Fix handling of invalid :pool_class symbols

Previously you couldn't hit the error, because the require
would raise a LoadError.  It doesn't make sense to allow requiring
random files if there is no way to register the entries.  In
Sequel 5, non default pool classes need to be specified by having
the :pool_class be a class instead of a non-default symbol.

lib/sequel/connection_pool.rb

@@ -26,6 +26,12 @@
 #                                     specified by the array of symbols.
 class Sequel::ConnectionPool
   OPTS = Sequel::OPTS
+  POOL_CLASS_MAP = {
+    :threaded => :ThreadedConnectionPool,
+    :single => :SingleConnectionPool,
+    :sharded_threaded => :ShardedThreadedConnectionPool,
+    :sharded_single => :ShardedSingleConnectionPool
+  }.freeze
 
   # Class methods used to return an appropriate pool subclass, separated
   # into a module for easier overridding by extensions.
@@ -44,20 +50,12 @@ def get_pool(db, opts = OPTS)
     def connection_pool_class(opts)
       if pc = opts[:pool_class]
         unless pc.is_a?(Class)
-          require("sequel/connection_pool/#{pc}")
-
-          pc = case pc
-          when :threaded
-            Sequel::ThreadedConnectionPool
-          when :single
-            Sequel::SingleConnectionPool
-          when :sharded_threaded
-            Sequel::ShardedThreadedConnectionPool
-          when :sharded_single
-            Sequel::ShardedSingleConnectionPool
-          else
+          unless name = POOL_CLASS_MAP[pc]
             raise Sequel::Error, "unsupported connection pool type, please pass appropriate class as the :pool_class option"
           end
+
+          require("sequel/connection_pool/#{pc}")
+          pc = Sequel.const_get(name)
         end
 
         pc

spec/core/connection_pool_spec.rb

@@ -31,6 +31,10 @@
   it "should have a size of zero" do
     @cpool.size.must_equal 0
   end
+
+  it "should raise Error for bad pool class" do
+    proc{Sequel::ConnectionPool.get_pool(mock_db.call, :pool_class=>:foo)}.must_raise Sequel::Error
+  end
 end
 
 describe "ConnectionPool options" do