Re: [PATCH 2/5] grep: stop modifying buffer in show_line()

From: Jeff King <peff@peff.net>
Date: Sep 21, 2021, 04:42 UTC
Message-ID: <YUlinQ7hPokFLgfm@coredump.intra.peff.net>
In-Reply-To: <YUleFU4QrKb28bDz@nand.local>

On Tue, Sep 21, 2021 at 12:22:45AM -0400, Taylor Blau wrote:

Show 13 quoted lines

> On Mon, Sep 20, 2021 at 11:48:09PM -0400, Jeff King wrote:
> > When showing lines via grep (or looking for funcnames), we call
> > show_line() on a multi-line buffer. It finds the end of line and marks
> > it with a NUL. However, we don't need to do so, as the resulting line is
> > only used along with its "eol" marker:
> >
> >  - we pass both to next_match(), which takes care to look at only the
> >    bytes we specified
> 
> Thinking aloud, next_match() calls match_next_pattern() which takes eol
> as non-const and passes it to match_one_pattern(). And that calls
> strip_timestamp(), which would be non-const, were it not the previous
> patch. So I think this conversion is safe.

To be a little nit-picky: this move would be OK even without the change to strip_timestamp(). The question is whether any of those sub-calls actually looks past the "eol" pointer we give it.

I didn't dig all the way down through the complete call-stack in an exhaustive way. But after having looked at the related functions when changing their const-ness elsewhere in the series, they'd have to be ignoring the "eol" parameter for it to be a problem. The irony, of course, is that they did ignore this parameter at one point! Because they'd all eventually end in regexec(), which would happily read past our "eol".

So really, all of this is predicated on those sub-functions behaving sensibly. I think they do. But if we found that they didn't, I'd much rather know that and fix them than live with this "this NUL may or may not be important" state forever.

Show 7 quoted lines

> >  - we pass the line to output_color() without its matching eol marker.
> >    However, we do use the "match" struct we got from next_match() to
> >    tell it how many bytes to look at (which can never exceed the string
> >    we passed it
> 
> Yep, makes sense. The patch looks good and matches your description
> here.

Thanks for looking it over (my nitpick aside). :)

-Peff