fix(query-frontend): restore trace ID in slow query logs

CHANGELOG.md

@@ -18,6 +18,7 @@ It is recommend to upgrade the storage components first (Receive, Store, etc.) a
 - [#8714](https://github.com/thanos-io/thanos/pull/8714): Tracing: Fix `tls_config` fields (`ca_file`, `cert_file`, `key_file`) being silently ignored when using the OTLP gRPC exporter. Previously, deployments using a private CA or mTLS client certificates had to work around this via `OTEL_EXPORTER_OTLP_CERTIFICATE` and related environment variables.
 - [#8128](https://github.com/thanos-io/thanos/issues/8128): Query-Frontend: Fix panic in `AnalyzesMerge` caused by indexing the wrong slice variable, leading to an out-of-range access when merging more than two query analyses.
 - [#8720](https://github.com/thanos-io/thanos/issues/8720): Receive: Fix 503 errors during restarts in some cases.
+- [#8762](https://github.com/thanos-io/thanos/pull/8762): Query-Frontend: Fix trace ID missing from slow query logs, regression from #8618.
 
 ### Added
 

internal/cortex/frontend/transport/handler.go

@@ -127,7 +127,7 @@ func (f *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	// Ensure we log slow queries and query statistics regardless of how the request completes.
 	defer func() {
-		f.reportQueryStatsAndSlowQueries(r, resp, buf, startTime, stats)
+		f.reportQueryStatsAndSlowQueries(r, w.Header(), buf, startTime, stats)
 	}()
 
 	var err error
@@ -153,7 +153,7 @@ func (f *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 }
 
 // reportQueryStatsAndSlowQueries handles logging of slow queries and query statistics.
-func (f *Handler) reportQueryStatsAndSlowQueries(r *http.Request, resp *http.Response, buf bytes.Buffer, startTime time.Time, stats *querier_stats.Stats) {
+func (f *Handler) reportQueryStatsAndSlowQueries(r *http.Request, responseHeaders http.Header, buf bytes.Buffer, startTime time.Time, stats *querier_stats.Stats) {
 	queryResponseTime := time.Since(startTime)
 
 	shouldReportSlowQuery := f.cfg.LogQueriesLongerThan != 0 &&
@@ -164,10 +164,6 @@ func (f *Handler) reportQueryStatsAndSlowQueries(r *http.Request, resp *http.Res
 		queryString := f.parseRequestQueryString(r, buf)
 
 		if shouldReportSlowQuery {
-			var responseHeaders http.Header
-			if resp != nil {
-				responseHeaders = resp.Header
-			}
 			f.reportSlowQuery(r, responseHeaders, queryString, queryResponseTime, stats)
 		}
 		if f.cfg.QueryStatsEnabled {

internal/cortex/frontend/transport/handler_test.go

@@ -50,6 +50,7 @@ func TestHandler_SlowQueryLog(t *testing.T) {
 				"param_query=absent(up)",
 				"param_start=1714262400",
 				"param_end=1714266000",
+				"trace_id=test-trace-id-123",
 			},
 		},
 		{
@@ -114,7 +115,9 @@ func TestHandler_SlowQueryLog(t *testing.T) {
 
 			handler := NewHandler(cfg, fakeRoundTripper, logger, prometheus.NewRegistry())
 
-			handler.ServeHTTP(httptest.NewRecorder(), httptest.NewRequest("GET", tt.url, nil))
+			recorder := httptest.NewRecorder()
+			recorder.Header().Set("X-Thanos-Trace-Id", "test-trace-id-123")
+			handler.ServeHTTP(recorder, httptest.NewRequest("GET", tt.url, nil))
 
 			for _, part := range tt.logParts {
 				require.Contains(t, logWriter.String(), part)
@@ -141,11 +144,19 @@ func TestHandler_SlowQueryLogOnError(t *testing.T) {
 	logger := log.NewLogfmtLogger(log.NewSyncWriter(logWriter))
 
 	handler := NewHandler(cfg, fakeRT, logger, prometheus.NewRegistry())
-	handler.ServeHTTP(httptest.NewRecorder(), httptest.NewRequest("GET", "/api/v1/query?query=up", nil))
+
+	// Simulate tracing middleware setting the trace ID on the ResponseWriter
+	// before the handler runs. The slow query log must include this trace ID
+	// even when the round-tripper returns an error (resp is nil).
+	recorder := httptest.NewRecorder()
+	recorder.Header().Set("X-Thanos-Trace-Id", "test-trace-abc123")
+
+	handler.ServeHTTP(recorder, httptest.NewRequest("GET", "/api/v1/query?query=up", nil))
 
 	// Verify slow query is logged even when round-tripper returns an error
 	require.Contains(t, logWriter.String(), "slow query detected")
 	require.Contains(t, logWriter.String(), "time_taken=")
 	require.Contains(t, logWriter.String(), "path=/api/v1/query")
 	require.Contains(t, logWriter.String(), "param_query=up")
+	require.Contains(t, logWriter.String(), "test-trace-abc123")
 }