[PATCH 1/2] builtin/rebase.c: Emit warning when rebasing without a forkpoint

[Wesley Schwengle <wesleys@opperschaap.net>]

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).
The behaviour of `git rebase' was that if you supply an upstream on the
command line that it behaves as if `--no-forkpoint' was supplied and if
you don't supply an upstream, it behaves as if `--forkpoint' was
supplied. This can result in a loss of commits if you don't know that
and if you don't know about `git reflog' or have other copies of your
changes. This can be seen with the following reproduction path:

    mkdir reproduction
    cd reproduction
    git init .
    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 --format='%C(yellow)%h%Creset %Cgreen%s%Creset'
    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 --format='%C(yellow)%h%Creset %Cgreen%s%Creset'
    03ad791 Second commit
    411e6d4 First commit

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
`--forkpoint' or `--no-forkpoint' command line option to your `git
rebase' invocation.

Signed-off-by: Wesley Schwengle <wesleys@opperschaap.net>
---
 builtin/rebase.c             | 15 ++++++++++-
 t/t3431-rebase-fork-point.sh | 50 ++++++++++++++++++++++++++++++++++++
 2 files changed, 64 insertions(+), 1 deletion(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 50cb85751f..41dd9b6256 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1604,8 +1604,21 @@ 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(_(
+					"Rebasing without specifying a forkpoint is discouraged. You can squelch\n"
+					"this message by running one of the following commands something before your\n"
+					"next rebase:\n"
+					"\n"
+					"  git config rebase.forkpoint = false # This will become the new default\n"
+					"  git config rebase.forkpoint = true  # This is the old default\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..a583ca6228 100755
--- a/t/t3431-rebase-fork-point.sh
+++ b/t/t3431-rebase-fork-point.sh
@@ -113,4 +113,54 @@ 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 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: Rebasing without specifying a forkpoint is discouraged. You can squelch
+	this message by running one of the following commands something before your
+	next rebase:
+
+	  git config rebase.forkpoint = false # This will become the new default
+	  git config rebase.forkpoint = true  # This is the old default
+
+	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
+'
+
 test_done
-- 
2.42.0.rc2.7.gf9972720e9.dirty