Run GC if no fiber stacks are available, before expanding pool.

Author: samuel-williams-shopify

cont.c

@@ -554,6 +554,7 @@ fiber_pool_expand(struct fiber_pool * fiber_pool, size_t count)
 
             if (!VirtualProtect(page, RB_PAGE_SIZE, PAGE_READWRITE | PAGE_GUARD, &old_protect)) {
                 VirtualFree(allocation->base, 0, MEM_RELEASE);
+                ruby_xfree(allocation);
                 rb_raise(rb_eFiberError, "can't set a guard page: %s", ERRNOMSG);
             }
 #elif defined(__wasi__)
@@ -666,13 +667,23 @@ fiber_pool_allocation_free(struct fiber_pool_allocation * allocation)
 static struct fiber_pool_stack
 fiber_pool_stack_acquire(struct fiber_pool * fiber_pool)
 {
-    struct fiber_pool_vacancy * vacancy ;
+    struct fiber_pool_vacancy * vacancy;
+
     RB_VM_LOCK_ENTER();
     {
         vacancy = fiber_pool_vacancy_pop(fiber_pool);
 
         if (DEBUG) fprintf(stderr, "fiber_pool_stack_acquire: %p used=%"PRIuSIZE"\n", (void*)fiber_pool->vacancies, fiber_pool->used);
 
+        // During allocation, if we run out of stacks, we may need to collect garbage to free up some stacks before trying to allocate more.
+        if (!vacancy) {
+            if (DEBUG)fprintf(stderr, "fiber_pool_stack_acquire: no stacks available, collecting garbage\n");
+            rb_gc();
+
+            // In the worst case, we garbage collect, but all fibers are reachable, so we don't free any stacks. We will try to expand the fiber pool:
+            vacancy = fiber_pool_vacancy_pop(fiber_pool);
+        }
+
         if (!vacancy) {
             const size_t maximum = FIBER_POOL_ALLOCATION_MAXIMUM_SIZE;
             const size_t minimum = fiber_pool->initial_count;
@@ -681,6 +692,7 @@ fiber_pool_stack_acquire(struct fiber_pool * fiber_pool)
             if (count > maximum) count = maximum;
             if (count < minimum) count = minimum;
 
+            if (DEBUG)fprintf(stderr, "fiber_pool_stack_acquire: expanding fiber pool to %"PRIuSIZE" stacks\n", count);
             fiber_pool_expand(fiber_pool, count);
 
             // The free list should now contain some stacks: