test: Ensure LIBCAMERA_BASE_PRIVATE isn't public

If LIBCAMERA_BASE_PRIVATE is ever exposed on the libcamera public dependencies,
then the private.h header protection will be circumvented.

Provide a test which will fail (at compile time) if the LIBCAMERA_BASE_PRIVATE
define ever leaks to the public dependencies.

Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

test/meson.build

@@ -25,6 +25,7 @@ subdir('v4l2_videodevice')
 
 public_tests = [
     ['geometry',                        'geometry.cpp'],
+    ['public-api',                      'public-api.cpp'],
     ['signal',                          'signal.cpp'],
     ['span',                            'span.cpp'],
 ]

test/public-api.cpp

@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Copyright (C) 2021, Google Inc.
+ *
+ * public-api.cpp - Public API validation
+ */
+
+#include <libcamera/libcamera.h>
+
+#include "test.h"
+
+class PublicAPITest : public Test
+{
+	int run()
+	{
+#ifdef LIBCAMERA_BASE_PRIVATE
+#error "Public interfaces should not be exposed to LIBCAMERA_BASE_PRIVATE"
+		return TestFail;
+#else
+		return TestPass;
+#endif
+	}
+};
+
+TEST_REGISTER(PublicAPITest)