subtree: fix squash merging after annotated tag was squashed merged

When 'git subtree merge --squash $ref' is invoked, either directly or
through 'git subtree pull --squash $repo $ref', the code looks for the
latest squash merge of the subtree in order to create the new merge
commit as a child of the previous squash merge.

This search is done in function 'process_subtree_split_trailer', invoked
by 'find_latest_squash', which looks for the most recent commit with a
'git-subtree-split' trailer; that trailer's value is the object name in
the subtree repository of the ref that was last squash-merged. The
function verifies that this object is present locally with 'git
rev-parse', and aborts if it's not.

The hash referenced by the 'git-subtree-split' trailer is guaranteed to
correspond to a commit since it is the result of running 'git rev-parse
-q --verify "$1^{commit}"' on the first argument of 'cmd_merge' (this
corresponds to 'rev' in 'cmd_merge' which is passed through to
'new_squash_commit' and 'squash_msg').

But this is only the case since e4f8baa88a (subtree: parse revs in
individual cmd_ functions, 2021-04-27), which went into Git 2.32. Before
that commit, 'cmd_merge' verified the revision it was given using 'git
rev-parse --revs-only "$@"'. Such an invocation, when fed the name of an
annotated tag, would return the hash of the tag, not of the commit
referenced by the tag.

This leads to a failure in 'find_latest_squash' when squash-merging if
the most recent squash-merge merged an annotated tag of the subtree
repository, using a pre-2.32 version of 'git subtree', unless that
previous annotated tag is present locally (which is not usually the
case).

We can fix this by fetching the object directly by its hash in
'process_subtree_split_trailer' when 'git rev-parse' fails, but in order
to do so we need to know the name or URL of the subtree repository.
This is not possible in general for 'git subtree merge', but is easy
when it is invoked through 'git subtree pull' since in that case the
subtree repository is passed by the user at the command line.

Allow the 'git subtree pull' scenario to work out-of-the-box by adding
an optional 'repository' argument to functions 'cmd_merge',
'find_latest_squash' and 'process_subtree_split_trailer', and invoke
'cmd_merge' with that 'repository' argument in 'cmd_pull'.

If 'repository' is absent in 'process_subtree_split_trailer', instruct
the user to try fetching the missing object directly.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 36 insertions, 0 deletions

diff --git a/contrib/subtree/t/t7900-subtree.sh b/contrib/subtree/t/t7900-subtree.sh
index 249743ab9a..d0671676c7 100755
--- a/contrib/subtree/t/t7900-subtree.sh
+++ b/contrib/subtree/t/t7900-subtree.sh
@@ -43,6 +43,30 @@ last_commit_subject () {
 	git log --pretty=format:%s -1
 }
 
+# Upon 'git subtree add|merge --squash' of an annotated tag,
+# pre-2.32.0 versions of 'git subtree' would write the hash of the tag
+# (sub1 below), instead of the commit (sub1^{commit}) in the
+# "git-subtree-split" trailer.
+# We immitate this behaviour below using a replace ref.
+# This function creates 3 repositories:
+# - $1
+# - $1-sub (added as subtree "sub" in $1)
+# - $1-clone (clone of $1)
+test_create_pre2_32_repo () {
+	subtree_test_create_repo "$1" &&
+	subtree_test_create_repo "$1-sub" &&
+	test_commit -C "$1" main1 &&
+	test_commit -C "$1-sub" --annotate sub1 &&
+	git -C "$1" subtree add --prefix="sub" --squash "../$1-sub" sub1 &&
+	tag=$(git -C "$1" rev-parse FETCH_HEAD) &&
+	commit=$(git -C "$1" rev-parse FETCH_HEAD^{commit}) &&
+	git -C "$1" log -1 --format=%B HEAD^2 >msg &&
+	test_commit -C "$1-sub" --annotate sub2 &&
+	git clone --no-local "$1" "$1-clone" &&
+	new_commit=$(cat msg | sed -e "s/$commit/$tag/" | git -C "$1-clone" commit-tree HEAD^2^{tree}) &&
+	git -C "$1-clone" replace HEAD^2 $new_commit
+}
+
 test_expect_success 'shows short help text for -h' '
 	test_expect_code 129 git subtree -h >out 2>err &&
 	test_must_be_empty err &&
@@ -264,6 +288,13 @@ test_expect_success 'merge new subproj history into subdir/ with a slash appende
 	)
 '
 
+test_expect_success 'merge with --squash after annotated tag was added/merged with --squash pre-v2.32.0 ' '
+	test_create_pre2_32_repo "$test_count" &&
+	git -C "$test_count-clone" fetch "../$test_count-sub" sub2  &&
+	test_must_fail git -C "$test_count-clone" subtree merge --prefix="sub" --squash FETCH_HEAD &&
+	git -C "$test_count-clone" subtree merge --prefix="sub" --squash FETCH_HEAD  "../$test_count-sub"
+'
+
 #
 # Tests for 'git subtree split'
 #
@@ -630,6 +661,11 @@ test_expect_success 'pull rejects flags for split' '
 	)
 '
 
+test_expect_success 'pull with --squash after annotated tag was added/merged with --squash pre-v2.32.0 ' '
+	test_create_pre2_32_repo "$test_count" &&
+	git -C "$test_count-clone" subtree -d pull --prefix="sub" --squash "../$test_count-sub" sub2
+'
+
 #
 # Tests for 'git subtree push'
 #