bug#64439: 28.2; auto-fill-mode gets turned on all over the place

[Jim Porter <jporterbugs@gmail.com>]

On 7/10/2023 4:59 AM, Eli Zaretskii wrote:
>> Date: Sun, 9 Jul 2023 11:00:46 -0700
>> Cc: dhowells@redhat.com, 64439@debbugs.gnu.org
>> From: Jim Porter <jporterbugs@gmail.com>
>>
>> The bug is that when this occurs, rather than setting
>> 'auto-fill-function' buffer-locally in text modes, it actually (somehow)
>> sets the default value of 'auto-fill-function'
> 
> I don't understand how this is possible with local variables.  Maybe
> because some code calls makunbound for it?  Does local-variable-p
> still return non-nil for auto-fill-function when this triggers?

I'll add a 'local-variable-p' check to my test code, but whatever's 
happening here, it only makes 'auto-fill-function' non-local 
temporarily. Immediately after the backtrace I posted, I evaled 
"(setq-default auto-fill-function nil)" to fix the default value, and 
then called 'normal-mode' in a text-mode buffer, and it correctly 
enabled auto-fill-mode in just that buffer.

> Btw, it is possible that your trap snaps too late: that the default
> value of auto-fill-function is non-nil does not yet mean this happens
> when your hook is called, it could have happened earlier.  Because
> setting the buffer-local value will DTRT regardless of the global
> value, AFAIU.

That's true, though I'm pretty sure it's not the case here, since I 
noticed my prog-mode buffers suddenly have 'auto-fill-mode' (which 
prompted me to check my *Messages* output to get the backtrace. I'll add 
some extra checks here in my test code.

>> I've only ever seen this happen when
>> 'ask-user-about-supersession-threat' is in the stack. The backtraces
>> I've captured all include Tramp too, but I'm not sure the latter is
>> actually necessary to reproduce this bug, or if it just changes the
>> timings to make it more likely.
> 
> "Timings"? is this stuff asynchronous in some sense?  Michael, any
> ideas?

It's just a guess, but if some other Lisp code can run while Tramp is 
fetching the file data, that could possibly cause some issue.