- Added changelog

- Added README entry for the feature flag
- Added missing mutex lock around reading trace data in `SetAttrOnScopeSpans`
- Added tests + benchmarks for `SetAttrOnScopeSpans`

.chloggen/tailsampling-record-policy.yaml

@@ -0,0 +1,15 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: processor/tailsampling
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: |
+  Adds support for optionally recording the policy (and any composite policy) associated with an inclusive tail processor sampling decision. 
+  This functionality is disabled by default, you can enable it by passing the following feature flag to the collector: `+processor.tailsamplingprocessor.recordpolicy`
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [35180]

processor/tailsamplingprocessor/README.md

@@ -528,6 +528,16 @@ sum (otelcol_processor_tail_sampling_count_traces_sampled) by (policy)
 
 As a reminder, a policy voting to sample the trace does not guarantee sampling; an "inverted not" decision from another policy would still discard the trace.
 
+### Tracking sampling policy
+To better understand _which_ sampling policy made the decision to include a trace, you can enable tracking the policy responsible for sampling a trace via the `processor.tailsamplingprocessor.recordpolicy` feature gate.
+
+When this feature gate is set, this will add additional attributes on each sampled span scope:
+
+| Attribute                       | Description                                                               | Present?                   |
+|---------------------------------|---------------------------------------------------------------------------|----------------------------|
+| `tailsampling.policy`           | Records the configured name of the policy that sampled a trace            | Always                     |
+| `tailsampling.composite_policy` | Records the configured name of a composite subpolicy that sampled a trace | When composite policy used |
+
 ### Policy Evaluation Errors
 
 ```

processor/tailsamplingprocessor/internal/sampling/util.go

@@ -95,9 +95,10 @@ func invertHasInstrumentationLibrarySpanWithCondition(ilss ptrace.ScopeSpansSlic
 }
 
 func SetAttrOnScopeSpans(data *TraceData, attrName string, attrKey string) {
+	data.Mutex.Lock()
+	defer data.Mutex.Unlock()
 
 	rs := data.ReceivedBatches.ResourceSpans()
-
 	for i := 0; i < rs.Len(); i++ {
 		rss := rs.At(i)
 		for j := 0; j < rss.ScopeSpans().Len(); j++ {

processor/tailsamplingprocessor/internal/sampling/util_test.go

@@ -0,0 +1,87 @@
+package sampling
+
+import (
+	"github.com/stretchr/testify/assert"
+	"go.opentelemetry.io/collector/pdata/pcommon"
+	"go.opentelemetry.io/collector/pdata/ptrace"
+	"testing"
+)
+
+func TestSetAttrOnScopeSpans_Empty(t *testing.T) {
+	traces := ptrace.NewTraces()
+	traceData := &TraceData{
+		ReceivedBatches: traces,
+	}
+
+	SetAttrOnScopeSpans(traceData, "test.attr", "value")
+}
+
+func TestSetAttrOnScopeSpans_Many(t *testing.T) {
+
+	assertAttrExists := func(t *testing.T, attrs pcommon.Map, key string, value string) {
+		v, ok := attrs.Get(key)
+		assert.True(t, ok)
+		assert.Equal(t, value, v.AsString())
+	}
+
+	traces := ptrace.NewTraces()
+
+	rs1 := traces.ResourceSpans().AppendEmpty()
+	ss1 := rs1.ScopeSpans().AppendEmpty()
+	span1 := ss1.Spans().AppendEmpty()
+	span2 := ss1.Spans().AppendEmpty()
+	ss2 := rs1.ScopeSpans().AppendEmpty()
+	span3 := ss2.Spans().AppendEmpty()
+	rs2 := traces.ResourceSpans().AppendEmpty()
+	ss3 := rs2.ScopeSpans().AppendEmpty()
+	span4 := ss3.Spans().AppendEmpty()
+
+	traceData := &TraceData{
+		ReceivedBatches: traces,
+	}
+
+	SetAttrOnScopeSpans(traceData, "test.attr", "value")
+
+	assertAttrExists(t, ss1.Scope().Attributes(), "test.attr", "value")
+	assertAttrExists(t, ss2.Scope().Attributes(), "test.attr", "value")
+	assertAttrExists(t, ss3.Scope().Attributes(), "test.attr", "value")
+
+	_, ok := span1.Attributes().Get("test.attr")
+	assert.False(t, ok)
+	_, ok = span2.Attributes().Get("test.attr")
+	assert.False(t, ok)
+	_, ok = span3.Attributes().Get("test.attr")
+	assert.False(t, ok)
+	_, ok = span4.Attributes().Get("test.attr")
+	assert.False(t, ok)
+}
+
+func BenchmarkSetAttrOnScopeSpans(b *testing.B) {
+	for n := 0; n < b.N; n++ {
+
+		traces := ptrace.NewTraces()
+
+		for i := 0; i < 5; i++ {
+			rs := traces.ResourceSpans().AppendEmpty()
+			ss1 := rs.ScopeSpans().AppendEmpty()
+			ss1.Spans().AppendEmpty()
+			ss1.Spans().AppendEmpty()
+			ss1.Spans().AppendEmpty()
+
+			ss2 := rs.ScopeSpans().AppendEmpty()
+			ss2.Spans().AppendEmpty()
+			ss2.Spans().AppendEmpty()
+
+			ss3 := rs.ScopeSpans().AppendEmpty()
+			ss3.Spans().AppendEmpty()
+		}
+
+		traceData := &TraceData{
+			ReceivedBatches: traces,
+		}
+
+		b.StartTimer()
+		SetAttrOnScopeSpans(traceData, "test.attr", "value")
+		b.StopTimer()
+	}
+}