[PATCH] commit-reach: fix sorting commits by generation

[Thomas Gummerer <t.gummerer@gmail.com>]

compare_commit_by_gen is used to sort a list of pointers to 'struct
commit'.  The comparison function for qsort is called with pointers to
the objects it needs to compare, so when sorting a list of 'struct
commit *', the arguments are of type 'struct commit **'.  However,
currently the comparison function casts it's arguments to 'struct
commit *' and uses those, leading to out of bounds memory access and
potentially to wrong results.  Fix that.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
---

I noticed this by running the test suite through valgrind.  I'm not
familiar with this code, so I'm not sure why this didn't cause any
issues or how they would manifest, but this seems like the right fix
for this function either way.

 commit-reach.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/commit-reach.c b/commit-reach.c
index bc522d6840..9efddfd7a0 100644
--- a/commit-reach.c
+++ b/commit-reach.c
@@ -516,8 +516,8 @@ int commit_contains(struct ref_filter *filter, struct commit *commit,
 
 static int compare_commits_by_gen(const void *_a, const void *_b)
 {
-	const struct commit *a = (const struct commit *)_a;
-	const struct commit *b = (const struct commit *)_b;
+	const struct commit *a = *(const struct commit **)_a;
+	const struct commit *b = *(const struct commit **)_b;
 
 	if (a->generation < b->generation)
 		return -1;
-- 
2.19.1.759.g500967bb5e