git-merge: --no-commit is not respected on a fresh repository

Jasmin Oster· Apr 3, 2024, 13:26 UTC · lore

## What did you do before the bug happened? (Steps to reproduce your issue)

When trying to perform a subtree merge, especially following the blog post on https://nuclearsquid.com/writings/subtree-merging-and-you/, I noticed that Git does not respect the --no-commit option if the repository does not have a history.

1. Create a new repository: $ git init
2. Add a remote: $ git remote add foo ../<path-to-another-repo>
3. Fetch everything: $ git fetch --all
4. Initiate a subtree merge: $ git merge -s our --no-commit foo/main

## What did you expect to happen? (Expected behavior)

The repository should've been in the "merging" state, in order to rewrite the paths using:

$ git read-tree --prefix=/project/ -u foo/main

It's only a minor annoyance that can be fixed by adding an empty commit in advance of performing the merge, yet it took me some minutes to figure out why Git does not behave as intended.

## What happened instead? (Actual behavior)

A merge commit was created.

## What's different between what you expected and what actually happened?

Said merge commit, even though the `--no-commit` option was set.

[System Info] git version: git version 2.44.0 cpu: x86_64 no commit associated with this build sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh uname: Linux 6.7.10-200.fc39.x86_64 #1 SMP PREEMPT_DYNAMIC Mon Mar 18 18:56:52 UTC 2024 x86_64 compiler info: gnuc: 13.2 libc info: glibc: 2.38 $SHELL (typically, interactive shell): /usr/bin/fish

-- 
Mit freundlichen Grüßen / kind regards

*Jasmin Oster*
Software Developer

ANEXIA Internetdienstleistungs GmbH

E-Mail: JOster@anexia.com <mailto:DName@anexia.com>
Web: anexia.com <https://anexia.com/>

Anschrift Hauptsitz Klagenfurt: Feldkirchner Straße 140, 9020 Klagenfurt
Geschäftsführer: Alexander Windbichler
Firmenbuch: FN 289918a | Gerichtsstand: Klagenfurt | UID-Nummer: AT 
U63216601

Junio C Hamano· Apr 3, 2024, 17:29 UTC · re: Jasmin Oster · lore

Re: git-merge: --no-commit is not respected on a fresh repository

Jasmin Oster <JOster@anexia.com> writes:

> 1. Create a new repository: $ git init
> 2. Add a remote: $ git remote add foo ../<path-to-another-repo>
> 3. Fetch everything: $ git fetch --all
> 4. Initiate a subtree merge: $ git merge -s our --no-commit foo/main

This is expected. We are not creating any new commit, so it is following the --no-commit option to the letter. Obviously it is different from what you expected to see, though ;-)

The "merge into void" first came to "git pull $URL $branch" (no other options) where people tried to pull into a freshly created repository, where it was very clear that they wanted to have a copy of the upstream branch. The "git pull" implementation of the "pulling into void" feature forgot that curious users may give "--no-commit", and just always fast-forwarded to the merged commit without checking if such an option was given.

The behaviour you are seeing was inherited into "git merge". It also always fast-forwards to the merged commit, ignoring the option.

In either case, there is no new commit created. The history you are seeing is the exact history of the upstream. If you want to dissociate your history from theirs and start your history anew:

    $ git merge foo/main
    $ git checkout --orphan master

would give you an unborn branch "master", with the merge result in the working tree and in the index, without any history behind it. If you wanted to, you can then rename it to main from this state with:

    $ git branch -M master main

We could make it more explicit with a patch like the following, but it probably is not worth it. I dunno.

 builtin/merge.c | 2 ++
 builtin/pull.c  | 2 ++
 2 files changed, 4 insertions(+)

diff --git c/builtin/merge.c w/builtin/merge.c
index 1cbd6a899c..1c3165e99a 100644
--- c/builtin/merge.c
+++ w/builtin/merge.c
@@ -1434,6 +1434,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		struct object_id *remote_head_oid;
 		if (squash)
 			die(_("Squash commit into empty head not supported yet"));
+		if (!option_commit)
+			die(_("--no-commit into empty head not supported"));
 		if (fast_forward == FF_NO)
 			die(_("Non-fast-forward commit does not make sense into "
 			    "an empty head"));
diff --git c/builtin/pull.c w/builtin/pull.c
index 72cbb76d52..98c11ecc55 100644
--- c/builtin/pull.c
+++ w/builtin/pull.c
@@ -1097,6 +1097,8 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 	if (is_null_oid(&orig_head)) {
 		if (merge_heads.nr > 1)
 			die(_("Cannot merge multiple branches into empty head."));
+		if (opt_commit && !strcmp("--no-commit", opt_commit))
+			die(_("--no-commit into empty head not supported"));
 		ret = pull_into_void(merge_heads.oid, &curr_head);
 		goto cleanup;
 	}