push: document & test --force-with-lease with multiple remotes

Document & test for cases where there are two remotes pointing to the
same URL, and a background fetch & subsequent `git push
--force-with-lease` shouldn't clobber un-updated references we haven't
fetched.

Some editors like Microsoft's VSC have a feature to auto-fetch in the
background, this bypasses the protections offered by
--force-with-lease & --force-with-lease=<refname>, as noted in the
documentation being added here.

See the 'Tools that do an automatic fetch defeat "git push
--force-with-lease"' (<1491617750.2149.10.camel@mattmccutchen.net>)
git mailing list thread for more details. Jakub Narębski suggested
this method of adding another remote to bypass this edge case,
document that & add a test for it.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 29 insertions, 0 deletions

diff --git a/t/t5533-push-cas.sh b/t/t5533-push-cas.sh
index a2c9e7439..d38ecee21 100755
--- a/t/t5533-push-cas.sh
+++ b/t/t5533-push-cas.sh
@@ -229,4 +229,33 @@ test_expect_success 'new branch already exists' '
 	)
 '
 
+test_expect_success 'background updates of REMOTE can be mitigated with a non-updated REMOTE-push' '
+	rm -rf src dst &&
+	git init --bare src.bare &&
+	test_when_finished "rm -rf src.bare" &&
+	git clone --no-local src.bare dst &&
+	test_when_finished "rm -rf dst" &&
+	(
+		cd dst &&
+		test_commit G &&
+		git remote add origin-push ../src.bare &&
+		git push origin-push master:master
+	) &&
+	git clone --no-local src.bare dst2 &&
+	test_when_finished "rm -rf dst2" &&
+	(
+		cd dst2 &&
+		test_commit H &&
+		git push
+	) &&
+	(
+		cd dst &&
+		test_commit I &&
+		git fetch origin &&
+		test_must_fail git push --force-with-lease origin-push &&
+		git fetch origin-push &&
+		git push --force-with-lease origin-push
+	)
+'
+
 test_done