thread_warning_example.rb

require "set"
require "rbconfig"

class Wrapper
  def initialize
    @a = Set.new
    @b = Set.new
  end

  def write_a
    # We would like a warning here
    @a << "a"
  end

  def write_b
    # We would like a warning here as well
    @b << "b"
  end
end

# No warnings are emitted until the warning is enabled
obj = Wrapper.new

Thread.new {
  obj.write_a # no warning
  obj.write_b # no warning
}.join

# Enable the warning
Warning[:non_owner_thread_writes] = true

obj = Wrapper.new

Thread.new {
  obj.write_a
  obj.write_b
}.join

# We would expect `write_a` and `write_b` to be blamed because they need a
# lock, but the output actually looks like this:
#
#   /Users/aaron/git/ruby/lib/set.rb:515: warning: Hash mutated from a thread that didn't create it
#   /Users/aaron/git/ruby/lib/set.rb:515: warning: Hash mutated from a thread that didn't create it
#   /Users/aaron/git/ruby/lib/set.rb:515: warning: Hash mutated from a thread that didn't create it
#   /Users/aaron/git/ruby/lib/set.rb:515: warning: Hash mutated from a thread that didn't create it
#
# It's because Set is implemented in Ruby, so it gets "blamed" for writing
# to the hash, but actually we need to do locking in `write_a` and `write_b`.
# To deal with this, we need to filter the warnings.  Below is an example for
# filtering the warnings.  It might need more logic for our app, but hopefully
# it can get the point across

module Warning
  class << self
    prepend Module.new {
      def warn(msg, category:)
        # We only care about non-owner writes
        return super unless category == :non_owner_thread_writes

        uplevel = 1

        # Add some filtering logic here
        caller_locations.each do |loc|
          path = loc.path

          # If the path starts with either of these directories, then we need
          # to go up a frame.
          if path.start_with?(RbConfig::CONFIG["topdir"]) || path.start_with?(RbConfig::CONFIG["rubylibdir"])
            uplevel += 1
          else
            break
          end
        end

        # If the first frame is _not_ in the above paths, then just emit
        return super if uplevel == 1

        # Otherwise, call Kernel.warn with the appropriate uplevel
        Kernel.warn(msg, uplevel:, category:)
      end
    }
  end
end

obj = Wrapper.new

Thread.new {
  obj.write_a
  obj.write_b
}.join

# Now the output looks like this:
#
# ./test.rb:12: warning: /Users/aaron/git/ruby/lib/set.rb:515: warning: Hash mutated from a thread that didn't create it
# ./test.rb:12: warning: /Users/aaron/git/ruby/lib/set.rb:515: warning: Hash mutated from a thread that didn't create it
# ./test.rb:17: warning: /Users/aaron/git/ruby/lib/set.rb:515: warning: Hash mutated from a thread that didn't create it
# ./test.rb:17: warning: /Users/aaron/git/ruby/lib/set.rb:515: warning: Hash mutated from a thread that didn't create it