[PATCH 1/2] pull: add proper error with --ff-only

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

The current error is not user-friendly:

  fatal: not possible to fast-forward, aborting.

We want something that actually explains what is going on:

  The pull was not fast-forward, please either merge or rebase.

The user can get rid of the warning by doing either --merge or
--rebase.

Except: doing "git pull --merge" is not actually enough; we would return
to the previous behavior: "fatal: not possible to fast-forward,
aborting". In order to do the right thing we will have to change the
semantics of --ff-only.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
---
 builtin/pull.c  | 33 ++++++++++++++++++--------------
 t/t5520-pull.sh | 50 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+), 14 deletions(-)

diff --git a/builtin/pull.c b/builtin/pull.c
index e0157d013f..44ec6e7216 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -1008,20 +1008,25 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 
 	can_ff = get_can_ff(&orig_head, &merge_heads.oid[0]);
 
-	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, a rebase, or a fast-forward.\n"
-			"You can squelch this message by running one of the following commands:\n"
-			"\n"
-			"  git config pull.rebase false  # merge (the default strategy)\n"
-			"  git config pull.rebase true   # rebase\n"
-			"  git config pull.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"
-			"If unsure, run \"git pull --merge\".\n"
-			"Read \"git pull --help\" for more information."
-			));
+	if (!can_ff && !opt_rebase) {
+		if (opt_ff && !strcmp(opt_ff, "--ff-only"))
+			die(_("The pull was not fast-forward, please either merge or rebase."));
+
+		if (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, a rebase, or a fast-forward.\n"
+				"You can squelch this message by running one of the following commands:\n"
+				"\n"
+				"  git config pull.rebase false  # merge (the default strategy)\n"
+				"  git config pull.rebase true   # rebase\n"
+				"  git config pull.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"
+				"If unsure, run \"git pull --merge\".\n"
+				"Read \"git pull --help\" for more information."
+				));
+		}
 	}
 
 	if (opt_rebase >= REBASE_TRUE) {
diff --git a/t/t5520-pull.sh b/t/t5520-pull.sh
index 9fae07cdfa..067780e658 100755
--- a/t/t5520-pull.sh
+++ b/t/t5520-pull.sh
@@ -819,4 +819,54 @@ 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 (ff-only)' '
+	test_config pull.ff only &&
+	setup_ff &&
+	git pull
+'
+
+test_expect_success 'non-fast-forward (ff-only)' '
+	test_config pull.ff only &&
+	setup_non_ff &&
+	test_must_fail git pull
+'
+
+test_expect_failure 'non-fast-forward with merge (ff-only)' '
+	test_config pull.ff only &&
+	setup_non_ff &&
+	git pull --merge
+'
+
+test_expect_success 'non-fast-forward with rebase (ff-only)' '
+	test_config pull.ff only &&
+	setup_non_ff &&
+	git pull --rebase
+'
+
+test_expect_success 'non-fast-forward error message (ff-only)' '
+	test_config pull.ff only &&
+	setup_non_ff &&
+	test_must_fail git pull 2> error &&
+	cat error &&
+	grep -q "The pull was not fast-forward" error
+'
+
 test_done
-- 
2.29.2