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>

Documentation/git-push.txt

@@ -217,6 +217,47 @@ with this feature.
 +
 "--no-force-with-lease" will cancel all the previous --force-with-lease on the
 command line.
++
+A general note on safety: supplying this option without an expected
+value, i.e. as `--force-with-lease` or `--force-with-lease=<refname>`
+interacts very badly with anything that implicitly runs `git fetch` on
+the remote to be pushed to in the background, e.g. `git fetch origin`
+on your repository in a cronjob.
++
+The protection it offers over `--force` is ensuring that subsequent
+changes your work wasn't based on aren't clobbered, but this is
+trivially defeated if some background process is updating refs in the
+background. We don't have anything except the remote tracking info to
+go by as a heuristic for refs you're expected to have seen & are
+willing to clobber.
++
+If your editor or some other system is running `git fetch` in the
+background for you a way to mitigate this is to simply set up another
+remote:
++
+	git remote add origin-push $(git config remote.origin.url)
+	git fetch origin-push
++
+Now when the background process runs `git fetch origin` the references
+on `origin-push` won't be updated, and thus commands like:
++
+	git push --force-with-lease origin-push
++
+Will fail unless you manually run `git fetch origin-push`. This method
+is of course entirely defeated by something that runs `git fetch
+--all`, in that case you'd need to either disable it or do something
+more tedious like:
++
+	git fetch              # update 'master' from remote
+	git tag base master    # mark our base point
+	git rebase -i master   # rewrite some commits
+	git push --force-with-lease=master:base master:master
++
+I.e. create a `base` tag for versions of the upstream code that you've
+seen and are willing to overwrite, then rewrite history, and finally
+force push changes to `master` if the remote version is still at
+`base`, regardless of what your local `remotes/origin/master` has been
+updated to in the background.
 
 -f::
 --force::

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