net: encapsulate fdwatch backend metadata

tests/smoke_auth.cpp

@@ -1,3 +1,4 @@
+#include <algorithm>
 #include <array>
 #include <cstdint>
 #include <cstring>
@@ -289,6 +290,24 @@ void TestFdwatchReadAndOneshotWrite()
 	close(sockets[0]);
 	close(sockets[1]);
 }
+
+void TestFdwatchBackendMetadata()
+{
+	LPFDWATCH fdw = fdwatch_new(4096);
+	Expect(fdw != nullptr, "fdwatch_new for backend metadata failed");
+
+#ifdef __USE_SELECT__
+	Expect(fdwatch_get_backend(fdw) == FDWATCH_BACKEND_SELECT, "Expected select backend");
+	Expect(std::strcmp(fdwatch_backend_name(fdwatch_get_backend(fdw)), "select") == 0, "Unexpected select backend name");
+	Expect(fdwatch_get_descriptor_limit(fdw) == std::min(4096, static_cast<int>(FD_SETSIZE)), "Unexpected select descriptor limit");
+#else
+	Expect(fdwatch_get_backend(fdw) == FDWATCH_BACKEND_KQUEUE, "Expected kqueue backend");
+	Expect(std::strcmp(fdwatch_backend_name(fdwatch_get_backend(fdw)), "kqueue") == 0, "Unexpected kqueue backend name");
+	Expect(fdwatch_get_descriptor_limit(fdw) == 4096, "Unexpected kqueue descriptor limit");
+#endif
+
+	fdwatch_delete(fdw);
+}
 }
 
 int main()
@@ -298,6 +317,7 @@ int main()
 		TestPacketLayouts();
 		TestSecureCipherRoundTrip();
 		TestSocketAuthWireFlow();
+		TestFdwatchBackendMetadata();
 		TestFdwatchReadAndOneshotWrite();
 		std::cout << "metin smoke tests passed\n";
 		return 0;