Temporarily lock msgpack version for Ruby 2.0 and 2.1

[marcotc]

The newest release of msgpack (1.4) is not currently compatible with Ruby 2.0 and 2.1.

This PR locks the msgpack version to < 1.4 until we know if support will be restored and completely dropped for these old rubies.

[ivoanjo]

Just a note: while this fix works for our CI, I'm not sure it's correct if we were to publish it to rubygems.org, because the metadata uploaded is the one that matches the ruby version used to build/upload the gem.

At least on the website it wouldn't be correct; not sure if bundler/rubygems would re-run the gemspec locally after downloading the gem and spot it.

[marcotc]

@ivoanjo, your assessment is correct. Having if conditions in the gemspec is not great: we should have these in the local Gemfile instead, that might have been a better approach here which I overlooked.

Overall, even if the gemspec is reevaluated locally, which I'm not sure it is, the index build by RubyGems is not and would download an unrestricted msgpack version for Ruby < 2.2.

We unfortunately don't have a way to tell RubyGems that for Ruby versions older than Ruby 2.2 we'd like to lock msgpack.

[ivoanjo]

I actually thought of a way we could fix this completely, with a bit of indirection.

The trick is to introduce a new gem dependency that we control. Let's call it ddtrace-internal-compat. Then, we release two versions for it on Rubygems.org:

• ddtrace-internal-compat 1.0.0, has required_ruby_version < 2.2, depends on msgpack < 1.4
• ddtrace-internal-compat 1.0.1, has required_ruby_version >= 2.2, depens on msgpack (no specific version)

And then in our gem, we just depend on ddtrace-internal-compat ~> 1.0. During dependency resolution, rubygems will pick 1.0.0 for the old rubies, thus forcing the constraint on msgpack.

(This solution would seem to work generically for any dependency on which you want to add a Ruby version constraint after the fact. For instance, we could have used this to make sure older rubies also don't pick up a too-modern rexml (see ruby/rexml#69))