fix(pool): returning closed resources to the pool (#154)

spec/pool_spec.cr

@@ -190,6 +190,23 @@ describe DB::Pool do
     all[2].closed?.should be_false
   end
 
+  it "should not return closed resources to the pool" do
+    pool = DB::Pool.new(max_pool_size: 1, max_idle_pool_size: 1) { Closable.new }
+
+    # pool size 1 should be reusing the one resource
+    resource1 = pool.checkout
+    pool.release resource1
+    resource2 = pool.checkout
+    resource1.should eq resource2
+
+    # it should not return a closed resource to the pool
+    resource2.close
+    pool.release resource2
+
+    resource2 = pool.checkout
+    resource1.should_not eq resource2
+  end
+
   it "should create resource after max_pool was reached if idle forced some close up" do
     all = [] of Closable
     pool = DB::Pool.new(max_pool_size: 3, max_idle_pool_size: 1) { Closable.new.tap { |c| all << c } }

src/db/pool.cr

@@ -141,7 +141,9 @@ module DB
       idle_pushed = false
 
       sync do
-        if can_increase_idle_pool
+        if resource.responds_to?(:closed?) && resource.closed?
+          @total.delete(resource)
+        elsif can_increase_idle_pool
           @idle << resource
           if resource.responds_to?(:after_release)
             resource.after_release