filter_chain: prioritize library filter over user ones

Fixes #195 (Question: Usage of add_filter)

We need to make sure that Airbrake library have higher precedence over
user filters to avoid confusion when expected data is not present in
the Notice object.

lib/airbrake-ruby/filter_chain.rb

@@ -7,6 +7,11 @@ module Airbrake
   # @api private
   # @since v1.0.0
   class FilterChain
+    ##
+    # @return [String] the namespace for reserved filters which are executed
+    #   before any other filters
+    PRIORITY_FILTER_NAME = '#<Airbrake::'.freeze
+
     ##
     # Filters to be executed last. By this time all permutations on a notice
     # should be done, so the final step is to blacklist/whitelist keys.
@@ -24,21 +29,26 @@ def initialize(config)
 
       [Airbrake::Filters::SystemExitFilter,
        Airbrake::Filters::GemRootFilter].each do |filter|
-        add_filter(filter.new)
+        @filters << filter.new
       end
 
       root_directory = config.root_directory
       return unless root_directory
 
-      add_filter(Airbrake::Filters::RootDirectoryFilter.new(root_directory))
+      @filters << Airbrake::Filters::RootDirectoryFilter.new(root_directory)
     end
 
     ##
     # Adds a filter to the filter chain.
+    #
     # @param [#call] filter The filter object (proc, class, module, etc)
+    # @return [void]
     def add_filter(filter)
       return @keys_filters << filter if KEYS_FILTERS.include?(filter.class)
-      @filters << filter
+      return @filters << filter unless filter.to_s.start_with?(PRIORITY_FILTER_NAME)
+
+      i = @filters.rindex { |f| f.to_s.start_with?(PRIORITY_FILTER_NAME) }
+      @filters.insert(i + 1, filter) if i
     end
 
     ##

spec/filter_chain_spec.rb

@@ -54,6 +54,22 @@
         @chain.refine(notice)
         expect(notice[:params][:bingo]).to eq('[Filtered]')
       end
+
+      it "executes library filters before user ones" do
+        nums = []
+
+        @chain.add_filter(proc { nums << 2 })
+
+        priority_filter = proc { nums << 1 }
+        def priority_filter.to_s
+          '#<Airbrake::'
+        end
+        @chain.add_filter(priority_filter)
+
+        @chain.refine(notice)
+
+        expect(nums).to eq([1, 2])
+      end
     end
 
     describe "default backtrace filters" do