[PATCH] git-rebase--merge: don't include absent parent as a base

From: Ben Woosley <ben.woosley@gmail.com>
Date: Apr 20, 2016, 18:20 UTC
Message-ID: <0102015434e7556a-2d9848cb-93c3-4883-96ec-c0c70098796b-000000@eu-west-1.amazonses.com>

From: Ben Woosley <ben.woosley@gmail.com>

Absent this fix, attempts to rebase an orphan branch with --strategy recursive will fail with:

    $ git rebase ORPHAN_TARGET_BASE -s recursive
    First, rewinding head to replay your work on top of it...
    fatal: Could not parse object 'ORPHAN_ROOT_SHA^'
    Unknown exit code (128) from command: git-merge-recursive ORPHAN_ROOT_SHA^ -- HEAD ORPHAN_ROOT_SHA

To fix, this will only include the rebase root's parent as a base if it exists, so that in cases of rebasing an orphan branch, it is a simple two-way merge.

Note the default rebase behavior does not fail:

    $ git rebase ORPHAN_TARGET_BASE
    First, rewinding head to replay your work on top of it...
    Applying: ORPHAN_ROOT_COMMIT_MSG
    Using index info to reconstruct a base tree...

Signed-off-by: Ben Woosley <ben.woosley@gmail.com>
---
 git-rebase--merge.sh    | 4 +++-
 t/t3402-rebase-merge.sh | 9 +++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/git-rebase--merge.sh b/git-rebase--merge.sh
index 2cc2a6d..8d43db9 100644
--- a/git-rebase--merge.sh
+++ b/git-rebase--merge.sh
@@ -67,7 +67,9 @@ call_merge () {
 		GIT_MERGE_VERBOSITY=1 && export GIT_MERGE_VERBOSITY
 	fi
 	test -z "$strategy" && strategy=recursive
-	eval 'git-merge-$strategy' $strategy_opts '"$cmt^" -- "$hd" "$cmt"'
+	# If cmt doesn't have a parent, don't include it as a base
+	base=$(git rev-parse --verify --quiet $cmt^)
+	eval 'git-merge-$strategy' $strategy_opts $base ' -- "$hd" "$cmt"'
 	rv=$?
 	case "$rv" in
 	0)
diff --git a/t/t3402-rebase-merge.sh b/t/t3402-rebase-merge.sh
index 8f64505..488945e 100755
--- a/t/t3402-rebase-merge.sh
+++ b/t/t3402-rebase-merge.sh
@@ -85,6 +85,15 @@ test_expect_success 'rebase -Xtheirs' '
 	! grep 11 original
 '
 
+test_expect_success 'rebase -Xtheirs from orphan' '
+	git checkout --orphan orphan-conflicting master~2 &&
+	echo "AB $T" >> original &&
+	git commit -morphan-conflicting original &&
+	git rebase -Xtheirs master &&
+	grep AB original &&
+	! grep 11 original
+'
+
 test_expect_success 'merge and rebase should match' '
 	git diff-tree -r test-rebase test-merge >difference &&
 	if test -s difference

--
https://github.com/git/git/pull/228