Metrics API: Fix text format parsing error (#6640)

* Metrics API: Fix text format parsing error resulting in unexpected end of input stream

Signed-off-by: rickbrouwer <[email protected]>

* Update an existing test to cover the empty new line case

Signed-off-by: Jorge Turrado <[email protected]>

---------

Signed-off-by: rickbrouwer <[email protected]>
Signed-off-by: Jorge Turrado <[email protected]>
Co-authored-by: Jorge Turrado <[email protected]>

Author: rickbrouwer

CHANGELOG.md

@@ -99,6 +99,7 @@ Here is an overview of all new **experimental** features:
 - **AWS Scalers**: Add AWS region to the AWS Config Cache key ([#6128](https://github.com/kedacore/keda/issues/6128))
 - **External Scaler**: Support server TLS without custom CA ([#6606](https://github.com/kedacore/keda/pull/6606))
 - **GCP Storage**: GCP Storage scaler ignores folders ([#6531](https://github.com/kedacore/keda/issues/6531))
+- **Metrics API**: Fix text format parsing error resulting in unexpected end of input stream ([#6559](https://github.com/kedacore/keda/issues/6559))
 - **NATS JetStream**: Support for looking up account using an ID ([#6611](https://github.com/kedacore/keda/pull/6611))
 - **Redis Streams**: Allow default value of 0 for activationLagCount ([#6478](https://github.com/kedacore/keda/issues/6478))
 - **Selenium Grid**: Scaler logic on platformName is set empty or `any` ([#6477](https://github.com/kedacore/keda/issues/6477))

pkg/scalers/metrics_api_scaler.go

@@ -274,13 +274,22 @@ func getValueFromPrometheusResponse(body []byte, valueLocation string) (float64,
 			metricName = v.Value
 		}
 	}
+
 	// Ensure EOL
-	reader := strings.NewReader(strings.ReplaceAll(string(body), "\r\n", "\n"))
+	bodyStr := strings.ReplaceAll(string(body), "\r\n", "\n")
+
+	// Check if newline is present
+	if len(bodyStr) > 0 && !strings.HasSuffix(bodyStr, "\n") {
+		bodyStr += "\n"
+	}
+
+	reader := strings.NewReader(bodyStr)
 	familiesParser := expfmt.TextParser{}
 	families, err := familiesParser.TextToMetricFamilies(reader)
 	if err != nil {
-		return 0, err
+		return 0, fmt.Errorf("prometheus format parsing error: %w", err)
 	}
+
 	family, ok := families[metricName]
 	if !ok {
 		return 0, fmt.Errorf("metric '%s' not found", metricName)

pkg/scalers/metrics_api_scaler_test.go

@@ -133,8 +133,9 @@ func TestGetValueFromResponse(t *testing.T) {
 	backend_queue_size{queueName="two", instance="zero"} 20
 	# HELP random_metric Random metric generate to include noise
 	# TYPE random_metric counter
-	random_metric 10
-	`)
+	random_metric 10`)
+	// Ending the file without new line is intended to verify
+	// https://github.com/kedacore/keda/issues/6559
 
 	testCases := []struct {
 		name      string