Raise Error when there is nothing to checkin

Now an Error is raised regardless of prior #checkout (to create a stack
of connections).  This required updating Wrapper#with to not attempt
to #checkin after a failed #checkout.

lib/connection_pool.rb

@@ -77,7 +77,8 @@ class ConnectionPool
 
   def checkin
     stack = ::Thread.current[@key]
-    raise ConnectionPool::Error, 'no connections are checked out' unless stack
+    raise ConnectionPool::Error, 'no connections are checked out' unless
+      stack and not stack.empty?
 
     conn = stack.pop
     if stack.empty?
@@ -98,9 +99,12 @@ class ConnectionPool
     end
 
     def with
-      yield @pool.checkout
-    ensure
-      @pool.checkin
+      conn = @pool.checkout
+      begin
+        yield conn
+      ensure
+        @pool.checkin
+      end
     end
 
     def pool_shutdown(&block)

test/test_connection_pool.rb

@@ -128,7 +128,7 @@ class TestConnectionPool < Minitest::Test
     assert_same conn, t2.value
   end
 
-  def test_checkin_no_checkout
+  def test_checkin_never_checkout
     pool = ConnectionPool.new(:timeout => 0, :size => 1) { Object.new }
 
     e = assert_raises ConnectionPool::Error do
@@ -138,6 +138,17 @@ class TestConnectionPool < Minitest::Test
     assert_equal 'no connections are checked out', e.message
   end
 
+  def test_checkin_no_current_checkout
+    pool = ConnectionPool.new(:timeout => 0, :size => 1) { Object.new }
+
+    pool.checkout
+    pool.checkin
+
+    assert_raises ConnectionPool::Error do
+      pool.checkin
+    end
+  end
+
   def test_checkin_twice
     pool = ConnectionPool.new(:timeout => 0, :size => 1) { Object.new }