feat: add stderr logging to its own file

main.go

@@ -34,6 +34,8 @@ func main() {
 		log.Fatal(err)
 	}
 
+	var panicLog *os.File
+
 	app := &cli.App{
 		Name:    "strimertul",
 		Usage:   "the small broadcasting suite for Twitch",
@@ -83,12 +85,27 @@ func main() {
 				level = zapcore.InfoLevel
 			}
 			initLogger(level)
+
+			// Create file for panics
+			panicLog, err = os.OpenFile("strimertul-panic.log", os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0o666)
+			if err != nil {
+				logger.Warn("could not create panic log", zap.Error(err))
+			} else {
+				utils.RedirectStderr(panicLog)
+			}
+
+			zap.RedirectStdLog(logger)()
+
 			ctx.Context = context.WithValue(ctx.Context, utils.ContextLogger, logger)
 			return nil
 		},
 		After: func(ctx *cli.Context) error {
+			if panicLog != nil {
+				utils.Close(panicLog, logger)
+			}
+
 			_ = logger.Sync()
-			zap.RedirectStdLog(logger)()
+
 			return nil
 		},
 	}

utils/stderr_unix.go

@@ -0,0 +1,17 @@
+//go:build unix
+
+package utils
+
+import "os"
+
+import (
+	"log"
+	"syscall"
+)
+
+func RedirectStderr(f *os.File) {
+	err := syscall.Dup2(int(f.Fd()), int(os.Stderr.Fd()))
+	if err != nil {
+		log.Fatalf("Failed to redirect stderr to file: %v", err)
+	}
+}

utils/stderr_windows.go

@@ -0,0 +1,34 @@
+//go:build windows
+
+package utils
+
+import (
+	"log"
+	"os"
+	"syscall"
+)
+
+var (
+	kernel32         = syscall.MustLoadDLL("kernel32.dll")
+	procSetStdHandle = kernel32.MustFindProc("SetStdHandle")
+)
+
+func setStdHandle(stdhandle int32, handle syscall.Handle) error {
+	r0, _, e1 := syscall.Syscall(procSetStdHandle.Addr(), 2, uintptr(stdhandle), uintptr(handle), 0)
+	if r0 == 0 {
+		if e1 != 0 {
+			return error(e1)
+		}
+		return syscall.EINVAL
+	}
+	return nil
+}
+
+func RedirectStderr(f *os.File) {
+	err := setStdHandle(syscall.STD_ERROR_HANDLE, syscall.Handle(f.Fd()))
+	if err != nil {
+		log.Fatalf("Failed to redirect stderr to file: %v", err)
+	}
+	// SetStdHandle does not affect prior references to stderr
+	os.Stderr = f
+}