Backtrace Cleaner

Backtraces often include many lines that are not relevant for the context under review. This makes it hard to find the signal amongst the backtrace noise, and adds debugging time. With a BacktraceCleaner, filters and silencers are used to remove the noisy lines, so that only the most relevant lines remain.

Filters are used to modify lines of data, while silencers are used to remove lines entirely. The typical filter use case is to remove lengthy path information from the start of each line, and view file paths relevant to the app directory instead of the file system root. The typical silencer use case is to exclude the output of a noisy library from the backtrace, so that you can focus on the rest.

bc = ActiveSupport::BacktraceCleaner.new
root = "#{Rails.root}/"
bc.add_filter   { |line| line.delete_prefix(root) } # strip the Rails.root prefix
bc.add_silencer { |line| /puma|rubygems/.match?(line) } # skip any lines from puma or rubygems
bc.clean(exception.backtrace) # perform the cleanup

To reconfigure an existing BacktraceCleaner (like the default one in Rails) and show as much data as possible, you can always call BacktraceCleaner#remove_silencers!, which will restore the backtrace to a pristine state. If you need to reconfigure an existing BacktraceCleaner so that it does not filter or modify the paths of any lines of the backtrace, you can call BacktraceCleaner#remove_filters! These two methods will give you a completely untouched backtrace.

Inspired by the Quiet Backtrace gem by thoughtbot.

Methods

Constants

FORMATTED_GEMS_PATTERN = /\A[^\/]+ \([\w.]+\) /

Class Public methods

new()

πŸ“ Source code 
# File activesupport/lib/active_support/backtrace_cleaner.rb, line 35
    def initialize
      @filters, @silencers = [], []
      add_core_silencer
      add_gem_filter
      add_gem_silencer
      add_stdlib_silencer
    end
πŸ”Ž See on GitHub

Instance Public methods

add_filter(&block)

Adds a filter from the block provided. Each line in the backtrace will be mapped against this filter.

# Will turn "/my/rails/root/app/models/person.rb" into "app/models/person.rb"
root = "#{Rails.root}/"
backtrace_cleaner.add_filter { |line| line.delete_prefix(root) }
πŸ“ Source code 
# File activesupport/lib/active_support/backtrace_cleaner.rb, line 154
    def add_filter(&block)
      @filters << block
    end
πŸ”Ž See on GitHub

add_silencer(&block)

Adds a silencer from the block provided. If the silencer returns true for a given line, it will be excluded from the clean backtrace.

# Will reject all lines that include the word "puma", like "/gems/puma/server.rb" or "/app/my_puma_server/rb"
backtrace_cleaner.add_silencer { |line| /puma/.match?(line) }
πŸ“ Source code 
# File activesupport/lib/active_support/backtrace_cleaner.rb, line 163
    def add_silencer(&block)
      @silencers << block
    end
πŸ”Ž See on GitHub

clean(backtrace, kind = :silent)

Returns the backtrace after all filters and silencers have been run against it. Filters run first, then silencers.

Also aliased as: filter
πŸ“ Source code 
# File activesupport/lib/active_support/backtrace_cleaner.rb, line 45
    def clean(backtrace, kind = :silent)
      filtered = filter_backtrace(backtrace)

      case kind
      when :silent
        silence(filtered)
      when :noise
        noise(filtered)
      else
        filtered
      end
    end
πŸ”Ž See on GitHub

clean_frame(frame, kind = :silent)

Returns the frame with all filters applied. returns nil if the frame was silenced.

πŸ“ Source code 
# File activesupport/lib/active_support/backtrace_cleaner.rb, line 73
    def clean_frame(frame, kind = :silent)
      frame = frame.to_s
      @filters.each do |f|
        frame = f.call(frame.to_s)
      end

      case kind
      when :silent
        frame unless @silencers.any? { |s| s.call(frame) }
      when :noise
        frame if @silencers.any? { |s| s.call(frame) }
      else
        frame
      end
    end
πŸ”Ž See on GitHub

clean_locations(locations, kind = :silent)

Given an array of Thread::Backtrace::Location objects, returns an array with the clean ones:

clean_locations = backtrace_cleaner.clean_locations(caller_locations)

Filters and silencers receive strings as usual. However, the path attributes of the locations in the returned array are the original, unfiltered ones, since locations are immutable.

πŸ“ Source code 
# File activesupport/lib/active_support/backtrace_cleaner.rb, line 67
    def clean_locations(locations, kind = :silent)
      locations.select { |location| clean_frame(location, kind) }
    end
πŸ”Ž See on GitHub

filter(backtrace, kind = :silent)

Alias for: clean

first_clean_frame(kind = :silent)

Returns the first clean frame of the caller’s backtrace, or nil.

Frames are strings.

πŸ“ Source code 
# File activesupport/lib/active_support/backtrace_cleaner.rb, line 94
      def first_clean_frame(kind = :silent)
        caller_location_skipped = false

        Thread.each_caller_location do |location|
          unless caller_location_skipped
            caller_location_skipped = true
            next
          end

          frame = clean_frame(location, kind)
          return frame if frame
        end
      end
πŸ”Ž See on GitHub

first_clean_location(kind = :silent)

Returns the first clean location of the caller’s call stack, or nil.

Locations are Thread::Backtrace::Location objects. Since they are immutable, their path attributes are the original ones, but filters are applied internally so silencers can still rely on them.

πŸ“ Source code 
# File activesupport/lib/active_support/backtrace_cleaner.rb, line 113
      def first_clean_location(kind = :silent)
        caller_location_skipped = false

        Thread.each_caller_location do |location|
          unless caller_location_skipped
            caller_location_skipped = true
            next
          end

          return location if clean_frame(location, kind)
        end
      end
πŸ”Ž See on GitHub

remove_filters!()

Removes all filters, but leaves in the silencers. Useful if you suddenly need to see entire filepaths in the backtrace that you had already filtered out.

πŸ“ Source code 
# File activesupport/lib/active_support/backtrace_cleaner.rb, line 177
    def remove_filters!
      @filters = []
    end
πŸ”Ž See on GitHub

remove_silencers!()

Removes all silencers, but leaves in the filters. Useful if your context of debugging suddenly expands as you suspect a bug in one of the libraries you use.

πŸ“ Source code 
# File activesupport/lib/active_support/backtrace_cleaner.rb, line 170
    def remove_silencers!
      @silencers = []
    end
πŸ”Ž See on GitHub