Fix auto reload when a config file with a syntax error is reverted

When we had a syntax error but restored the old file, we did not
re-trigger the config reload, so the config reload metric was showing
that config reload was unsucessful.

I made magic to handle logs in cmd/prometheus.

For now it is a separate file so we can backport this easily.

I will generalize the helper in another PR.

Signed-off-by: Julien <roidelapluie@o11y.eu>

cmd/prometheus/main.go

@@ -1153,9 +1153,8 @@ func main() {
 						if err := reloadConfig(cfg.configFile, cfg.tsdb.EnableExemplarStorage, logger, noStepSubqueryInterval, callback, reloaders...); err != nil {
 							logger.Error("Error reloading config", "err", err)
 						} else if cfg.enableAutoReload {
-							if currentChecksum, err := config.GenerateChecksum(cfg.configFile); err == nil {
+							checksum, err = config.GenerateChecksum(cfg.configFile)
-								checksum = currentChecksum
+							if err != nil {
-							} else {
 								logger.Error("Failed to generate checksum during configuration reload", "err", err)
 							}
 						}
@@ -1166,9 +1165,8 @@ func main() {
 						} else {
 							rc <- nil
 							if cfg.enableAutoReload {
-								if currentChecksum, err := config.GenerateChecksum(cfg.configFile); err == nil {
+								checksum, err = config.GenerateChecksum(cfg.configFile)
-									checksum = currentChecksum
+								if err != nil {
-								} else {
 									logger.Error("Failed to generate checksum during configuration reload", "err", err)
 								}
 							}
@@ -1179,6 +1177,7 @@ func main() {
 						}
 						currentChecksum, err := config.GenerateChecksum(cfg.configFile)
 						if err != nil {
+							checksum = currentChecksum
 							logger.Error("Failed to generate checksum during configuration reload", "err", err)
 						} else if currentChecksum == checksum {
 							continue

