fix nil pointer error on trace by id path (grafana#1441)

* fix nil pointer error on trace by id path

Signed-off-by: Joe Elliott <number101010@gmail.com>

* changelog

Signed-off-by: Joe Elliott <number101010@gmail.com>

CHANGELOG.md

@@ -1,4 +1,11 @@
-## main / unreleased
+[##](##) main / unreleased
+
+* [CHANGE] metrics-generator: Changed added metric label `instance` to `__metrics_gen_instance` to reduce collisions with custom dimensions. [#1439](https://github.com/grafana/tempo/pull/1439) (@joe-elliott)
+* [FEATURE] metrics-generator: support per-tenant processor configuration [#1434](https://github.com/grafana/tempo/pull/1434) (@kvrhdn)
+* [ENHANCEMENT] Added the ability to have a per tenant max search duration. [#1421](https://github.com/grafana/tempo/pull/1421) (@joe-elliott)
+* [BUGFIX] Fix nil pointer panic when the trace by id path errors. [1441](https://github.com/grafana/tempo/pull/1441) (@joe-elliott)
+
+## v1.4.1 / 2022-05-05
 
 * [BUGFIX] metrics-generator: don't inject X-Scope-OrgID header for single-tenant setups [1417](https://github.com/grafana/tempo/pull/1417) (@kvrhdn)
 * [BUGFIX] compactor: populate `compaction_objects_combined_total` and `tempo_discarded_spans_total{reason="trace_too_large_to_compact"}` metrics again [1420](https://github.com/grafana/tempo/pull/1420) (@mdisibio)

modules/frontend/frontend.go

@@ -160,14 +160,16 @@ func newTraceByIDMiddleware(cfg Config, logger log.Logger) Middleware {
 					}
 					resp.Body = io.NopCloser(bytes.NewReader(traceBuffer))
 				}
+
+				if resp.Header != nil {
+					resp.Header.Set(api.HeaderContentType, marshallingFormat)
+				}
 			}
 			span := opentracing.SpanFromContext(r.Context())
 			if span != nil {
 				span.SetTag("contentType", marshallingFormat)
 			}
 
-			resp.Header.Set(api.HeaderContentType, marshallingFormat)
-
 			return resp, err
 		})
 	})