Return an existing filter instance in Filters.build

lib/rackstash/filter_chain.rb

@@ -302,10 +302,8 @@ module Rackstash
     # @return [#call] a filter instance
     def build_filter(filter_spec, &block)
       if filter_spec.empty?
-        return block if block_given?
+        return Rackstash::Filters.build(block) if block_given?
         raise ArgumentError, 'Need to specify a filter'
-      elsif filter_spec.length == 1 && filter_spec.first.respond_to?(:call)
-        filter_spec.first
       else
         Rackstash::Filters.build(*filter_spec, &block)
       end

lib/rackstash/filters.rb

@@ -34,11 +34,13 @@ module Rackstash
     # filter in which case we are resolving it to a class defined inside the
     # {Rackstash::Filters} namespace.
     #
-    # @param klass [Class, Symbol, String] a description of the class from which
-    #   we are creating a new filter object. When giving a `Class`, we are using
-    #   it as is. When giving a `String` or `Symbol`, we are determining the
-    #   associated class from the {Rackstash::Filters} module and create an
-    #   instance of that.
+    # @param klass [Class, Symbol, String, #call] a description of the class
+    #   from which we are creating a new filter object. When giving a `Class`,
+    #   we are using it as is. When giving a `String` or `Symbol`, we are
+    #   determining the associated class from the {Rackstash::Filters} module
+    #   and create an instance of that. When giving an object which responds to
+    #   `call` already, we return it unchanged, ignoring any additional passed
+    #   `args`.
     # @param args [Array] an optional list of arguments which is passed to the
     #   initializer for the new filter object.
     # @raise [TypeError] if we can not create a new Filter object from `class`
@@ -56,6 +58,8 @@ module Rackstash
           .to_sym
         filter_class = const_get(filter_class_name, false)
         filter_class.new(*args, &block)
+      when ->(filter) { filter.respond_to?(:call) }
+        klass
       else
         raise TypeError, "Can not build filter for #{klass.inspect}"
       end

spec/rackstash/filters_spec.rb

@@ -43,10 +43,19 @@ describe Rackstash::Filters do
       described_class.build("filter_class#{random}", *args)
     end
 
+    it 'returns an existing filter' do
+      filter = -> {}
+      expect(described_class.build(filter)).to equal filter
+      expect(described_class.build(filter, :ignored, 42)).to equal filter
+    end
+
     it 'raises a TypeError with different arguments' do
       expect { described_class.build(123) }.to raise_error(TypeError)
       expect { described_class.build(nil) }.to raise_error(TypeError)
       expect { described_class.build(true) }.to raise_error(TypeError)
+
+      expect { described_class.build('MissingFilter') }.to raise_error(NameError)
+      expect { described_class.build(:missing_filter) }.to raise_error(NameError)
     end
   end