pull: require choice between rebase/merge on non-fast-forward pulljc/pull-training-wheel

Because it is so easy to let Git handle automatically a trivial
merge with "git pull", a person who is new to Git may not realize
that the project s/he is interacting with may prefer a "rebase"
workflow.

Add a safety valve to fail "git pull" that does not explicitly
specify what branch from which repository to integrate your history
with, when it is neither a fast-forward or "already up-to-date",
until/unless the user expressed her preference between the two ways
of integration.

This can be an irritating backward incompatible change for old
timers, but it can be a one time irritation by doing:

    git config --global pull.rebase false

once to say "I will always --merge", and they'd not even notice.

    http://thread.gmane.org/gmane.comp.version-control.git/225146/focus=225326

for a full discussion.

Helped-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

4 files changed, 100 insertions, 2 deletions

diff --git a/Documentation/git-pull.txt b/Documentation/git-pull.txt
index 24ab07a3f8..8b3acf5b11 100644
--- a/Documentation/git-pull.txt
+++ b/Documentation/git-pull.txt
@@ -97,6 +97,14 @@ must be given before the options meant for 'git fetch'.
 Options related to merging
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+With no repository or branch on the command line, `git pull` needs
+to be told how to integrate the changes with your history.
+
+This can be done via either `--merge` or `--rebase` option, but most
+people would want to decide which method matches the workflow of the
+project once, and set the configuration variable `pull.rebase` or
+`branch.<name>.rebase` to stick to it; see linkgit:git-config[1].
+
 include::merge-options.txt[]
 
 :git-pull: 1
@@ -119,6 +127,7 @@ It rewrites history, which does not bode well when you
 published that history already.  Do *not* use this option
 unless you have read linkgit:git-rebase[1] carefully.
 
+--merge::
 --no-rebase::
 	Override earlier --rebase.
 
diff --git a/git-pull.sh b/git-pull.sh
index 638aabb7b3..88c198fa66 100755
--- a/git-pull.sh
+++ b/git-pull.sh
@@ -41,13 +41,21 @@ test -f "$GIT_DIR/MERGE_HEAD" && die_merge
 strategy_args= diffstat= no_commit= squash= no_ff= ff_only=
 log_arg= verbosity= progress= recurse_submodules= verify_signatures=
 merge_args= edit=
+
 curr_branch=$(git symbolic-ref -q HEAD)
 curr_branch_short="${curr_branch#refs/heads/}"
+
+# See if we are configured to rebase by default.
+# The value $rebase is, throughout the main part of the code:
+#    (empty) - the user did not have any preference
+#    true    - the user told us to integrate by rebasing
+#    false   - the user told us to integrate by merging
 rebase=$(git config --bool branch.$curr_branch_short.rebase)
 if test -z "$rebase"
 then
 	rebase=$(git config --bool pull.rebase)
 fi
+
 dry_run=
 while :
 do
@@ -113,7 +121,8 @@ do
 	-r|--r|--re|--reb|--reba|--rebas|--rebase)
 		rebase=true
 		;;
-	--no-r|--no-re|--no-reb|--no-reba|--no-rebas|--no-rebase)
+	--no-r|--no-re|--no-reb|--no-reba|--no-rebas|--no-rebase|\
+	-m|--m|--me|--mer|--merg|--merge)
 		rebase=false
 		;;
 	--recurse-submodules)
@@ -219,6 +228,7 @@ test true = "$rebase" && {
 		fi
 	done
 }
+
 orig_head=$(git rev-parse -q --verify HEAD)
 git fetch $verbosity $progress $dry_run $recurse_submodules --update-head-ok "$@" || exit 1
 test -z "$dry_run" || exit 0
@@ -264,6 +274,34 @@ case "$merge_head" in
 		die "$(gettext "Cannot rebase onto multiple branches")"
 	fi
 	;;
+*)
+	# integrating with a single other history; be careful not to
+	# trigger this check when we will say "fast-forward" or "already
+	# up-to-date".
+	merge_head=${merge_head% }
+	if test -z "$rebase$no_ff$ff_only${squash#--no-squash}" &&
+		test -n "$orig_head" &&
+		test $# = 0 &&
+		! git merge-base --is-ancestor "$orig_head" "$merge_head" &&
+		! git merge-base --is-ancestor "$merge_head" "$orig_head"
+	then
+echo >&2 "orig-head was $orig_head"
+echo >&2 "merge-head is $merge_head"
+git show >&2 --oneline -s "$orig_head" "$merge_head"
+
+		die "The pull does not fast-forward; please specify
+if you want to merge or rebase.
+
+Use either
+
+    git pull --rebase
+    git pull --merge
+
+You can also use 'git config pull.rebase true' (if you want --rebase) or
+'git config pull.rebase false' (if you want --merge) to set this once for
+this project and forget about it."
+	fi
+	;;
 esac
 
 if test -z "$orig_head"
diff --git a/t/t5520-pull.sh b/t/t5520-pull.sh
index 6af6c63350..1e91ecae7e 100755
--- a/t/t5520-pull.sh
+++ b/t/t5520-pull.sh
@@ -255,4 +255,55 @@ test_expect_success 'git pull --rebase against local branch' '
 	test file = "$(cat file2)"
 '
 
+test_expect_success 'git pull that does not say how to integrate' '
+	git checkout -b other master^1 &&
+	>new &&
+	git add new &&
+	git commit -m "add new file" &&
+
+	git checkout -b test-to-integrate master &&
+
+	test_config branch.test-to-integrate.remote . &&
+	test_config branch.test-to-integrate.merge other &&
+
+	# need real integration
+	test_must_fail git pull &&
+	git reset --hard master &&
+
+
+	# configuration is explicit enough
+	for how in false true
+	do
+		test_config pull.rebase $how &&
+		git pull &&
+		git reset --hard master || break
+	done &&
+
+	# per branch configuration is explicit enough
+	test_unconfig pull.rebase &&
+	for how in false true
+	do
+		test_config branch.test-to-integrate.rebase $how &&
+		git pull &&
+		git reset --hard master || break
+	done &&
+
+	test_unconfig pull.rebase &&
+	test_unconfig branch.test-to-integrate &&
+
+	# already up to date
+	git reset --hard master &&
+	git branch -f other master^1
+	git pull &&
+
+	# fast forward
+	git reset --hard master &&
+	git checkout -B other master &&
+	>new &&
+	git add new &&
+	git commit -m "add new file" &&
+	git checkout -B test-to-integrate master &&
+	git pull
+'
+
 test_done
diff --git a/t/t5524-pull-msg.sh b/t/t5524-pull-msg.sh
index 8cccecc2fc..660714b1ad 100755
--- a/t/t5524-pull-msg.sh
+++ b/t/t5524-pull-msg.sh
@@ -25,7 +25,7 @@ test_expect_success setup '
 test_expect_success pull '
 (
 	cd cloned &&
-	git pull --log &&
+	git pull --log --merge &&
 	git log -2 &&
 	git cat-file commit HEAD >result &&
 	grep Dollar result