fix: Unset Content-Encoding header when uncompressed

CHANGELOG.md

@@ -1,5 +1,9 @@
 ## Unreleased
 
+## 1.20.2 / 2024-08-23
+
+* [BUGFIX] promhttp: Unset Content-Encoding header when data is uncompressed. #1596
+
 ## 1.20.1 / 2024-08-20
 
 * [BUGFIX] process-collector: Fixed unregistered descriptor error when using process collector with `PedanticRegistry` on linux machines. #1587

VERSION

@@ -1 +1 @@
-1.20.1
+1.20.2

prometheus/promhttp/http.go

@@ -203,8 +203,10 @@ func HandlerForTransactional(reg prometheus.TransactionalGatherer, opts HandlerO
 
 		defer closeWriter()
 
-		rsp.Header().Set(contentEncodingHeader, encodingHeader)
-
+		// Set Content-Encoding only when data is compressed
+		if encodingHeader != string(Identity) {
+			rsp.Header().Set(contentEncodingHeader, encodingHeader)
+		}
 		enc := expfmt.NewEncoder(w, contentType)
 
 		// handleError handles the error according to opts.ErrorHandling

prometheus/promhttp/http_test.go

@@ -267,7 +267,7 @@ func TestInstrumentMetricHandler(t *testing.T) {
 		t.Errorf("got HTTP status code %d, want %d", got, want)
 	}
 
-	if got, want := writer.Header().Get(contentEncodingHeader), string(Identity); got != want {
+	if got, want := writer.Header().Get(contentEncodingHeader), ""; got != want {
 		t.Errorf("got HTTP content encoding header %s, want %s", got, want)
 	}