Allow RateSampler with a sampling rate of zero

lib/datadog/tracing/sampling/rate_sampler.rb

@@ -13,24 +13,13 @@ class RateSampler < Sampler
         # This sampler keeps a random subset of the traces. Its main purpose is to
         # reduce the instrumentation footprint.
         #
-        # * +sample_rate+: the sample rate as a {Float} between 0.0 and 1.0. 0.0
-        #   means that no trace will be sampled; 1.0 means that all traces will be
-        #   sampled.
-        #
-        # DEV-2.0: Allow for `sample_rate` zero (drop all) to be allowed. This eases
-        # DEV-2.0: usage for all internal users of the {RateSampler} class: both
-        # DEV-2.0: RuleSampler and Single Span Sampling leverage the RateSampler, but want
-        # DEV-2.0: `sample_rate` zero to mean "drop all". They work around this by hard-
-        # DEV-2.0: setting the `sample_rate` to zero like so:
-        # DEV-2.0: ```
-        # DEV-2.0: sampler = RateSampler.new
-        # DEV-2.0: sampler.sample_rate = sample_rate
-        # DEV-2.0: ```
+        # @param sample_rate [Numeric] the sample rate between 0.0 and 1.0, inclusive.
+        #   0.0 means that no trace will be sampled; 1.0 means that all traces will be  sampled.
         def initialize(sample_rate = 1.0, decision: nil)
           super()
 
-          unless sample_rate > 0.0 && sample_rate <= 1.0
-            Datadog.logger.error('sample rate is not between 0 and 1, disabling the sampler')
+          unless sample_rate >= 0.0 && sample_rate <= 1.0
+            Datadog.logger.error('sample rate is not between 0 and 1, falling back to 1')
             sample_rate = 1.0
           end
 

lib/datadog/tracing/sampling/rule.rb

@@ -53,18 +53,7 @@ class SimpleRule < Rule
         #                defaults to always match
         # @param sample_rate [Float] Sampling rate between +[0,1]+
         def initialize(name: SimpleMatcher::MATCH_ALL, service: SimpleMatcher::MATCH_ALL, sample_rate: 1.0)
-          # We want to allow 0.0 to drop all traces, but {Datadog::Tracing::Sampling::RateSampler}
-          # considers 0.0 an invalid rate and falls back to 100% sampling.
-          #
-          # We address that here by not setting the rate in the constructor,
-          # but using the setter method.
-          #
-          # We don't want to make this change directly to {Datadog::Tracing::Sampling::RateSampler}
-          # because it breaks its current contract to existing users.
-          sampler = RateSampler.new
-          sampler.sample_rate = sample_rate
-
-          super(SimpleMatcher.new(name: name, service: service), sampler)
+          super(SimpleMatcher.new(name: name, service: service), RateSampler.new(sample_rate))
         end
       end
     end

lib/datadog/tracing/sampling/span/rule.rb

@@ -29,12 +29,7 @@ def initialize(
             @sample_rate = sample_rate
             @rate_limit = rate_limit
 
-            @sampler = Sampling::RateSampler.new
-            # Set the sample_rate outside of the initializer to allow for
-            # zero to be a "drop all".
-            # The RateSampler initializer enforces non-zero, falling back to 100% sampling
-            # if zero is provided.
-            @sampler.sample_rate = sample_rate
+            @sampler = Sampling::RateSampler.new(sample_rate)
             @rate_limiter = Sampling::TokenBucket.new(rate_limit)
           end
 

spec/datadog/tracing/sampling/rate_sampler_spec.rb

@@ -27,6 +27,12 @@
       context 'that is 0' do
         let(:sample_rate) { 0.0 }
 
+        it_behaves_like 'sampler with sample rate', 0.0
+      end
+
+      context 'that is 1' do
+        let(:sample_rate) { 1.0 }
+
         it_behaves_like 'sampler with sample rate', 1.0
       end