Check flat node file format for forwards compatibility

In a flat node file the first location is that of node id 0, which does
not exist. So the first location must always be the undefined location,
otherwise we might be looking at a different kind of file.

This adds a check for forwards compatibility. If and when we change the
file format, we can use the first 8 bytes to differentiate the file
formats.

src/node-persistent-cache.cpp

@@ -45,6 +45,15 @@ node_persistent_cache::node_persistent_cache(std::string file_name,
     }
 
     m_index = std::make_unique<index_t>(m_fd);
+
+    // First location must always be the undefined location, otherwise we
+    // might be looking at a different kind of file. This check here is for
+    // forwards compatibility. If and when we change the file format, we
+    // can use the first 8 bytes to differentiate the file format.
+    auto const loc = get(0);
+    if (loc.is_defined()) {
+        throw fmt_error("Not a version 1 flatnode file '{}'", m_file_name);
+    }
 }
 
 node_persistent_cache::~node_persistent_cache() noexcept