Release aggregate instances when sqlite finishes with them

ext/sqlite3/aggregator.c

@@ -97,7 +97,7 @@ rb_sqlite3_aggregate_instance_destroy(sqlite3_context *ctx)
     VALUE *inst_ptr = sqlite3_aggregate_context(ctx, 0);
     VALUE inst;
 
-    if (!inst_ptr || (inst = *inst_ptr)) {
+    if (!inst_ptr || !(inst = *inst_ptr)) {
         return;
     }
 

test/test_integration_aggregate.rb

@@ -354,6 +354,18 @@ def finalize
     end
   end
 
+  # Used by one test only, so a live count of 1 means just the template.
+  class ReleasableAggregator
+    def step(*args)
+      @sum ||= 0
+      args.each { |a| @sum += a.to_i }
+    end
+
+    def finalize
+      @sum
+    end
+  end
+
   class AccumulateAggregator2
     def step(a, b)
       @sum ||= 1
@@ -396,6 +408,15 @@ def test_define_aggregator_does_not_use_moved_instances_after_gc_compaction
     assert_equal 33, values[1]
   end
 
+  def test_aggregate_instances_are_released_after_each_query
+    @db.define_aggregator("accumulate", ReleasableAggregator.new)
+
+    5.times { assert_equal 33, @db.get_first_value("select accumulate(c) from foo") }
+    GC.start(full_mark: true, immediate_sweep: true)
+
+    assert_equal 1, ObjectSpace.each_object(ReleasableAggregator).count
+  end
+
   def test_step_on_statement_whose_database_was_closed_does_not_use_freed_aggregator
     @db.define_aggregator("accumulate", AccumulateAggregator.new)
     stmt = @db.prepare("select accumulate(c) from foo")