Re: [PATCH] diff.c: increment buffer pointer in all code path

[Jeff King]

On Thu, Oct 19, 2017 at 02:30:08PM +0900, Junio C Hamano wrote:

> Jeff King <p...@peff.net> writes:
> 
> > it does. It just adjusts our "end pointer" to point to the last valid
> > character in the string (rather than one past), which seems to be the
> > convention that those loops (and next_byte) expect.
> 
> Yeah I am not sure if I like this comparison at the beginning of the
> function:
> 
>         static int next_byte(const char **cp, const char **endp,
>                              const struct diff_options *diffopt)
>         {
>                 int retval;
> 
>                 if (*cp > *endp)
>                         return -1;
> 
> but it says endp _is_ part of valid input, contrary to my intuition.

Actually, I think even this function is confused about its convention.
In the line you quote, we clearly treat *endp as part of the input. But
later we do:

  while (*cp < *endp && isspace(**cp))
          (*cp)++;

meaning that we'd fail to soak up whitespace at *endp. That wouldn't be
so bad if not for the other bug which fails to eat whitespace at endp in
the first place. :)

So I think the right fix is this:

diff --git a/diff.c b/diff.c
index 6fd288420b..09081a207c 100644
--- a/diff.c
+++ b/diff.c
@@ -712,7 +712,7 @@ static int next_byte(const char **cp, const char **endp,
 {
        int retval;
 
-       if (*cp > *endp)
+       if (*cp >= *endp)
                return -1;
 
        if (isspace(**cp)) {
@@ -729,7 +729,12 @@ static int next_byte(const char **cp, const char **endp,
                if (DIFF_XDL_TST(diffopt, IGNORE_WHITESPACE)) {
                        while (*cp < *endp && isspace(**cp))
                                (*cp)++;
-                       /* return the first non-ws character via the usual 
below */
+                       /*
+                        * return the first non-ws character via the usual
+                        * below, unless we ate all of the bytes
+                        */
+                       if (*cp >= *endp)
+                               return -1;
                }
        }
 
@@ -750,9 +755,9 @@ static int moved_entry_cmp(const struct diff_options 
*diffopt,
                return a->es->len != b->es->len  || memcmp(ap, bp, a->es->len);
 
        if (DIFF_XDL_TST(diffopt, IGNORE_WHITESPACE_AT_EOL)) {
-               while (ae > ap && isspace(*ae))
+               while (ae > ap && isspace(ae[-1]))
                        ae--;
-               while (be > bp && isspace(*be))
+               while (be > bp && isspace(be[-1]))
                        be--;
        }
 
@@ -775,7 +780,7 @@ static unsigned get_string_hash(struct emitted_diff_symbol 
*es, struct diff_opti
                int c;
 
                strbuf_reset(&sb);
-               while (ae > ap && isspace(*ae))
+               while (ae > ap && isspace(ae[-1]))
                        ae--;
                while ((c = next_byte(&ap, &ae, o)) > 0)
                        strbuf_addch(&sb, c);

It's late here, so I'll wait for comments from Stefan and then try to
wrap it up with a commit message and test tomorrow.

-Peff