[PATCH v2 2/3] builtin/rebase.c: Emit warning when rebasing without a forkpoint

[Wesley Schwengle <wesleys@opperschaap.net>]

This patch adds a warning where it will indicate that `rebase.forkpoint'
must be set in the git configuration and/or that you can supply a
`--fork-point' or `--no-fork-point' command line option to your `git
rebase' invocation.

When commit d1e894c6d7 (Document `rebase.forkpoint` in rebase man page,
2021-09-16) was submitted there was a discussion on if the forkpoint
behaviour of `git rebase' was sane. In my experience this wasn't sane.

Git rebase doesn't work if you don't have an upstream branch configured
(or something that says `merge = refs/heads/master' in the git config).
You would than need to use `git rebase <upstream>' to rebase. If you
configure an upstream it would seem logical to be able to run `git
rebase' without arguments. However doing so would trigger a different
kind of behavior.  `git rebase <upstream>' behaves as if
`--no-fork-point' was supplied and without it behaves as if
`--fork-point' was supplied. This behavior can result in a loss of
commits and can surprise users. The following reproduction path exposes
this behavior:

    git init reproduction
    cd reproduction
    echo "commit a" > file.txt
    git add file.txt
    git commit -m "First commit" file.txt
    echo "commit b" >> file.txt
    git commit -m "Second commit" file.txt

    git switch -c foo
    echo "commit c" >> file.txt"
    git commit -m "Third commit" file.txt
    git branch --set-upstream-to=master

    git status
    On branch foo
    Your branch is ahead of 'master' by 1 commit.

    git switch master
    git merge foo
    git reset --hard HEAD^
    git switch foo
    Switched to branch 'foo'
    Your branch is ahead of 'master' by 1 commit.

    git log --oneline
    5f427e3 Third commit
    03ad791 Second commit
    411e6d4 First commit

    git rebase
    git status
    On branch foo
    Your branch is up to date with 'master'.

    git log --oneline
    03ad791 Second commit
    411e6d4 First commit

Signed-off-by: Wesley Schwengle <wesleys@opperschaap.net>
---
 builtin/rebase.c             | 16 +++++++++-
 t/t3431-rebase-fork-point.sh | 62 ++++++++++++++++++++++++++++++++++++
 2 files changed, 77 insertions(+), 1 deletion(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 2108001600..ee7db9ba0c 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1608,8 +1608,22 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 								    NULL);
 			if (!options.upstream_name)
 				error_on_missing_default_upstream();
-			if (options.fork_point < 0)
+			if (options.fork_point < 0) {
+				warning(_(
+					"When \"git rebase\" is run without specifying <upstream> on the\n"
+					"command line, the current default is to use the fork-point\n"
+					"heuristics. This is expected to change in a future version\n"
+					"of Git, and you will have to explicitly give \"--fork-point\" from\n"
+					"the command line if you keep using the fork-point mode.  You can\n"
+					"run \"git config rebase.forkpoint false\" to adopt the new default\n"
+					"in advance and that will also squelch the message.\n"
+					"\n"
+					"You can replace \"git config\" with \"git config --global\" to set a default\n"
+					"preference for all repositories. You can also pass --no-fork-point, --fork-point\n"
+					"on the command line to override the configured default per invocation.\n"
+				));
 				options.fork_point = 1;
+			}
 		} else {
 			options.upstream_name = argv[0];
 			argc--;
diff --git a/t/t3431-rebase-fork-point.sh b/t/t3431-rebase-fork-point.sh
index 4bfc779bb8..908867ae0f 100755
--- a/t/t3431-rebase-fork-point.sh
+++ b/t/t3431-rebase-fork-point.sh
@@ -113,4 +113,66 @@ test_expect_success 'rebase.forkPoint set to true and --root given' '
 	git rebase --root
 '
 
+# The use of the diff -qw is because there is some kind of whitespace character
+# magic going on which probably has to do with the tabs. It only occurs when we
+# check STDERR
+test_expect_success 'rebase without rebase.forkpoint' '
+	git init rebase-forkpoint &&
+	cd rebase-forkpoint &&
+	git status >/tmp/foo &&
+	echo "commit a" > file.txt &&
+	git add file.txt &&
+	git commit -m "First commit" file.txt &&
+	echo "commit b" >> file.txt &&
+	git commit -m "Second commit" file.txt &&
+	git switch -c foo &&
+	echo "commit c" >> file.txt &&
+	git commit -m "Third commit" file.txt &&
+	git branch --set-upstream-to=main &&
+	git switch main &&
+	git merge foo &&
+	git reset --hard HEAD^ &&
+	git switch foo &&
+	commit=$(git log -n1 --format="%h") &&
+	git rebase >out 2>err &&
+	test_must_be_empty out &&
+	cat <<-\OEF > expect &&
+	warning: When "git rebase" is run without specifying <upstream> on the
+	command line, the current default is to use the fork-point
+	heuristics. This is expected to change in a future version
+	of Git, and you will have to explicitly give "--fork-point" from
+	the command line if you keep using the fork-point mode.  You can
+	run "git config rebase.forkpoint false" to adopt the new default
+	in advance and that will also squelch the message.
+
+	You can replace "git config" with "git config --global" to set a default
+	preference for all repositories. You can also pass --no-fork-point, --fork-point
+	on the command line to override the configured default per invocation.
+
+	Successfully rebased and updated refs/heads/foo.
+	OEF
+	diff -qw expect err &&
+	git reset --hard $commit &&
+	git rebase --fork-point >out 2>err &&
+	test_must_be_empty out &&
+	echo "Successfully rebased and updated refs/heads/foo." > expect &&
+	diff -qw expect err &&
+	git reset --hard $commit &&
+	git rebase --no-fork-point >out 2>err &&
+	test_must_be_empty err &&
+	echo "Current branch foo is up to date." > expect &&
+	test_cmp out expect &&
+	git config --add rebase.forkpoint true &&
+	git rebase >out 2>err &&
+	test_must_be_empty out &&
+	echo "Successfully rebased and updated refs/heads/foo." > expect &&
+	diff -qw expect err &&
+	git reset --hard $commit &&
+	git config --replace-all rebase.forkpoint false &&
+	git rebase >out 2>err &&
+	test_must_be_empty err &&
+	echo "Current branch foo is up to date." > expect &&
+	test_cmp out expect
+'
+
 test_done
-- 
2.42.0.103.g5622fd1409.dirty