Span#deep_dup is a shallow dup

[SeanLF]

Span#deep_dup (span.rb:245) is def deep_dup; dup; end. The copy shares @data, @tags, and @span_recorder with the original, so writing to the copy mutates the original.

Scope#dup calls it on every push_scope and every capture_event, so a "copied" scope holding a plain child span shares the parent's recorder.

Making it a real deep copy changes allocation behaviour on a hot path. Happy to open a PR to fix this once maintainers weigh in

Failing spec

Drop in sentry-ruby/spec/ and run bundle exec rspec spec/span_deep_dup_spec.rb. Both examples fail on master.

# Run from sentry-ruby/: bundle exec rspec path/to/span_deep_dup_spec.rb
require "spec_helper"

RSpec.describe "Span#deep_dup" do
  before { perform_basic_setup { |config| config.traces_sample_rate = 1.0 } }

  let(:span) do
    transaction = Sentry.start_transaction(name: "t", op: "op")
    transaction.start_child(op: "child")
  end

  it "does not share mutable state with the original" do
    span.set_data("original", true)
    copy = span.deep_dup

    copy.set_data("written_via_copy", true)

    expect(span.data).not_to have_key("written_via_copy")
  end

  it "gives the copy its own data, tags and span recorder" do
    copy = span.deep_dup

    aggregate_failures do
      expect(copy.data).not_to be(span.data)
      expect(copy.tags).not_to be(span.tags)
      expect(copy.span_recorder).not_to be(span.span_recorder)
    end
  end
end

[linear-code]

RUBY-204