diff: prevent pprint_rename from underrunning input

The logic described in d020e27 (diff: Fix rename pretty-print when
suffix and prefix overlap, 2013-02-23) is wrong: The proof in the
comment is valid only if both strings are the same length.  *One* of
old/new can reach a-1 (b-1, resp.) if 'a' is a suffix of 'b' (or vice
versa).

Since the intent was to let the loop run down to the '/' at the end of
the common prefix, fix it by making that distinction explicit: if
there is no prefix, allow no underrun.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/diff.c b/diff.c
index e1d82c9..d641c26 100644 (file)
--- a/diff.c
+++ b/diff.c
@@ -1151,6 +1151,7 @@ static char *pprint_rename(const char *a, const char *b)
        const char *new = b;
        struct strbuf name = STRBUF_INIT;
        int pfx_length, sfx_length;
+       int pfx_adjust_for_slash;
        int len_a = strlen(a);
        int len_b = strlen(b);
        int a_midlen, b_midlen;
@@ -1178,14 +1179,16 @@ static char *pprint_rename(const char *a, const char *b)
        new = b + len_b;
        sfx_length = 0;
        /*
-        * Note:
-        * if pfx_length is 0, old/new will never reach a - 1 because it
-        * would mean the whole string is common suffix. But then, the
-        * whole string would also be a common prefix, and we would not
-        * have pfx_length equals 0.
+        * If there is a common prefix, it must end in a slash.  In
+        * that case we let this loop run 1 into the prefix to see the
+        * same slash.
+        *
+        * If there is no common prefix, we cannot do this as it would
+        * underrun the input strings.
         */
-       while (a + pfx_length - 1 <= old &&
-              b + pfx_length - 1 <= new &&
+       pfx_adjust_for_slash = (pfx_length ? 1 : 0);
+       while (a + pfx_length - pfx_adjust_for_slash <= old &&
+              b + pfx_length - pfx_adjust_for_slash <= new &&
               *old == *new) {
                if (*old == '/')
                        sfx_length = len_a - (old - a);