fix: log startup errors to the event log when running as a service (#2506)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Jan-Otto Kröpke <mail@jkroepke.de>

.github/workflows/pr-check.yaml

@@ -7,6 +7,7 @@ on:
       - synchronize
       - labeled
       - unlabeled
+      - edited
 
 permissions: {}
 

cmd/windows_exporter/0_service.go

@@ -18,8 +18,10 @@
 package main
 
 import (
+	"context"
 	"errors"
 	"fmt"
+	"log/slog"
 	"os"
 	"strings"
 	"unsafe"
@@ -161,6 +163,22 @@ func logToEventToLog(eType uint16, msg string) error {
 	return nil
 }
 
+// logStartupError logs an error that occurs before the configured logger exists.
+// A service has no console to receive the default stderr output,
+// so the error is written to the event log as well.
+func logStartupError(ctx context.Context, msg string, err error) {
+	if IsService {
+		if logErr := logToEventToLog(windows.EVENTLOG_ERROR_TYPE, fmt.Sprintf("%s: %v", msg, err)); logErr != nil {
+			logToFile(fmt.Sprintf("%s: %v", msg, err))
+		}
+	}
+
+	//nolint:sloglint // We do not have a logger yet.
+	slog.LogAttrs(ctx, slog.LevelError, msg,
+		slog.Any("err", err),
+	)
+}
+
 func logToFile(msg string) {
 	if file, err := os.CreateTemp("", "windows_exporter.service.error.log"); err == nil {
 		_, _ = file.WriteString(msg)

cmd/windows_exporter/main.go

@@ -128,10 +128,7 @@ func run(ctx context.Context, args []string) int {
 	collectors := collector.NewWithFlags(app)
 
 	if err := config.Parse(app, args); err != nil {
-		//nolint:sloglint // we do not have an logger yet
-		slog.LogAttrs(ctx, slog.LevelError, "Failed to load configuration",
-			slog.Any("err", err),
-		)
+		logStartupError(ctx, "Failed to load configuration", err)
 
 		return 1
 	}
@@ -140,9 +137,7 @@ func run(ctx context.Context, args []string) int {
 
 	logger, err := log.New(logConfig)
 	if err != nil {
-		logger.LogAttrs(ctx, slog.LevelError, "failed to create logger",
-			slog.Any("err", err),
-		)
+		logStartupError(ctx, "failed to create logger", err)
 
 		return 1
 	}