Subject: Re: [PATCH 4/6] am --abort: revert changes introduced by failed 3way merge

From: Junio C Hamano <[email-masked]>
Date: Mon, 08 Jun 2015 13:09:13 -0700

Paul Tan <[email-masked]> writes:

> Even when a merge conflict occurs with am --3way, the index will be
> modified with the results of any successfully merged files. These
> changes to the index will not be reverted with a
> "git read-tree --reset -u HEAD ORIG_HEAD", as git read-tree will not be
> aware of how the current index differs from HEAD or ORIG_HEAD.
>
> To fix this, we first reset any conflicting entries in the index. The
> resulting index will contain the results of successfully merged files
> introduced by the failed merge. We write this index to a tree, and then
> use git read-tree to fast-forward this "index tree" back to ORIG_HEAD,
> thus undoing all the changes from the failed merge.
>
> When we are on an unborn branch, HEAD and ORIG_HEAD will not point to
> valid trees. In this case, use an empty tree.
>
> Signed-off-by: Paul Tan <[email-masked]>
> ---
>  git-am.sh           |  6 +++++-
>  t/t4151-am-abort.sh | 23 +++++++++++++++++++++++
>  2 files changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/git-am.sh b/git-am.sh
> index 67f4f25..e4fe3ed 100755
> --- a/git-am.sh
> +++ b/git-am.sh
> @@ -519,7 +519,11 @@ then
>  		git rerere clear
>  		if safe_to_abort
>  		then
> -			git read-tree --reset -u HEAD ORIG_HEAD
> +			head_tree=$(git rev-parse --verify -q HEAD || echo $empty_tree) &&
> +			git read-tree --reset -u $head_tree $head_tree &&
> +			index_tree=$(git write-tree) &&
> +			orig_head=$(git rev-parse --verify -q ORIG_HEAD || echo $empty_tree) &&
> +			git read-tree -m -u $index_tree $orig_head
>  			git reset ORIG_HEAD

What would the last "reset" do when ORIG_HEAD is invalid?  In other
words, does it make sense to worry about the case where ORIG_HEAD
does not exist?  At which point in the flow does it get written using
what information?

>  		fi
>  		rm -fr "$dotest"
> diff --git a/t/t4151-am-abort.sh b/t/t4151-am-abort.sh
> index ea4a49e..2366042 100755
> --- a/t/t4151-am-abort.sh
> +++ b/t/t4151-am-abort.sh
> @@ -70,6 +70,17 @@ test_expect_success 'am -3 --skip removes otherfile-4' '
>  	test 4 = "$(cat otherfile-4)" &&
>  	git am --skip &&
>  	test_cmp_rev initial HEAD &&
> +	test -z "$(git ls-files -u)" &&
> +	test_path_is_missing otherfile-4
> +'
> +
> +test_expect_success 'am -3 --abort removes otherfile-4' '
> +	git reset --hard initial &&
> +	test_must_fail git am -3 0003-*.patch &&
> +	test 3 -eq $(git ls-files -u | wc -l) &&
> +	test 4 = "$(cat otherfile-4)" &&
> +	git am --abort &&
> +	test_cmp_rev initial HEAD &&
>  	test -z $(git ls-files -u) &&
>  	test_path_is_missing otherfile-4
>  '
> @@ -102,4 +113,16 @@ test_expect_success 'am -3 --skip clears index on unborn branch' '
>  	test_path_is_missing tmpfile
>  '
>  
> +test_expect_success 'am -3 --abort removes otherfile-4 on unborn branch' '
> +	git checkout -f --orphan orphan &&
> +	git reset &&
> +	rm -f otherfile-4 file-1 &&
> +	test_must_fail git am -3 0003-*.patch &&
> +	test 2 -eq $(git ls-files -u | wc -l) &&
> +	test 4 = "$(cat otherfile-4)" &&
> +	git am --abort &&
> +	test -z "$(git ls-files -u)" &&
> +	test_path_is_missing otherfile-4
> +'
> +
>  test_done