testutil: Add ScrapeAndCompare (#1043)

* testutil: Add ScrapeAndCompare

Signed-off-by: sazary <soroosh@azary.ir>

* testutil: Use %w verb wherever we're using an error in fmt.Errorf

Signed-off-by: sazary <soroosh@azary.ir>

* Format

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

Co-authored-by: Kemal Akkoyun <kakkoyun@users.noreply.github.com>
Co-authored-by: Kemal Akkoyun <kakkoyun@gmail.com>

prometheus/testutil/testutil.go

@@ -41,12 +41,12 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"net/http"
 	"reflect"
 
 	"github.com/davecgh/go-spew/spew"
-	"github.com/prometheus/common/expfmt"
-
 	dto "github.com/prometheus/client_model/go"
+	"github.com/prometheus/common/expfmt"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/internal"
@@ -155,6 +155,34 @@ func GatherAndCount(g prometheus.Gatherer, metricNames ...string) (int, error) {
 	return result, nil
 }
 
+// ScrapeAndCompare calls a remote exporter's endpoint which is expected to return some metrics in
+// plain text format. Then it compares it with the results that the `expected` would return.
+// If the `metricNames` is not empty it would filter the comparison only to the given metric names.
+func ScrapeAndCompare(url string, expected io.Reader, metricNames ...string) error {
+	resp, err := http.Get(url)
+	if err != nil {
+		return fmt.Errorf("scraping metrics failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("the scraping target returned a status code other than 200: %d",
+			resp.StatusCode)
+	}
+
+	scraped, err := convertReaderToMetricFamily(resp.Body)
+	if err != nil {
+		return err
+	}
+
+	wanted, err := convertReaderToMetricFamily(expected)
+	if err != nil {
+		return err
+	}
+
+	return compareMetricFamilies(scraped, wanted, metricNames...)
+}
+
 // CollectAndCompare registers the provided Collector with a newly created
 // pedantic Registry. It then calls GatherAndCompare with that Registry and with
 // the provided metricNames.
@@ -184,17 +212,35 @@ func TransactionalGatherAndCompare(g prometheus.TransactionalGatherer, expected
 	if err != nil {
 		return fmt.Errorf("gathering metrics failed: %w", err)
 	}
+
+	wanted, err := convertReaderToMetricFamily(expected)
+	if err != nil {
+		return err
+	}
+
+	return compareMetricFamilies(got, wanted, metricNames...)
+}
+
+// convertReaderToMetricFamily would read from a io.Reader object and convert it to a slice of
+// dto.MetricFamily.
+func convertReaderToMetricFamily(reader io.Reader) ([]*dto.MetricFamily, error) {
+	var tp expfmt.TextParser
+	notNormalized, err := tp.TextToMetricFamilies(reader)
+	if err != nil {
+		return nil, fmt.Errorf("converting reader to metric families failed: %w", err)
+	}
+
+	return internal.NormalizeMetricFamilies(notNormalized), nil
+}
+
+// compareMetricFamilies would compare 2 slices of metric families, and optionally filters both of
+// them to the `metricNames` provided.
+func compareMetricFamilies(got, expected []*dto.MetricFamily, metricNames ...string) error {
 	if metricNames != nil {
 		got = filterMetrics(got, metricNames)
 	}
-	var tp expfmt.TextParser
-	wantRaw, err := tp.TextToMetricFamilies(expected)
-	if err != nil {
-		return fmt.Errorf("parsing expected metrics failed: %w", err)
-	}
-	want := internal.NormalizeMetricFamilies(wantRaw)
 
-	return compare(got, want)
+	return compare(got, expected)
 }
 
 // compare encodes both provided slices of metric families into the text format,

prometheus/testutil/testutil_test.go

@@ -14,6 +14,9 @@
 package testutil
 
 import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
 	"strings"
 	"testing"
 
@@ -308,6 +311,61 @@ Diff:
 	}
 }
 
+func TestScrapeAndCompare(t *testing.T) {
+	const expected = `
+		# HELP some_total A value that represents a counter.
+		# TYPE some_total counter
+
+		some_total{ label1 = "value1" } 1
+	`
+
+	expectedReader := strings.NewReader(expected)
+
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprintln(w, expected)
+	}))
+	defer ts.Close()
+
+	if err := ScrapeAndCompare(ts.URL, expectedReader, "some_total"); err != nil {
+		t.Errorf("unexpected scraping result:\n%s", err)
+	}
+}
+
+func TestScrapeAndCompareFetchingFail(t *testing.T) {
+	err := ScrapeAndCompare("some_url", strings.NewReader("some expectation"), "some_total")
+	if err == nil {
+		t.Errorf("expected an error but got nil")
+	}
+	if !strings.HasPrefix(err.Error(), "scraping metrics failed") {
+		t.Errorf("unexpected error happened: %s", err)
+	}
+}
+
+func TestScrapeAndCompareBadStatusCode(t *testing.T) {
+	const expected = `
+		# HELP some_total A value that represents a counter.
+		# TYPE some_total counter
+
+		some_total{ label1 = "value1" } 1
+	`
+
+	expectedReader := strings.NewReader(expected)
+
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusBadGateway)
+		fmt.Fprintln(w, expected)
+	}))
+	defer ts.Close()
+
+	err := ScrapeAndCompare(ts.URL, expectedReader, "some_total")
+	if err == nil {
+		t.Errorf("expected an error but got nil")
+	}
+	if !strings.HasPrefix(err.Error(), "the scraping target returned a status code other than 200") {
+		t.Errorf("unexpected error happened: %s", err)
+	}
+}
+
 func TestCollectAndCount(t *testing.T) {
 	c := prometheus.NewCounterVec(
 		prometheus.CounterOpts{