Is it by design that opening with -S doesn't account for changes in file permission?

Question

If a_session is created via :mksession, when reopening that session via vim -S a_session, Vim doesn't take into account that file permissions of some of the opened files might have changed in the meanwhile.

Here are the repro steps.

1. Open a_file which is not readonly, and make sure that's the case by verifying :echo &readonly gives 0;
2. save the session, :mksession;
3. close Vim, :q;
4. in the terminal, make the file readonly chmod -w a_file;
5. reopen Vim via vim -S Session.vim;
6. verify that :echo &readonly still returns 0, which I find strange
7. run :e
8. verify that :echo &readonly returns 1 now.

Is what happens at step 6 by design?

Answer 1

Session is simply a VimScript file. You can read through it and see what is going on.

What it does, it re-creates buffer(s) and after that it attempts to restore all previously saved options. By doing so it may overwrite any automatic settings such as &readonly that were set while the new buffer was being loaded.

IMO, saving options into session script should be avoided.

Refer to :h 'sessionoptions' to do adjustments.