filters/thread_filter: fixed segfault on Ruby 2.1.3

Fixes #229 (Another segfault report)

Ruby segfaults whenever it tries to access `:__recursive_key__`.
Unfortunately, there's no other good way to avoid this segfault.
`:__recursive_key__` is set by PrettyPrint, so it's very common.

CHANGELOG.md

@@ -3,6 +3,9 @@ Airbrake Ruby Changelog
 
 ### master
 
+* Fixed segfault in `ThreadFilter` on Ruby 2.1.3
+  ([#228](https://github.com/airbrake/airbrake-ruby/pull/228))
+
 ### [v2.2.5][v2.2.5] (May 23, 2017)
 
 * Fixed bug when the block form of `notify` would run its block for ignored

lib/airbrake-ruby/filters/thread_filter.rb

@@ -21,6 +21,13 @@ class ThreadFilter
         Numeric
       ].freeze
 
+      ##
+      # @return [Array<Symbol>] the list of ignored fiber variables
+      IGNORED_FIBER_VARIABLES = [
+        # https://github.com/airbrake/airbrake-ruby/issues/229
+        :__recursive_key__
+      ].freeze
+
       def initialize
         @weight = 110
       end
@@ -57,6 +64,7 @@ def thread_variables(th)
 
       def fiber_variables(th)
         th.keys.map.with_object({}) do |key, h|
+          next if IGNORED_FIBER_VARIABLES.any? { |v| v == key }
           h[key] = sanitize_value(th[key])
         end
       end

spec/filters/thread_filter_spec.rb

@@ -119,6 +119,18 @@ def new_thread
         )
       end
     end
+
+    it "ignores the :__recursive_key__ fiber variable" do
+      key = :__recursive_key__
+
+      new_thread do |th|
+        th[key] = key
+        subject.call(notice)
+      end
+
+      fiber_variables = notice[:params][:thread][:fiber_variables]
+      expect(fiber_variables[key]).to be_nil
+    end
   end
 
   describe "fiber variables" do