Account for nil sample_rate when sampling

Author: solnic

lib/sentry/opentelemetry/sampler.ex

@@ -81,23 +81,25 @@ if Code.ensure_loaded?(:otel_sampler) do
     end
 
     defp make_sampling_decision(sample_rate) do
-      {sampled, random_value} =
-        cond do
-          sample_rate == 0.0 ->
-            {false, 1.0}
-
-          sample_rate == 1.0 ->
-            {true, 0.0}
+      cond do
+        is_nil(sample_rate) ->
+          {:drop, [], []}
 
-          true ->
-            random_value = :rand.uniform()
-            {random_value < sample_rate, random_value}
-        end
+        sample_rate == 0.0 ->
+          tracestate = build_tracestate(sample_rate, 1.0, false)
+          {:drop, [], tracestate}
 
-      tracestate = build_tracestate(sample_rate, random_value, sampled)
-      decision = if sampled, do: :record_and_sample, else: :drop
+        sample_rate == 1.0 ->
+          tracestate = build_tracestate(sample_rate, 0.0, true)
+          {:record_and_sample, [], tracestate}
 
-      {decision, [], tracestate}
+        true ->
+          random_value = :rand.uniform()
+          sampled = random_value < sample_rate
+          tracestate = build_tracestate(sample_rate, random_value, sampled)
+          decision = if sampled, do: :record_and_sample, else: :drop
+          {decision, [], tracestate}
+      end
     end
 
     defp build_tracestate(sample_rate, random_value, sampled) do

test/sentry/opentelemetry/sampler_test.exs

@@ -123,6 +123,20 @@ defmodule Sentry.Opentelemetry.SamplerTest do
       discarded_count = Map.get(state, {:sample_rate, "transaction"}, 0)
       assert discarded_count > 0, "Expected some spans to be dropped and recorded"
     end
+
+    test "always drops when sample rate is nil (tracing disabled) and records discarded event" do
+      :sys.replace_state(ClientReport.Sender, fn _ -> %{} end)
+
+      Sentry.Config.put_config(:traces_sample_rate, nil)
+
+      test_ctx = create_test_span_context()
+
+      assert {:drop, [], []} =
+               Sampler.should_sample(test_ctx, 123, nil, "test span", nil, nil, drop: [])
+
+      state = :sys.get_state(ClientReport.Sender)
+      assert state == %{{:sample_rate, "transaction"} => 1}
+    end
   end
 
   describe "trace-level sampling consistency" do