Some bug fixes:

- logging: LogToFile shadowed the package-level fileLogger, so
  IsFileLogger was always false and Shutdown never closed the file
- logging: replace IsJSONHandler's unsafe reflection into slog
  internals with a *slog.JSONHandler type check
- cli: fail if the log file can't be opened instead of silently
  logging to the terminal; return the log shutdown error from Finish
- build.sh: -X flags used the wrong module path, so the version, build
  time and commit were never stamped

internal/logging/logging.go

@@ -3,8 +3,6 @@ package logging
 import (
 	"log/slog"
 	"os"
-	"reflect"
-	"unsafe"
 
 	"github.com/lmittmann/tint"
 )
@@ -33,7 +31,8 @@ func LogToFile(logfile string) error {
 	}
 
 	// Open the log file
-	fileLogger, err := os.OpenFile(logfile, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0666)
+	var err error
+	fileLogger, err = os.OpenFile(logfile, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0o600)
 	if err != nil {
 		return err
 	}
@@ -67,35 +66,10 @@ func LogToTerminal() {
 	slog.SetDefault(logger)
 }
 
+// IsJSONHandler reports whether h writes JSON, as the file logger does.
 func IsJSONHandler(h slog.Handler) bool {
-	val := reflect.ValueOf(h)
-
-	// Ensure we are working with a pointer to a struct (*JSONHandler or *TextHandler or derived)
-	if val.Kind() != reflect.Pointer || val.Elem().Kind() != reflect.Struct {
-		return false
-	}
-
-	// Both JSONHandler and TextHandler wrap an unexported field named 'commonHandler'
-	commonHandlerField := val.Elem().FieldByName("commonHandler")
-	if !commonHandlerField.IsValid() {
-		return false
-	}
-
-	// Get the underlying *commonHandler struct pointer
-	// commonHandlerField is unexported, so we must use unsafe to bypass Go's safety checks
-	ptr := reflect.NewAt(
-		commonHandlerField.Type().Elem(),
-		unsafe.Pointer(commonHandlerField.Pointer()),
-	)
-
-	// Find the unexported 'json' boolean field inside commonHandler
-	jsonField := ptr.Elem().FieldByName("json")
-	if !jsonField.IsValid() {
-		return false
-	}
-
-	// Read the boolean value using unsafe
-	return *(*bool)(unsafe.Pointer(jsonField.UnsafeAddr()))
+	_, ok := h.(*slog.JSONHandler)
+	return ok
 }
 
 func IsFileLogger() bool {