Fix data-race in metric without code and method but with `WithLabelFr…

…omCtx` (prometheus#1318) This commit fixes a data race that exists when the metric used in any `promhttp` middleware doesn't collect the `code` and `method` but uses `WithLabelFromCtx` to collect values from context. The problem happens because when no `code` and `method` tags are collected, the `labels` function returns a pre-initialized map `emptyLabels` for every request. When one or multipe `WithLabelFromCtx` options are configured, the returned map from the `labels` function call is used to collect the metrics from context which creates a multi-write data race. Signed-off-by: Tiago Silva <[email protected]>
msam1r · Aug 1, 2023 · 59c00e3 · 59c00e3
1 parent 7f2db5f
commit 59c00e3
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Showing 2 changed files with 4 additions and 7 deletions.
diff --git a/prometheus/promhttp/instrument_server.go b/prometheus/promhttp/instrument_server.go
@@ -389,15 +389,12 @@ func isLabelCurried(c prometheus.Collector, label string) bool {
 	return true
 }
 
-// emptyLabels is a one-time allocation for non-partitioned metrics to avoid
-// unnecessary allocations on each request.
-var emptyLabels = prometheus.Labels{}
-
 func labels(code, method bool, reqMethod string, status int, extraMethods ...string) prometheus.Labels {
+	labels := prometheus.Labels{}
+
 	if !(code || method) {
-		return emptyLabels
+		return labels
 	}
-	labels := prometheus.Labels{}
 
 	if code {
 		labels["code"] = sanitizeCode(status)

diff --git a/prometheus/promhttp/instrument_server_test.go b/prometheus/promhttp/instrument_server_test.go
@@ -250,7 +250,7 @@ func TestLabels(t *testing.T) {
 	}{
 		"empty": {
 			varLabels:  []string{},
-			wantLabels: emptyLabels,
+			wantLabels: prometheus.Labels{},
 			reqMethod:  "GET",
 			respStatus: 200,
 			ok:         true,