change: replace log with log/slog for logging (#1280)

* change: replace log with log/slog for logging
* add TODO about slog.DiscardHandler

logging/logger_test.go

@@ -0,0 +1,60 @@
+package logging_test
+
+import (
+	"os"
+	"testing"
+
+	"github.com/mgechev/revive/logging"
+)
+
+func TestGetLogger(t *testing.T) {
+	t.Run("no debug", func(t *testing.T) {
+		t.Setenv("DEBUG", "")
+
+		logger, err := logging.GetLogger()
+
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+		if logger == nil {
+			t.Fatal("expected logger to be non-nil")
+		}
+		logger.Info("msg") // no output
+	})
+
+	t.Run("debug", func(t *testing.T) {
+		t.Setenv("DEBUG", "1")
+		t.Cleanup(func() { os.Remove("revive.log") })
+
+		logger, err := logging.GetLogger()
+
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+		if logger == nil {
+			t.Fatal("expected logger to be non-nil")
+		}
+		if _, err := os.Stat("revive.log"); os.IsNotExist(err) {
+			t.Error("expected revive.log file to be created")
+		}
+	})
+
+	t.Run("reuse logger", func(t *testing.T) {
+		t.Setenv("DEBUG", "1")
+		t.Cleanup(func() { os.Remove("revive.log") })
+
+		logger1, err := logging.GetLogger()
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+
+		logger2, err := logging.GetLogger()
+		if err != nil {
+			t.Fatalf("expected no error, got %v", err)
+		}
+
+		if logger1 != logger2 {
+			t.Errorf("expected the same logger instance to be returned: logger1=%+v, logger2=%+v", logger1, logger2)
+		}
+	})
+}