Re: [PATCH] range-diff: support reading mbox files

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

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Hi Phillip,

On Thu, 17 Nov 2022, Phillip Wood wrote:

> On 16/11/2022 14:40, Phillip Wood wrote:
> > > +        } else if (state == MBOX_IN_DIFF) {
> > > +            switch (line[0]) {
> > > +            case '\0':
> > > +                continue; /* ignore empty lines after diff */
> > > +            case '+':
> > > +            case '-':
> > > +            case ' ':
> > > +                if (!old_count && !new_count)
> > > +                    break;
> > 
> > This shouldn't happen in a well formed diff. Below we happily accept bad
> > counts, is there a reason to reject them here?
> 
> I think this might be picking up the "--" at the end of the patch as we don't
> want to break here at the end of a hunk. If so then a comment would be
> helpful.

Agreed. And yes, it is picking up the "-- " line at the end of the patch.

> > > +                if (old_count && line[0] != '+')
> > > +                    old_count--;
> > > +                if (new_count && line[0] != '-')
> > > +                    new_count--;
> > 
> > The diff is malformed if old_count == 0 and we see '-' or ' ' or new_count
> > == 0 and we see '+' or ' '. The code is careful not to decrement the count
> > in that case so I think it is harmless to accept diffs with bad line counts
> > in the hunk header.

I might be overly cautious here, but as you mentioned elsewhere, it is
really bad if a `size_t` is decremented below 0, and
`new_count`/`old_count` are of that type.

> > > +                /* fallthrough */
> > > +            case '\\':
> > > +                strbuf_addstr(&buf, line);
> > > +                strbuf_addch(&buf, '\n');
> > > +                util->diffsize++;
> > 
> > I think this might be a better place to break if old_count and new_count are
> > both zero.
> 
> It would be the right place to break at the end of each hunk, but I don't
> think we want to do that.

It would not even be the right place to break here then: think of the
`\ No newline at end of file` lines: they come after the preceding line
decremented `old_count`/`new_count`, yet we still want them to be part of
the diff.

> 
> > > +                continue;
> > > +            case '@':
> > > +                if (parse_hunk_header(line, &old_count,
> > > +                              &new_count, &p))
> > > +                    break;
> > > +
> > > +                strbuf_addstr(&buf, "@@");
> > > +                if (current_filename && *p)
> > > +                    strbuf_addf(&buf, " %s:",
> > > +                            current_filename);
> > > +                strbuf_addstr(&buf, p);
> > > +                strbuf_addch(&buf, '\n');
> > > +                util->diffsize++;
> > > +                continue;
> > > +            }
> > 
> > This is effectively the `default:` clause as it is executed when we don't
> > handle the line above. We ignore the contents of this line which makes me
> > wonder what happens if it is the start of another diff.
> 
> We'll pick that up earlier with "if (starts_with(line, "diff --git"))"
> 
> We only get here at the end of a patch (assuming it has the "--" line from
> format-patch)

We also get here in case of garbage in the middle of a diff ;-)

Thank you for setting a fantastic example how to review code in a
constructive, helpful manner!
Dscho