cmd/prometheus/reload_test.go

@@ -0,0 +1,229 @@
+// Copyright 2024 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"bufio"
+	"encoding/json"
+	"io"
+	"net/http"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/prometheus/prometheus/util/testutil"
+)
+
+const configReloadMetric = "prometheus_config_last_reload_successful"
+
+func TestAutoReloadConfig_ValidToValid(t *testing.T) {
+	steps := []struct {
+		configText       string
+		expectedInterval string
+		expectedMetric   float64
+	}{
+		{
+			configText: `
+global:
+  scrape_interval: 30s
+`,
+			expectedInterval: "30s",
+			expectedMetric:   1,
+		},
+		{
+			configText: `
+global:
+  scrape_interval: 15s
+`,
+			expectedInterval: "15s",
+			expectedMetric:   1,
+		},
+		{
+			configText: `
+global:
+  scrape_interval: 30s
+`,
+			expectedInterval: "30s",
+			expectedMetric:   1,
+		},
+	}
+
+	runTestSteps(t, steps)
+}
+
+func TestAutoReloadConfig_ValidToInvalidToValid(t *testing.T) {
+	steps := []struct {
+		configText       string
+		expectedInterval string
+		expectedMetric   float64
+	}{
+		{
+			configText: `
+global:
+  scrape_interval: 30s
+`,
+			expectedInterval: "30s",
+			expectedMetric:   1,
+		},
+		{
+			configText: `
+global:
+  scrape_interval: 15s
+invalid_syntax
+`,
+			expectedInterval: "30s",
+			expectedMetric:   0,
+		},
+		{
+			configText: `
+global:
+  scrape_interval: 30s
+`,
+			expectedInterval: "30s",
+			expectedMetric:   1,
+		},
+	}
+
+	runTestSteps(t, steps)
+}
+
+func runTestSteps(t *testing.T, steps []struct {
+	configText       string
+	expectedInterval string
+	expectedMetric   float64
+},
+) {
+	configDir := t.TempDir()
+	configFilePath := filepath.Join(configDir, "prometheus.yml")
+
+	t.Logf("Config file path: %s", configFilePath)
+
+	require.NoError(t, os.WriteFile(configFilePath, []byte(steps[0].configText), 0o644), "Failed to write initial config file")
+
+	port := testutil.RandomUnprivilegedPort(t)
+	runPrometheusWithLogging(t, configFilePath, port)
+
+	baseURL := "http://localhost:" + strconv.Itoa(port)
+	require.Eventually(t, func() bool {
+		resp, err := http.Get(baseURL + "/-/ready")
+		if err != nil {
+			return false
+		}
+		defer resp.Body.Close()
+		return resp.StatusCode == http.StatusOK
+	}, 5*time.Second, 100*time.Millisecond, "Prometheus didn't become ready in time")
+
+	for i, step := range steps {
+		t.Logf("Step %d", i)
+		require.NoError(t, os.WriteFile(configFilePath, []byte(step.configText), 0o644), "Failed to write config file for step")
+
+		require.Eventually(t, func() bool {
+			return verifyScrapeInterval(t, baseURL, step.expectedInterval) &&
+				verifyConfigReloadMetric(t, baseURL, step.expectedMetric)
+		}, 10*time.Second, 500*time.Millisecond, "Prometheus config reload didn't happen in time")
+	}
+}
+
+func verifyScrapeInterval(t *testing.T, baseURL, expectedInterval string) bool {
+	resp, err := http.Get(baseURL + "/api/v1/status/config")
+	require.NoError(t, err)
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	require.NoError(t, err)
+
+	config := struct {
+		Data struct {
+			YAML string `json:"yaml"`
+		} `json:"data"`
+	}{}
+
+	require.NoError(t, json.Unmarshal(body, &config))
+	return strings.Contains(config.Data.YAML, "scrape_interval: "+expectedInterval)
+}
+
+func verifyConfigReloadMetric(t *testing.T, baseURL string, expectedValue float64) bool {
+	resp, err := http.Get(baseURL + "/metrics")
+	require.NoError(t, err)
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	require.NoError(t, err)
+
+	lines := string(body)
+	var actualValue float64
+	found := false
+
+	for _, line := range strings.Split(lines, "\n") {
+		if strings.HasPrefix(line, configReloadMetric) {
+			parts := strings.Fields(line)
+			if len(parts) >= 2 {
+				actualValue, err = strconv.ParseFloat(parts[1], 64)
+				require.NoError(t, err)
+				found = true
+				break
+			}
+		}
+	}
+
+	return found && actualValue == expectedValue
+}
+
+func captureLogsToTLog(t *testing.T, r io.Reader) {
+	scanner := bufio.NewScanner(r)
+	for scanner.Scan() {
+		t.Log(scanner.Text())
+	}
+	if err := scanner.Err(); err != nil {
+		t.Logf("Error reading logs: %v", err)
+	}
+}
+
+func runPrometheusWithLogging(t *testing.T, configFilePath string, port int) {
+	stdoutPipe, stdoutWriter := io.Pipe()
+	stderrPipe, stderrWriter := io.Pipe()
+
+	var wg sync.WaitGroup
+	wg.Add(2)
+
+	prom := exec.Command(promPath, "-test.main", "--enable-feature=auto-reload-config", "--config.file="+configFilePath, "--config.auto-reload-interval=1s", "--web.listen-address=0.0.0.0:"+strconv.Itoa(port))
+	prom.Stdout = stdoutWriter
+	prom.Stderr = stderrWriter
+
+	go func() {
+		defer wg.Done()
+		captureLogsToTLog(t, stdoutPipe)
+	}()
+	go func() {
+		defer wg.Done()
+		captureLogsToTLog(t, stderrPipe)
+	}()
+
+	t.Cleanup(func() {
+		prom.Process.Kill()
+		prom.Wait()
+		stdoutWriter.Close()
+		stderrWriter.Close()
+		wg.Wait()
+	})
+
+	require.NoError(t, prom.Start())
+}