[PATCH v6 5/7] pull: add merge-ff-only option

[Felipe Contreras <felipe.contreras@gmail.com>]

It is very typical for Git newcomers to inadvertently create merges and
worst; inadvertently pushing them. This is one of the reasons many
experienced users prefer to avoid 'git pull', and recommend newcomers to
avoid it as well.

To avoid these problems and keep 'git pull' useful, it has been
suggested that 'git pull' barfs by default if the merge is
non-fast-forward, which unfortunately would break backwards
compatibility.

This patch leaves everything in place to enable this new mode, but it
only gets enabled if the user specifically configures it; pull.mode =
merge-ff-only.

Later on this mode can be enabled by default.

For the full discussion you can read:

http://thread.gmane.org/gmane.comp.version-control.git/225146/focus=225305

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
---
 Documentation/config.txt |  7 +++++--
 git-pull.sh              | 15 ++++++++++++++-
 t/t5520-pull.sh          | 36 ++++++++++++++++++++++++++++++++++++
 3 files changed, 55 insertions(+), 3 deletions(-)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index c028aeb..b391ec1 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -1886,8 +1886,11 @@ pretty.<name>::
 pull.mode::
 	When "git pull" is run, this determines if it would either merge or
 	rebase the fetched branch. The possible values are 'merge',
-	'rebase', and 'rebase-preserve'. See "branch.<name>.pullmode" for doing
-	this in a non branch-specific manner.
+	'rebase', 'ff-only,' and 'rebase-preserve'.
+	If 'ff-only' is specified, the merge will only succeed if it's
+	fast-forward.
+	See "branch.<name>.pullmode" for doing this in a non branch-specific
+	manner.
 +
 	When 'rebase-preserve', also pass `--preserve-merges` along to
 	'git rebase' so that locally committed merge commits will not be
diff --git a/git-pull.sh b/git-pull.sh
index e7e52ec..2446417 100755
--- a/git-pull.sh
+++ b/git-pull.sh
@@ -57,7 +57,7 @@ then
 	mode=$(git config pull.mode)
 fi
 case "$mode" in
-merge|rebase|'')
+merge|rebase|ff-only|'')
 	;;
 rebase-preserve)
 	mode="rebase"
@@ -78,6 +78,7 @@ then
 		rebase=$(bool_or_string_config pull.rebase)
 	fi
 fi
+test -z "$mode" && mode=merge
 dry_run=
 while :
 do
@@ -313,6 +314,18 @@ case "$merge_head" in
 		die "$(gettext "Cannot rebase onto multiple branches")"
 	fi
 	;;
+*)
+	# check if a non-fast-foward merge would be needed
+	merge_head=${merge_head% }
+	if test "$mode" = 'ff-only' && test -z "$no_ff$ff_only${squash#--no-squash}" &&
+		test -n "$orig_head" &&
+		! git merge-base --is-ancestor "$orig_head" "$merge_head" &&
+		! git merge-base --is-ancestor "$merge_head" "$orig_head"
+	then
+		die "$(gettext "The pull was not fast-forward, please either merge or rebase.
+If unsure, run 'git pull --merge'.")"
+	fi
+	;;
 esac
 
 # Pulling into unborn branch: a shorthand for branching off
diff --git a/t/t5520-pull.sh b/t/t5520-pull.sh
index 01ad17a..a548c1b 100755
--- a/t/t5520-pull.sh
+++ b/t/t5520-pull.sh
@@ -365,4 +365,40 @@ test_expect_success 'git pull --rebase against local branch' '
 	test file = "$(cat file2)"
 '
 
+test_expect_success 'git pull fast-forward (ff-only)' '
+	test_when_finished "git checkout master && git branch -D other test" &&
+	test_config pull.mode ff-only &&
+	git checkout -b other master &&
+	>new &&
+	git add new &&
+	git commit -m new &&
+	git checkout -b test -t other &&
+	git reset --hard master &&
+	git pull
+'
+
+test_expect_success 'git pull non-fast-forward (ff-only)' '
+	test_when_finished "git checkout master && git branch -D other test" &&
+	test_config pull.mode ff-only &&
+	git checkout -b other master^ &&
+	>new &&
+	git add new &&
+	git commit -m new &&
+	git checkout -b test -t other &&
+	git reset --hard master &&
+	test_must_fail git pull
+'
+
+test_expect_success 'git pull non-fast-forward with merge (ff-only)' '
+	test_when_finished "git checkout master && git branch -D other test" &&
+	test_config pull.mode ff-only &&
+	git checkout -b other master^ &&
+	>new &&
+	git add new &&
+	git commit -m new &&
+	git checkout -b test -t other &&
+	git reset --hard master &&
+	git pull --merge
+'
+
 test_done
-- 
1.9.2+fc1.19.g85b6256