bug#71604: diff--get-revision-properties calls set-auto-mode twice

[Dmitry Gutov <dmitry@gutov.dev>]

On 17/06/2024 19:33, Juri Linkov wrote:
>> vc-find-revision-no-save calls set-auto-mode, and then we repeat that call
>> in diff-syntax-fontify-props.
>>
>> Which is a problem with CC Mode because it parses the buffer inside the
>> major mode function, and ends up doing that twice, at least the first time
>> the buffer is initialized before being saved to revision cache. Not exactly
>> double the work, but more work nevertheless.
>>
>> This is a split off from https://debbugs.gnu.org/61667#587. I thought
>> I recalled an existing report of vc-diff being slow, but apparently not.
>>
>> @@ -2883,9 +2883,6 @@ diff--get-revision-properties
>>            (buffer (cdr entry)))
>>       (if (buffer-live-p buffer)
>>           (progn
>> -          ;; Don't re-initialize the buffer (which would throw
>> -          ;; away the previous fontification work).
>> -          (setq file nil)
>>             (setq diff--cached-revision-buffers
>>                   (cons entry
>>                         (delq entry diff--cached-revision-buffers))))
>> @@ -2905,7 +2902,8 @@ diff--get-revision-properties
>>       (diff--cache-schedule-clean)
>>       (and buffer
>>            (with-current-buffer buffer
>> -           (diff-syntax-fontify-props file text line-nb)))))
>> +           ;; major mode is set in vc-find-revision-no-save already.
>> +           (diff-syntax-fontify-props nil text line-nb)))))
> 
> Thanks, looks correct.

Pushed to master, thanks.