[PATCH] checkout -p: handle tree arguments correctly again

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

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

This fixes a segmentation fault.

The bug is caused by dereferencing `new_branch_info->commit` when it is
`NULL`, which is the case when the tree-ish argument is actually a tree,
not a commit-ish. This was introduced in 5602b500c3c (builtin/checkout:
fix `git checkout -p HEAD...` bug, 2020-10-07), where we tried to ensure
that the special tree-ish `HEAD...` is handled correctly.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
    checkout -p: handle tree arguments correctly again
    
    I literally just ran into this segmentation fault after rebasing Git for
    Windows onto -rc1, and did not really think that the regression was
    introduced in the v2.30.0 cycle, but was proven wrong by my
    investigation: it was introduced by v2.30.0-rc0151^23.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-819%2Fdscho%2Fcheckout-p-empty-tree-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-819/dscho/checkout-p-empty-tree-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/819

 builtin/checkout.c        | 2 +-
 t/t2016-checkout-patch.sh | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index 9b82119129a..8b567b0424d 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -482,7 +482,7 @@ static int checkout_paths(const struct checkout_opts *opts,
 		 * properly. However, there is special logic for the HEAD case
 		 * so we mustn't replace that.
 		 */
-		if (rev && strcmp(rev, "HEAD"))
+		if (rev && new_branch_info->commit && strcmp(rev, "HEAD"))
 			rev = oid_to_hex_r(rev_oid, &new_branch_info->commit->object.oid);
 
 		if (opts->checkout_index && opts->checkout_worktree)
diff --git a/t/t2016-checkout-patch.sh b/t/t2016-checkout-patch.sh
index d91a329eb31..abfd586c32b 100755
--- a/t/t2016-checkout-patch.sh
+++ b/t/t2016-checkout-patch.sh
@@ -123,4 +123,9 @@ test_expect_success PERL 'none of this moved HEAD' '
 	verify_saved_head
 '
 
+test_expect_success PERL 'empty tree can be handled' '
+	test_when_finished "git reset --hard" &&
+	git checkout -p $(test_oid empty_tree) --
+'
+
 test_done

base-commit: 6d3ef5b467eccd2769f1aa1c555d317d3c8dc707
-- 
gitgitgadget