcast(&safepoint_cond_begin);
+        uv_mutex_unlock(&safepoint_lock);
+        uv_mutex_lock(&ptls->sleep_lock);
+    }
+    return gc_state;
+}
 
 // n.b. suspended threads may still run in the GC or GC safe regions
 // but shouldn't be observable, depending on which enum the user picks (only 1 and 2 are typically recommended here)

src/scheduler.c

@@ -499,8 +499,7 @@ JL_DLLEXPORT jl_task_t *jl_task_get_next(jl_value_t *trypoptask, jl_value_t *q,
 
                 // the other threads will just wait for an individual wake signal to resume
                 JULIA_DEBUG_SLEEPWAKE( ptls->sleep_enter = cycleclock() );
-                int8_t gc_state = jl_gc_safe_enter(ptls);
-                uv_mutex_lock(&ptls->sleep_lock);
+                int8_t gc_state = jl_safepoint_take_sleep_lock(ptls); // This puts the thread in GC_SAFE and takes the sleep lock
                 while (may_sleep(ptls)) {
                     if (ptls->tid == 0) {
                         task = wait_empty;