filters/thread_filter: don't serialise IO objects

Fixes #204 (Airbrake gem segfaults on Circle CI)

I stumbled upon an edge case while running the thread filter against a
Rails app. The app would stuck because `io_obj.to_json` would never
return. The `io_obj` was open. Closed IO objects raise IOError, which
we already handle.

To make sure we avoid any kind of segfaults, we ignore
`:__recursive_key__`, which holds data for PrettyPrint.

CHANGELOG.md

@@ -3,6 +3,9 @@ Airbrake Ruby Changelog
 
 ### master
 
+* Fixed segfault on Ruby 2.1 while using the thread filter
+  ([#206](https://github.com/airbrake/airbrake-ruby/pull/206))
+
 ### [v2.2.0][v2.2.0] (May 1, 2017)
 
 * Make `notify/notify_sync` accept a block, which yields an `Airbrake::Notice`

lib/airbrake-ruby/filters/thread_filter.rb

@@ -38,12 +38,17 @@ def call(notice)
 
       def thread_variables(th)
         th.thread_variables.map.with_object({}) do |var, h|
-          h[var] = th.thread_variable_get(var).inspect
+          next if (value = th.thread_variable_get(var)).is_a?(IO)
+          h[var] = value
         end
       end
 
       def fiber_variables(th)
-        th.keys.map.with_object({}) { |key, h| h[key] = th[key].inspect }
+        th.keys.map.with_object({}) do |key, h|
+          next if key == :__recursive_key__
+          next if (value = th[key]).is_a?(IO)
+          h[key] = value
+        end
       end
 
       def add_thread_info(th, thread_info)

spec/filters/thread_filter_spec.rb

@@ -11,15 +11,15 @@
     subject.call(notice)
     th.thread_variable_set(:bingo, nil)
 
-    expect(notice[:params][:thread][:thread_variables][:bingo]).to eq(':bango')
+    expect(notice[:params][:thread][:thread_variables][:bingo]).to eq(:bango)
   end
 
   it "appends fiber variables" do
     th[:bingo] = :bango
     subject.call(notice)
     th[:bingo] = nil
 
-    expect(notice[:params][:thread][:fiber_variables][:bingo]).to eq(':bango')
+    expect(notice[:params][:thread][:fiber_variables][:bingo]).to eq(:bango)
   end
 
   it "appends name", skip: !Thread.current.respond_to?(:name) do
@@ -49,4 +49,32 @@
     subject.call(notice)
     expect(notice[:params][:thread][:safe_level]).to eq(0)
   end
+
+  context "when an IO-like object is stored" do
+    let(:io_obj) do
+      Class.new(IO) do
+        def initialize; end
+      end.new
+    end
+
+    before do
+      expect(io_obj).to be_is_a(IO)
+    end
+
+    it "doesn't append the IO object to thread variables" do
+      th.thread_variable_set(:io, io_obj)
+      subject.call(notice)
+      th.thread_variable_set(:io, nil)
+
+      expect(notice[:params][:thread][:thread_variables][:io]).to be_nil
+    end
+
+    it "doesn't append the IO object to thread variables" do
+      th[:io] = io_obj
+      subject.call(notice)
+      th[:io] = nil
+
+      expect(notice[:params][:thread][:fiber_variables][:io]).to be_nil
+    end
+  end
 end