bug#30786: Save text properties in desktop

[Juri Linkov <juri@linkov.net>]

>>>> Do you think this patch covers all possible unreadable cases on writing?
>>>>
>>>> +     ;; Get rid of unreadable text properties.
>>>> +     (if (ignore-errors (read (format "%S" value)))
>>>> +         (cons 'may value)
>>>> +       (let ((copy (copy-sequence value)))
>>>> +         (set-text-properties 0 (length copy) nil copy)
>>>> +         (cons 'may copy))))
>>>
>>> I think it won't cover the case where an object's print syntax is only
>>> readable by the current Emacs version, and not an earlier one.  To
>>> handle that you'll need to call desktop--v2s recursively, like in the
>>> vectorp and consp branches.
>>
>> I don't understand how calling desktop--v2s recursively will ensure its
>> readability in earlier versions?  For example, if Emacs 27 will write to
>> the desktop file ‘#("foo" 0 3 (bar 42))’ how Emacs 19 can read it?
>
> Ah, I wasn't thinking of the #(...) specifically, rather about the
> values of text properties, e.g.,
>
>     (cl-defstruct foo f1 f2)
>     (desktop--v2s (make-foo :f1 1 :f2 2)) ;=> (may . "Unprintable entity")
>
> So desktop--v2s considers structs as unprintable, unlike read, hence
> using desktop--v2s recursively instead of read will catch properties
> that would otherwise be unreadable in Emacs 25.
>
> If we want to support Emacs versions that don't even know about #(...)
> then we can't write string properties at all, I guess.

If we can afford continuing incrementing the desktop file version in
‘desktop-file-version’ doing this every time when support for reading more
syntaxes is added, thus preventing incompatibilities, then the patch above
would be the most reliable solution.