using text properties in a buffer that is older than its file

using text properties in a buffer that is older than its file

[Drew Adams]

Suppose some code just modifies some text properties.  This is
considered by Emacs to be a buffer modification, and if the
file has been changed externally since it was last visited in
the Emacs session, then trying to modify the text properties
prompts the user with:

  ... changed on disk; really edit the buffer?

(This is done by `ask-user-about-supersession-threat'.)

Saving, resetting, and restoring `buffer-modified-p' is not
sufficient to inhibit this user prompting.  And if the code
is called multiple times then the user gets prompted multiple
times (assuming a response that keeps the buffer unsynced).

What is a good way to inhibit this prompting?  Binding
`buffer-file-name' to nil works, but is there a better way?

Is there now (shouldn't there be?) a way to tell Emacs that
within some scope it should not consider text-property changes
to be buffer modifications - at least for purposes of
consideration by `ask-user-about-supersession-threat'?

That would be much better than binding `buffer-file-name', as
it could be done around code that might let a user modify the
buffer (text changes, not just property changes), and the user
would still get prompted for any real text change.

I'm hoping that there is already a feature for handling this
kind of use case, which I'm just unaware of.

Re: using text properties in a buffer that is older than its file

[João Távora]

On Fri, Jul 31, 2015 at 7:20 PM, Drew Adams <drew.adams@oracle.com> wrote:
> Saving, resetting, and restoring `buffer-modified-p' is not
> sufficient to inhibit this user prompting.
> [...]
> I'm hoping that there is already a feature for handling this
> kind of use case, which I'm just unaware of.

`with-silent-modifications' does the trick for me.

I tried with an externally modified file don't get the prompt.

João

RE: using text properties in a buffer that is older than its file

[Drew Adams]

> `with-silent-modifications' does the trick for me.
> I tried with an externally modified file don't get the prompt.

Thank you, João.

I've filed bug # , to get this documented in the Elisp manual.
It's apparently been around since Emacs 23.

RE: using text properties in a buffer that is older than its file

[Drew Adams]

> > `with-silent-modifications'... 
>
> I've filed bug #     , to get this documented in the Elisp manual.
                  21171

Re: using text properties in a buffer that is older than its file

[Stefan Monnier]

> Saving, resetting, and restoring `buffer-modified-p' is not
> sufficient to inhibit this user prompting.

C-h f with-silent-modifications RET


        Stefan

RE: using text properties in a buffer that is older than its file

[Drew Adams]

> > Saving, resetting, and restoring `buffer-modified-p' is not
> > sufficient to inhibit this user prompting.
> 
> C-h f with-silent-modifications RET

Yes, thanks.  (João beat you to it - thanks to both.)
And please see bug #21171.