[PATCH v4 17/19] pull: add pull.mode=ff-only

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It is very typical for Git newcomers to inadvertently create merges and
worse; 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 escape 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 = ff-only.

Later on this mode can be enabled by default.

For the full discussion you can read:

https://lore.kernel.org/git/5363BB9F.40102@xxxxxxxxxxx/

Signed-off-by: Felipe Contreras <felipe.contreras@xxxxxxxxx>
---
 Documentation/config/branch.txt |  4 +--
 Documentation/config/pull.txt   |  4 +--
 builtin/pull.c                  |  5 +++-
 builtin/remote.c                |  3 +++
 rebase.c                        |  2 ++
 rebase.h                        |  3 ++-
 t/t5520-pull.sh                 | 45 +++++++++++++++++++++++++++++++++
 7 files changed, 60 insertions(+), 6 deletions(-)

diff --git a/Documentation/config/branch.txt b/Documentation/config/branch.txt
index b57bf2c308..39f60cd8f7 100644
--- a/Documentation/config/branch.txt
+++ b/Documentation/config/branch.txt
@@ -98,8 +98,8 @@ for details).
 
 branch.<name>.pullmode::
 	When "git pull" is run, this determines if it would either merge or
-	rebase the fetched branch. The possible values are 'merge', and
-	'rebase'. See "pull.mode" for doing this in a non
+	rebase the fetched branch. The possible values are 'merge',
+	'rebase', and 'ff-only'. See "pull.mode" for doing this in a non
 	branch-specific manner.
 
 branch.<name>.description::
diff --git a/Documentation/config/pull.txt b/Documentation/config/pull.txt
index f4385cde33..40778d9120 100644
--- a/Documentation/config/pull.txt
+++ b/Documentation/config/pull.txt
@@ -32,8 +32,8 @@ for details).
 pull.mode::
 	When "git pull" is run, this determines if it would either merge or
 	rebase the fetched branch. The possible values are 'merge',
-	and 'rebase'. See "branch.<name>.pullmode" for setting this on a
-	per-branch basis.
+	'rebase', and 'ff-only'. See "branch.<name>.pullmode" for setting
+	this on a per-branch basis.
 
 pull.octopus::
 	The default merge strategy to use when pulling multiple branches
diff --git a/builtin/pull.c b/builtin/pull.c
index 7756c8aea1..5a67667b79 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -1088,6 +1088,9 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 
 	can_ff = get_can_ff(&orig_head, &merge_heads.oid[0]);
 
+	if (mode == PULL_MODE_FF_ONLY && !can_ff)
+		die(_("The pull was not fast-forward, please either merge or rebase.\n"));
+
 	if (!opt_rebase && !can_ff && opt_verbosity >= 0 && (!opt_ff || !strcmp(opt_ff, "--ff"))) {
 		advise(_("Pulling without specifying how to reconcile divergent branches is discouraged;\n"
 			"you need to specify if you want a merge, or a rebase.\n"
@@ -1095,7 +1098,7 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 			"\n"
 			"  git config pull.mode merge    # (the default strategy)\n"
 			"  git config pull.mode rebase\n"
-			"  git config pull.ff only       # fast-forward only\n"
+			"  git config pull.mode ff-only  # fast-forward only\n"
 			"\n"
 			"You can replace \"git config\" with \"git config --global\" to set a default\n"
 			"preference for all repositories.\n"
diff --git a/builtin/remote.c b/builtin/remote.c
index 51b1e675e3..34d3ea9d38 100644
--- a/builtin/remote.c
+++ b/builtin/remote.c
@@ -335,6 +335,9 @@ static int config_read_branches(const char *key, const char *value, void *cb)
 		case PULL_MODE_REBASE:
 			info->rebase = REBASE_TRUE;
 			break;
+		case PULL_MODE_FF_ONLY:
+			info->rebase = REBASE_TRUE;
+			break;
 		default:
 			info->rebase = REBASE_INVALID;
 			break;
diff --git a/rebase.c b/rebase.c
index bdfca49886..c6233e888f 100644
--- a/rebase.c
+++ b/rebase.c
@@ -40,6 +40,8 @@ enum pull_mode_type pull_mode_parse_value(const char *value)
 		return PULL_MODE_MERGE;
 	else if (!strcmp(value, "rebase") || !strcmp(value, "r"))
 		return PULL_MODE_REBASE;
+	else if (!strcmp(value, "ff-only") || !strcmp(value, "f"))
+		return PULL_MODE_FF_ONLY;
 
 	return PULL_MODE_INVALID;
 }
diff --git a/rebase.h b/rebase.h
index 5ab8f4ddd5..432bcb55c4 100644
--- a/rebase.h
+++ b/rebase.h
@@ -17,7 +17,8 @@ enum pull_mode_type {
 	PULL_MODE_INVALID = -1,
 	PULL_MODE_DEFAULT = 0,
 	PULL_MODE_MERGE,
-	PULL_MODE_REBASE
+	PULL_MODE_REBASE,
+	PULL_MODE_FF_ONLY
 };
 
 enum pull_mode_type pull_mode_parse_value(const char *value);
diff --git a/t/t5520-pull.sh b/t/t5520-pull.sh
index eb0086bd1c..b5b007b175 100755
--- a/t/t5520-pull.sh
+++ b/t/t5520-pull.sh
@@ -861,4 +861,49 @@ test_expect_success 'git pull --rebase against local branch' '
 	test_cmp expect file2
 '
 
+setup_other () {
+	test_when_finished "git checkout master && git branch -D other test" &&
+	git checkout -b other $1 &&
+	>new &&
+	git add new &&
+	git commit -m new &&
+	git checkout -b test -t other &&
+	git reset --hard master
+}
+
+setup_ff () {
+	setup_other master
+}
+
+setup_non_ff () {
+	setup_other master^
+}
+
+test_expect_success 'fast-forward (pull.mode=ff-only)' '
+	setup_ff &&
+	git -c pull.mode=ff-only pull
+'
+
+test_expect_success 'non-fast-forward (pull.mode=ff-only)' '
+	setup_non_ff &&
+	test_must_fail git -c pull.mode=ff-only pull
+'
+
+test_expect_success 'non-fast-forward with merge (pull.mode=ff-only)' '
+	setup_non_ff &&
+	git -c pull.mode=ff-only pull --merge
+'
+
+test_expect_success 'non-fast-forward with rebase (pull.mode=ff-only)' '
+	setup_non_ff &&
+	git -c pull.mode=ff-only pull --rebase
+'
+
+test_expect_success 'non-fast-forward error message (pull.mode=ff-only)' '
+	setup_non_ff &&
+	test_must_fail git -c pull.mode=ff-only pull 2> error &&
+	cat error &&
+	test_i18ngrep "The pull was not fast-forward" error
+'
+
 test_done
-- 
2.29.2




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