fix(tracer): use configurable MaxTagsHeaderLen for x-datadog-tags extraction

mtoffl01 · claude · mtoffl01 · commit aeda5a9d9cf0 · 2026-05-05T15:36:36.000-04:00
unmarshalPropagatingTags now uses the configured PropagatorConfig
MaxTagsHeaderLen as the size cap on incoming x-datadog-tags headers
instead of a hardcoded 512-byte constant. A non-positive MaxTagsHeaderLen
disables extraction, mirroring the existing inject-side disable behavior.
The DD_TRACE_X_DATADOG_TAGS_MAX_LENGTH environment variable now governs
both inject and extract.

Co-Authored-By: Claude Opus 4.7 (1M context) &lt;noreply@anthropic.com&gt;
diff --git a/ddtrace/tracer/textmap.go b/ddtrace/tracer/textmap.go
@@ -103,9 +103,6 @@ const originHeader = "x-datadog-origin"
 // traceTagsHeader holds the propagated trace tags
 const traceTagsHeader = "x-datadog-tags"
 
-// propagationExtractMaxSize limits the total size of incoming propagated tags to parse
-const propagationExtractMaxSize = 512
-
 // PropagatorConfig defines the configuration for initializing a propagator.
 type PropagatorConfig struct {
 	// BaggagePrefix specifies the prefix that will be used to store baggage
@@ -539,7 +536,7 @@ func (p *propagator) extractTextMap(reader TextMapReader) (*SpanContext, error)
 		case originHeader:
 			ctx.origin = v // +checklocksignore - Initialization time, freshly extracted ctx not yet shared.
 		case traceTagsHeader:
-			unmarshalPropagatingTags(&ctx, v)
+			unmarshalPropagatingTags(&ctx, v, p.cfg.MaxTagsHeaderLen)
 		default:
 			if after, ok := strings.CutPrefix(key, p.cfg.BaggagePrefix); ok {
 				ctx.setBaggageItem(after, v)
@@ -603,13 +600,18 @@ func overrideDatadogParentID(ctx, w3cCtx, ddCtx *SpanContext) {
 	}
 }
 
-// unmarshalPropagatingTags unmarshals tags from v into ctx
-func unmarshalPropagatingTags(ctx *SpanContext, v string) {
+// unmarshalPropagatingTags unmarshals tags from v into ctx, dropping the
+// entire header if its length exceeds maxLen. A non-positive maxLen disables
+// extraction (mirroring the inject side).
+func unmarshalPropagatingTags(ctx *SpanContext, v string, maxLen int) {
 	if ctx.trace == nil {
 		ctx.trace = newTrace()
 	}
-	if len(v) > propagationExtractMaxSize {
-		log.Warn("Did not extract %s, size limit exceeded: %d. Incoming tags will not be propagated further.", traceTagsHeader, propagationExtractMaxSize)
+	if maxLen <= 0 {
+		return
+	}
+	if len(v) > maxLen {
+		log.Warn("Did not extract %s, size limit exceeded: %d. Incoming tags will not be propagated further.", traceTagsHeader, maxLen)
 		ctx.trace.setTag(keyPropagationError, "extract_max_size")
 		return
 	}
diff --git a/ddtrace/tracer/textmap_test.go b/ddtrace/tracer/textmap_test.go
@@ -675,6 +675,48 @@ func TestTextMapPropagator(t *testing.T) {
 	})
 }
 
+func TestExtractTraceTagsHeaderUsesMaxTagsHeaderLen(t *testing.T) {
+	t.Setenv(headerPropagationStyleExtract, "datadog")
+	const customMax = 200
+	propagator := NewPropagator(&PropagatorConfig{
+		MaxTagsHeaderLen: customMax,
+	})
+
+	// Header is longer than the configured cap but shorter than the previous
+	// hardcoded 512 cap, to confirm extract now honors the configured maxLen.
+	tags := "_dd.p.k=" + strings.Repeat("a", customMax)
+	require.Greater(t, len(tags), customMax)
+	require.Less(t, len(tags), 512)
+
+	src := TextMapCarrier(map[string]string{
+		DefaultTraceIDHeader:  "1",
+		DefaultParentIDHeader: "1",
+		traceTagsHeader:       tags,
+	})
+	ctx, err := propagator.Extract(src)
+	require.NoError(t, err)
+	assert.Equal(t, "extract_max_size", ctx.trace.tags["_dd.propagation_error"])
+}
+
+func TestExtractTraceTagsHeaderDisabled(t *testing.T) {
+	t.Setenv(headerPropagationStyleExtract, "datadog")
+	propagator := NewPropagator(&PropagatorConfig{
+		MaxTagsHeaderLen: 0, // disable, mirroring inject
+	})
+
+	src := TextMapCarrier(map[string]string{
+		DefaultTraceIDHeader:  "1",
+		DefaultParentIDHeader: "1",
+		traceTagsHeader:       "_dd.p.k=v",
+	})
+	ctx, err := propagator.Extract(src)
+	require.NoError(t, err)
+	// Disabled extract: no propagating tags, no error tag.
+	_, hasErr := ctx.trace.tags["_dd.propagation_error"]
+	assert.False(t, hasErr)
+	assert.Empty(t, ctx.trace.propagatingTags)
+}
+
 func TestEnvVars(t *testing.T) {
 	var testEnvs []map[string]string
 
@@ -2434,7 +2476,7 @@ func FuzzMarshalPropagatingTags(f *testing.F) {
 		if _, ok := sendCtx.trace.tags[keyPropagationError]; ok {
 			t.Skipf("Skipping invalid tags")
 		}
-		unmarshalPropagatingTags(recvCtx, marshal)
+		unmarshalPropagatingTags(recvCtx, marshal, pConfig.MaxTagsHeaderLen)
 		marshaled := sendCtx.trace.propagatingTags
 		unmarshaled := recvCtx.trace.propagatingTags
 		if !reflect.DeepEqual(sendCtx.trace.propagatingTags, recvCtx.trace.propagatingTags) {
