add precondition for mount/unmount in commands response

src/command/AllCommands.cxx

@@ -234,6 +234,12 @@ command_available([[maybe_unused]] const Partition &partition,
 		return neighbor_commands_available(partition.instance);
 #endif
 
+#ifdef ENABLE_DATABASE
+	if (StringIsEqual(cmd->cmd, "mount") ||
+		StringIsEqual(cmd->cmd, "unmount"))
+		return mount_commands_available(partition.instance);
+#endif
+
 	if (StringIsEqual(cmd->cmd, "save") ||
 	    StringIsEqual(cmd->cmd, "rm") ||
 	    StringIsEqual(cmd->cmd, "rename") ||

src/command/StorageCommands.cxx

@@ -275,3 +275,15 @@ handle_unmount(Client &client, Request args, Response &r)
 
 	return CommandResult::OK;
 }
+
+bool
+mount_commands_available(Instance &instance) noexcept
+{
+#ifdef ENABLE_DATABASE
+	if (auto *db = dynamic_cast<SimpleDatabase *>(instance.GetDatabase())) {
+		return db->HasCache();
+	}
+#endif
+
+	return false;
+}

src/command/StorageCommands.hxx

@@ -22,6 +22,7 @@
 
 #include "CommandResult.hxx"
 
+struct Instance;
 class Client;
 class Storage;
 class Request;
@@ -42,4 +43,8 @@ handle_mount(Client &client, Request request, Response &response);
 CommandResult
 handle_unmount(Client &client, Request request, Response &response);
 
+[[gnu::pure]]
+bool
+mount_commands_available(Instance &instance) noexcept;
+
 #endif

src/db/plugins/simple/SimpleDatabasePlugin.hxx

@@ -86,6 +86,10 @@ public:
 		return *root;
 	}
 
+	bool HasCache() const noexcept {
+		return !cache_path.IsNull();
+	}
+
 	void Save();
 
 	/**