Workaround buggy Method#arity in JRuby

JRuby apparently didn't even properly support #arity for Java proxy methods before 9.0.3.0 (see jruby/jruby@013e288), so this implements a workaround for these versions.

The workaround essentially makes use of Java's introspection to figure out matching methods (which could be more than one partly because Java supports multiple overloads, and partly because JRuby introduces aliases to make method names look more Rubyesque). If there is only a single match, we can use that methods arity directly instead of the default -1 arity.

One could in theory do better than -1 for various overload sets with multiple methods in them as well, but we have to draw a line somewhere.

Given that this issue also impacts JRuby 1.7.x in 1.8-mode, it would require guards around all the #parameters calls to integrate it into the existing workaround, and thus it is implemented as a separate workaround.

Author: grddev

lib/rspec/support/method_signature_verifier.rb

@@ -154,25 +154,56 @@ def classify_parameters
       INFINITY = 1 / 0.0
     end
 
-    # JRuby has only partial support for UnboundMethod#parameters, so we fall back on using #arity
-    # https://github.com/jruby/jruby/issues/2816 and https://github.com/jruby/jruby/issues/2817
-    if RSpec::Support::Ruby.jruby? &&
-       RubyFeatures.optional_and_splat_args_supported? &&
-       Java::JavaLang::String.instance_method(:char_at).parameters == []
+    if RSpec::Support::Ruby.jruby?
+      # JRuby has only partial support for UnboundMethod#parameters, so we fall back on using #arity
+      # https://github.com/jruby/jruby/issues/2816 and https://github.com/jruby/jruby/issues/2817
+      if RubyFeatures.optional_and_splat_args_supported? &&
+         Java::JavaLang::String.instance_method(:char_at).parameters == []
+
+        class MethodSignature < remove_const(:MethodSignature)
+        private
+
+          def classify_parameters
+            super
+            if (arity = @method.arity) != 0 && @method.parameters.empty?
+              if arity < 0
+                @min_non_kw_args = ~arity
+                @max_non_kw_args = INFINITY
+              else
+                @min_non_kw_args = arity
+                @max_non_kw_args = arity
+              end
+            end
+          end
+        end
+      end
 
-      class MethodSignature < remove_const(:MethodSignature)
-      private
+      # JRuby used to always report -1 arity for Java proxy methods
+      if Java::JavaLang::String.instance_method(:char_at).arity == -1
+        class MethodSignature < remove_const(:MethodSignature)
+        private
+
+          def classify_parameters
+            super
+            if @method.arity == -1
+              arity = java_single_overload_arity
+              if arity < 0
+                @min_non_kw_args = ~arity
+                @max_non_kw_args = INFINITY
+              else
+                @min_non_kw_args = arity
+                @max_non_kw_args = arity
+              end
+            end
+          end
 
-        def classify_parameters
-          super
-          if (arity = @method.arity) != 0 && @method.parameters.empty?
-            if arity < 0
-              @min_non_kw_args = ~arity
-              @max_non_kw_args = INFINITY
-            else
-              @min_non_kw_args = arity
-              @max_non_kw_args = arity
+          def java_single_overload_arity
+            return @method.arity unless @method.owner.respond_to?(:java_class)
+            java_instance_methods = @method.owner.java_class.java_instance_methods
+            compatible_overloads = java_instance_methods.select do |java_method|
+              @method == @method.owner.instance_method(java_method.name)
             end
+            (compatible_overloads.size == 1) ? compatible_overloads.first.arity : @method.arity
           end
         end
       end

spec/rspec/support/method_signature_verifier_spec.rb

@@ -727,6 +727,14 @@ def arity_block(_, &block); end
             it 'validates against a single argument' do
               expect(valid_non_kw_args?(1)).to eq true
             end
+
+            it 'fails validation against 0 arguments' do
+              expect(valid_non_kw_args?(0)).to eq false
+            end
+
+            it 'fails validation against 2 arguments' do
+              expect(valid_non_kw_args?(2)).to eq false
+            end
           end
         end
       end