From: Tom Jones <thj@freebsd.org>
Subject: Re: diff: handle missing new lines in trailing context
To: gameoftrees@openbsd.org
Date: Fri, 30 Sep 2022 14:05:50 +0100

On Thu, Sep 29, 2022 at 12:37:46PM +0200, Stefan Sperling wrote:
> On Thu, Sep 29, 2022 at 12:24:22PM +0200, Stefan Sperling wrote:
> > Well, our loops certainly take great care not to index result->chunks.head
> > in the way you are proposing. For example:
> > 
> >         for (c_idx = cc->chunk.start; c_idx < cc->chunk.end; c_idx++) {
> >                 const struct diff_chunk *c = &result->chunks.head[c_idx];
> > 
> > This keeps c_idx < chunk.end, not <= chunk.end.
> > 
> > Can you show what you are seeing in the debugger? What are the contents
> > of the final chunk? In particular, what is that chunk's type?
> 
> Some more hints:
> 
> Keep in mind that an arraylist might have more memory allocated than
> needed to represent the elements of the array (the elements being diff
> chunks in this case).
> 
> What is important is that we never index at or beyond the chunk arraylist's
> len field. If there is a chunk at index 'len' it should not be considered
> valid. Because it is not part of the user-visible data stored on the array.
> 
> The 'chunk.end' index points to the first chunk array element following this
> chunk. If chunk.end equals the chunk arraylist's len, we are at end-of-file.

Is the correct final chunk in a result the one at result->chunks.len -
1?

This works:
	rc = diff_output_trailing_newline_msg(outinfo, dest,          
			&result->chunks.head[result->chunks.len - 1]);

and doesn't violate the public length of the array list.

- Tom