Re: [Emacs-diffs] emacs-26 9bf66c6: Don't run FOR_EACH_FRAME when there's no frame left (Bug#29961)

[Paul Eggert <eggert@cs.ucla.edu>]

martin rudalics wrote:
>  > I recognized callers that either --enable-gcc-warnings complained
>  > about (because code after the FOR_EACH_FRAME loop clearly had
>  > undefined behavior unless the loop iterated at least once),
> 
> Which ones were these?

The code with undefined behavior is in delete_frame, after its 3rd use of 
FOR_EACH_FRAME. This loop head is of the form 'FOR_EACH_FRAME (tail, frame1) 
...' and the code after the loop assumes that frame1 is initialized, an 
assumption that is false if Vframe_list is nil.

> Neither next_frame nor prev_frame exhibit such behavior IMO.

That's right. However, commit 8720f601e715e5f1d41f7cf863a525a1cc1bc12c removed 
these functions' assertions that frame-list is non-nil, so I thought it wise to 
resurrect them.

>  > and
>  > callers where historically there was an eassert that checked the
>  > assumption.
> 
> How far did you go back in history?

To commit 8720f601e715e5f1d41f7cf863a525a1cc1bc12c, which is the commit that 
inserted the (now-removed) assumption that frame-list is non-nil when 
FOR_EACH_FRAME is first executed.