Add loaded specs fallback

[ericmustin]

this pr addresses #1505. Within the core tracer we should generally try to avoid Gem.loaded_specs if a known libraries version constant exists, as loaded_specs may not work depending on how the user is using Rubygems.

This work builds on top of #1506 and the next step here would be to add fallback code, where possible, for each contrib integration. Doing this repeatedly would be a chore so it's preferable to have a helper class for this that safely checks the version constant, maybe something like

module Datadog
  module Utils
    # Common gem-related utility functions.
    module Gem
      module_function

      # Safely falls back to checking if a gem has been loaded
      def loaded_specs_present?(gem_name = '', &block)
        begin
          Gem.loaded_specs[gem_name] || block.call

        # catch errors for an undefined or unsupported version constant
        rescue NameError, ArgumentError
        end
      end


      # Safely falls back to checking a gem's version constant if defined
      def loaded_specs_version(gem_name = '', &block)
        begin
          (Gem.loaded_specs[gem_name] && Gem.loaded_specs[gem_name].version) || Gem::Version.new(block.call)

        # catch errors for an undefined or unsupported version constant
        rescue NameError, ArgumentError
        end
      end
    end
  end
end

Datadog::Utils::Gem.loaded_specs_version('dogstatsd-ruby') { Datadog::Statsd::VERSION }

[codecov-commenter]

Codecov Report

Merging #1510 (7ab4f8d) into master (88d321b) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1510      +/-   ##
==========================================
- Coverage   98.22%   98.22%   -0.01%     
==========================================
  Files         862      862              
  Lines       41686    41685       -1     
==========================================
- Hits        40945    40944       -1     
  Misses        741      741              

Impacted Files	Coverage Δ
lib/ddtrace/metrics.rb	96.52% <100.00%> (ø)
spec/ddtrace/metrics_spec.rb	100.00% <100.00%> (ø)

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[ivoanjo]

I'm... still a bit unconvinced that this is correct:

~/datadog/dd-trace-rb$ bundle exec ruby -e "puts defined?(Datadog::Statsd::VERSION).inspect; puts Gem.loaded_specs['dogstatsd-ruby']"
nil
#<Bundler::StubSpecification name=dogstatsd-ruby version=5.0.1 platform=ruby>

But I admit some of the details of the inner workings of rubygems and Ruby are unknown to me. My understanding is that Gem.loaded_specs is available as long as Ruby is booted with rubygems enabled (which is the default); but the version constant only really shows up after the gem is required (or at least, after the file containing the version is required).

So I think this replacement is not a direct equivalent. This is why in #1506 I've tried to wire things up such as if the library appears to be loaded (and protobuf doesn't provide a version constant, so "appears to be loaded" was what I had to work with), then we avoid the use of Gem.loaded_specs, but there's a fallback to Gem.loaded_specs whenever the gem was not yet loaded.

[ericmustin]

@ivoanjo yes i've been thinking about this incorrectly, will update issue + this pr

[ericmustin]

@ivoanjo what do u think now, better?

[ivoanjo]