net: speed up select fdwatch lookups

tests/smoke_auth.cpp

@@ -308,6 +308,46 @@ void TestFdwatchBackendMetadata()
 
 	fdwatch_delete(fdw);
 }
+
+void TestFdwatchSlotReuseAfterDelete()
+{
+	int sockets_a[2] = { -1, -1 };
+	int sockets_b[2] = { -1, -1 };
+	Expect(socketpair(AF_UNIX, SOCK_STREAM, 0, sockets_a) == 0, "socketpair A failed");
+	Expect(socketpair(AF_UNIX, SOCK_STREAM, 0, sockets_b) == 0, "socketpair B failed");
+
+	LPFDWATCH fdw = fdwatch_new(64);
+	Expect(fdw != nullptr, "fdwatch_new for slot reuse failed");
+
+	int marker_a = 11;
+	int marker_b = 22;
+
+	fdwatch_add_fd(fdw, sockets_a[1], &marker_a, FDW_READ, false);
+	fdwatch_add_fd(fdw, sockets_b[1], &marker_b, FDW_READ, false);
+	fdwatch_del_fd(fdw, sockets_a[1]);
+
+	const uint8_t byte = 0x51;
+	Expect(write(sockets_b[0], &byte, sizeof(byte)) == sizeof(byte), "socketpair B write failed");
+
+	timeval timeout {};
+	timeout.tv_sec = 0;
+	timeout.tv_usec = 200000;
+
+	const int num_events = fdwatch(fdw, &timeout);
+	Expect(num_events == 1, "Expected one read event after slot reuse");
+	Expect(fdwatch_get_client_data(fdw, 0) == &marker_b, "Unexpected client data after slot reuse");
+	Expect(fdwatch_check_event(fdw, sockets_b[1], 0) == FDW_READ, "Expected FDW_READ after slot reuse");
+
+	uint8_t read_back = 0;
+	Expect(read(sockets_b[1], &read_back, sizeof(read_back)) == sizeof(read_back), "socketpair B read failed");
+	Expect(read_back == byte, "Read payload mismatch after slot reuse");
+
+	fdwatch_delete(fdw);
+	close(sockets_a[0]);
+	close(sockets_a[1]);
+	close(sockets_b[0]);
+	close(sockets_b[1]);
+}
 }
 
 int main()
@@ -319,6 +359,7 @@ int main()
 		TestSocketAuthWireFlow();
 		TestFdwatchBackendMetadata();
 		TestFdwatchReadAndOneshotWrite();
+		TestFdwatchSlotReuseAfterDelete();
 		std::cout << "metin smoke tests passed\n";
 		return 0;
 	}