Add guard against releasing slots that haven't been reserved

lib/kredis/types/slots.rb

@@ -19,27 +19,36 @@ def reserve
           release
         end
       else
-        if incr <= available
+        if available?
+          incr
           true
         else
-          release
           false
         end
       end
     end
   end
 
   def release
-    decr
+    if taken > 0
+      decr
+      true
+    else
+      false
+    end
   end
 
   def available?
     failsafe returning: false do
-      get.to_i < available
+      taken < available
     end
   end
 
   def reset
     del
   end
+
+  def taken
+    get.to_i
+  end
 end

test/types/slots_test.rb

@@ -26,6 +26,22 @@ class SlotsTest < ActiveSupport::TestCase
     assert @slots.available?
   end
 
+  test "release when slots are reserved" do
+    assert_not @slots.release
+
+    3.times do
+      assert @slots.reserve
+    end
+
+    3.times do
+      assert @slots.release
+    end
+
+    assert_not @slots.release
+
+    assert_equal 0, @slots.taken
+  end
+
   test "reserve with block" do
     assert @slots.reserve
     assert @slots.reserve
@@ -70,6 +86,17 @@ class SlotsTest < ActiveSupport::TestCase
     assert_not slot.available?
   end
 
+  test "release single slot when reserved" do
+    slot = Kredis.slot "myslot"
+
+    assert_not slot.release
+
+    assert slot.reserve
+    assert slot.release
+
+    assert_not slot.release
+  end
+
   test "failing open" do
     stub_redis_down(@slots) do
       assert_not @slots.available?