[PATCH] rebase --autosquash: fix a potential segfault

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

When rearranging the todo list so that the fixups/squashes are reordered
just after the commits they intend to fix up, we use two arrays to
maintain that list: `next` and `tail`.

The idea is that `next[i]`, if set to a non-negative value, contains the
index of the item that should be rearranged just after the `i`th item.

To avoid having to walk the entire `next` chain when appending another
fixup/squash, we also store the end of the `next` chain in `last[i]`.

The logic we currently use to update these array items is based on the
assumption that given a fixup/squash item at index `i`, we just found
the index `i2` indicating the first item in that fixup chain.

However, as reported by Paul Ganssle, that need not be true: the special
form `fixup! <commit-hash>` is allowed to point to _another_ fixup
commit in the middle of the fixup chain.

Example:

	* 0192a To fixup
	* 02f12 fixup! To fixup
	* 03763 fixup! To fixup
	* 04ecb fixup! 02f12

Note how the fourth commit targets the second commit, which is already a
fixup that targets the first commit.

The good news is that it is easy to fix this: we can detect the
situation by looking at `last[i2]` (which will be `-1` if `i2` is
actually in the middle of a fixup chain), and in that case we simply
need to squeeze the current item into the middle of the `next` chain,
without touching `last` (i.e. leaving the end index of the fixup chain
alone).

Reported-by: Paul Ganssle <paul@ganssle.io>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
    rebase --autosquash: fix a potential segfault
    
    This patch is in response to 
    https://lore.kernel.org/git/017dbc40-8d21-00fb-7b0e-6708d2dcb366@ganssle.io/
    .

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-625%2Fdscho%2Fautosquash-segfault-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-625/dscho/autosquash-segfault-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/625

 sequencer.c                  | 11 ++++++++++-
 t/t3415-rebase-autosquash.sh | 14 ++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/sequencer.c b/sequencer.c
index e528225e787..0d4d53d2a49 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -5266,8 +5266,17 @@ int todo_list_rearrange_squash(struct todo_list *todo_list)
 				TODO_FIXUP : TODO_SQUASH;
 			if (next[i2] < 0)
 				next[i2] = i;
-			else
+			else if (tail[i2] >= 0)
 				next[tail[i2]] = i;
+			else {
+				/*
+				 * i2 refers to a fixup commit in the middle of
+				 * a fixup chain
+				 */
+				next[i] = next[i2];
+				next[i2] = i;
+				continue;
+			}
 			tail[i2] = i;
 		} else if (!hashmap_get_from_hash(&subject2item,
 						strhash(subject), subject)) {
diff --git a/t/t3415-rebase-autosquash.sh b/t/t3415-rebase-autosquash.sh
index 093de9005b7..ca135349346 100755
--- a/t/t3415-rebase-autosquash.sh
+++ b/t/t3415-rebase-autosquash.sh
@@ -424,4 +424,18 @@ test_expect_success 'abort last squash' '
 	! grep first actual
 '
 
+test_expect_success 'fixup a fixup' '
+	echo 0to-fixup >file0 &&
+	test_tick &&
+	git commit -m "to-fixup" file0 &&
+	test_tick &&
+	git commit --squash HEAD -m X --allow-empty &&
+	test_tick &&
+	git commit --squash HEAD^ -m Y --allow-empty &&
+	test_tick &&
+	git commit -m "squash! $(git rev-parse HEAD^)" -m Z --allow-empty &&
+	git rebase -ki --autosquash HEAD~4 &&
+	test XZY = $(git show | tr -cd X-Z)
+'
+
 test_done

base-commit: af6b65d45ef179ed52087e80cb089f6b2349f4ec
-- 
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