Add deprecation warning for Ruby 2.0 support

docs/GettingStarted.md

@@ -98,7 +98,7 @@ To contribute, check out the [contribution guidelines][contribution docs] and [d
 |       |                            | 2.3     | Full                                 | Latest              |
 |       |                            | 2.2     | Full                                 | Latest              |
 |       |                            | 2.1     | Full                                 | Latest              |
-|       |                            | 2.0     | Full                                 | Latest              |
+|       |                            | 2.0     | Deprecated                           | < 0.50.0            |
 |       |                            | 1.9.3   | EOL since August 6th, 2020           | < 0.27.0            |
 |       |                            | 1.9.1   | EOL since August 6th, 2020           | < 0.27.0            |
 | JRuby | https://www.jruby.org      | 9.2     | Full                                 | Latest              |

lib/ddtrace/configuration.rb

@@ -2,10 +2,11 @@
 require 'ddtrace/configuration/pin_setup'
 require 'ddtrace/configuration/settings'
 require 'ddtrace/configuration/components'
+require 'ddtrace/utils/only_once'
 
 module Datadog
   # Configuration provides a unique access point for configurations
-  module Configuration
+  module Configuration # rubocop:disable Metrics/ModuleLength
     extend Forwardable
 
     # Used to ensure that @components initialization/reconfiguration is performed one-at-a-time, by a single thread.
@@ -42,6 +43,8 @@ def configuration
     end
 
     def configure(target = configuration, opts = {})
+      ruby_version_deprecation_warning
+
       if target.is_a?(Settings)
         yield(target) if block_given?
 
@@ -168,5 +171,24 @@ def logger_without_components
         logger
       end
     end
+
+    # Perform version check only once
+    DEPRECATED_RUBY_VERSION = Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.1')
+    private_constant :DEPRECATED_RUBY_VERSION
+
+    RUBY_VERSION_DEPRECATION_ONLY_ONCE = Datadog::Utils::OnlyOnce.new
+    private_constant :RUBY_VERSION_DEPRECATION_ONLY_ONCE
+
+    def ruby_version_deprecation_warning
+      return unless DEPRECATED_RUBY_VERSION
+
+      RUBY_VERSION_DEPRECATION_ONLY_ONCE.run do
+        Datadog.logger.warn(
+          "Support for Ruby versions < 2.1 in dd-trace-rb is DEPRECATED.\n" \
+          "Last version to support Ruby < 2.1 will be 0.49.x, which will only receive critical bugfixes.\n" \
+          'Support for Ruby versions < 2.1 will be REMOVED in version 0.50.0.'
+        )
+      end
+    end
   end
 end

lib/ddtrace/version.rb

@@ -7,6 +7,8 @@ module VERSION
 
     STRING = [MAJOR, MINOR, PATCH, PRE].compact.join('.')
 
+    # Support for Ruby < 2.1 is currently deprecated in the tracer.
+    # Support will be dropped in the near future.
     MINIMUM_RUBY_VERSION = '2.0.0'.freeze
   end
 end

spec/ddtrace/configuration_spec.rb

@@ -348,6 +348,30 @@
           end
         end
       end
+
+      context 'deprecation warning' do
+        context 'with a deprecated Ruby version' do
+          before { skip unless Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.1') }
+
+          it 'emits deprecation warning once' do
+            expect(Datadog.logger).to receive(:warn)
+              .with(/Support for Ruby versions < 2\.1 in dd-trace-rb is DEPRECATED/).once
+
+            test_class.configure
+            test_class.configure
+          end
+        end
+
+        context 'with a supported Ruby version' do
+          before { skip if Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.1') }
+
+          it 'emits no warnings' do
+            expect(Datadog.logger).to_not receive(:warn)
+
+            configure
+          end
+        end
+      end
     end
 
     describe '#health_metrics' do