Fix growth in minor GC when we have initial slots

If initial slots is set, then during a minor GC, if we have allocatable
pages but the heap is mostly full, then we will set `grow_heap` to true
since `total_slots` does not count allocatable pages so it will be less
than `init_slots`. This can cause `allocatable_pages` to grow to much
higher than desired since it will appear that the heap is mostly full.

gc.c

@@ -5689,12 +5689,11 @@ gc_sweep_finish_size_pool(rb_objspace_t *objspace, rb_size_pool_t *size_pool)
     if (swept_slots < min_free_slots) {
         bool grow_heap = is_full_marking(objspace);
 
-        if (!is_full_marking(objspace)) {
-            /* The heap is a growth heap if it freed more slots than had empty
-             * slots and used up all of its allocatable pages. */
-            bool is_growth_heap = (size_pool->empty_slots == 0 ||
-                                       size_pool->freed_slots > size_pool->empty_slots) &&
-                                   size_pool->allocatable_pages == 0;
+        /* Consider growing or starting a major GC if we are not currently in a
+         * major GC and we can't allocate any more pages. */
+        if (!is_full_marking(objspace) && size_pool->allocatable_pages == 0) {
+            /* The heap is a growth heap if it freed more slots than had empty slots. */
+            bool is_growth_heap = size_pool->empty_slots == 0 || size_pool->freed_slots > size_pool->empty_slots;
 
             /* Grow this heap if we haven't run at least RVALUE_OLD_AGE minor
              * GC since the last major GC or if this heap is smaller than the

test/ruby/test_gc.rb

@@ -492,6 +492,45 @@ class TestGc < Test::Unit::TestCase
         assert_in_epsilon(SIZES[i], total_slots, 0.01, s)
       end
     RUBY
+
+    # Check that we don't grow the heap in minor GC if we have alloctable pages.
+    env["RUBY_GC_HEAP_FREE_SLOTS_MIN_RATIO"] = "0.3"
+    env["RUBY_GC_HEAP_FREE_SLOTS_GOAL_RATIO"] = "0.99"
+    env["RUBY_GC_HEAP_FREE_SLOTS_MAX_RATIO"] = "1.0"
+    env["RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR"] = "100" # Large value to disable major GC
+    assert_separately([env, "-W0"], __FILE__, __LINE__, <<~RUBY)
+      SIZES = #{sizes}
+
+      # Run a major GC to clear out dead objects.
+      GC.start
+
+      # Disable GC so we can control when GC is ran.
+      GC.disable
+
+      # Run minor GC enough times so that we don't grow the heap because we
+      # haven't yet ran RVALUE_OLD_AGE minor GC cycles.
+      GC::INTERNAL_CONSTANTS[:RVALUE_OLD_AGE].times { GC.start(full_mark: false) }
+
+      # Fill size pool 0 to over 50% full so that the number of allocatable
+      # pages that will be created will be over the number in heap_allocatable_pages
+      # (calculated using RUBY_GC_HEAP_FREE_SLOTS_MIN_RATIO).
+      # 70% was chosen here to guarantee that.
+      ary = []
+      while GC.stat_heap(0, :heap_allocatable_pages) >
+          (GC.stat_heap(0, :heap_allocatable_pages) + GC.stat_heap(0, :heap_eden_pages)) * 0.3
+        ary << Object.new
+      end
+
+      GC.start(full_mark: false)
+
+      # Check that we still have the same number of slots as initially configured.
+      GC.stat_heap.each do |i, s|
+        # Sometimes pages will have 1 less slot due to alignment, so always increase slots_per_page by 1.
+        slots_per_page = (s[:heap_eden_slots] / s[:heap_eden_pages]) + 1
+        total_slots = s[:heap_eden_slots] + s[:heap_allocatable_pages] * slots_per_page
+        assert_in_epsilon(SIZES[i], total_slots, 0.01, s)
+      end
+    RUBY
   end
 
   def test_profiler_enabled