Add specs and update comment

spec/database_spec.cr

@@ -179,6 +179,25 @@ describe DB::Database do
     end
   end
 
+  it "should not checkout multiple connections if there is a statement error" do
+    with_dummy "dummy://localhost:1027?initial_pool_size=1&max_pool_size=10&retry_attempts=10" do |db|
+      expect_raises DB::Error do
+        db.exec("syntax error")
+      end
+      DummyDriver::DummyConnection.connections.size.should eq(1)
+    end
+  end
+
+  it "should attempt all retries if connection is lost" do
+    with_dummy "dummy://localhost:1027?initial_pool_size=1&max_pool_size=1&retry_attempts=10" do |db|
+      expect_raises DB::PoolRetryAttemptsExceeded do
+        db.exec("raise ConnectionLost")
+      end
+      # 1 initial + 10 retries
+      DummyDriver::DummyConnection.connections.size.should eq(11)
+    end
+  end
+
   describe "prepared_statements connection option" do
     it "defaults to true" do
       with_dummy "dummy://localhost:1027" do |db|

spec/dummy_driver.cr

@@ -142,6 +142,7 @@ class DummyDriver < DB::Driver
       super(connection, command)
       @@statements_count.add(1)
       raise DB::Error.new(command) if command == "syntax error"
+      raise DB::ConnectionLost.new(connection) if command == "raise ConnectionLost"
     end
 
     def self.statements_count

src/db/pool.cr

@@ -206,8 +206,6 @@ module DB
           # if the connection is lost it will be closed by
           # the exception to release resources
           # we still need to remove it from the known pool.
-          # Closed connection will be evicted from statement cache
-          # in PoolPreparedStatement#clean_connections
           sync { delete(e.resource) }
         rescue e : PoolResourceRefused
           # a ConnectionRefused means a new connection