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>
Ævar Arnfjörð Bjarmason committed
Apr 19, 2017 at 09:22 UTC
f17d642d3b0fa64879d59b311e596949f2a1f6d2
2 files changed
+70
Documentation/git-push.txt
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@@ -217,6 +217,47 @@ with this feature.
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+
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"--no-force-with-lease" will cancel all the previous --force-with-lease on the
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command line.
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++
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+A general note on safety: supplying this option without an expected
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+value, i.e. as `--force-with-lease` or `--force-with-lease=<refname>`
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+interacts very badly with anything that implicitly runs `git fetch` on
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+the remote to be pushed to in the background, e.g. `git fetch origin`
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+on your repository in a cronjob.
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++
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+The protection it offers over `--force` is ensuring that subsequent
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+changes your work wasn't based on aren't clobbered, but this is
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+trivially defeated if some background process is updating refs in the
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+background. We don't have anything except the remote tracking info to
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+go by as a heuristic for refs you're expected to have seen & are
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+willing to clobber.
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++
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+If your editor or some other system is running `git fetch` in the
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+background for you a way to mitigate this is to simply set up another
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+remote:
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++
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+ git remote add origin-push $(git config remote.origin.url)
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+ git fetch origin-push
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++
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+Now when the background process runs `git fetch origin` the references
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+on `origin-push` won't be updated, and thus commands like:
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++
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+ git push --force-with-lease origin-push
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++
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+Will fail unless you manually run `git fetch origin-push`. This method
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+is of course entirely defeated by something that runs `git fetch
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+--all`, in that case you'd need to either disable it or do something
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+more tedious like:
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++
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+ git fetch # update 'master' from remote
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+ git tag base master # mark our base point
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+ git rebase -i master # rewrite some commits
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+ git push --force-with-lease=master:base master:master
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++
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+I.e. create a `base` tag for versions of the upstream code that you've
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+seen and are willing to overwrite, then rewrite history, and finally
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+force push changes to `master` if the remote version is still at
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+`base`, regardless of what your local `remotes/origin/master` has been
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+updated to in the background.
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-f::
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--force::
t/t5533-push-cas.sh
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@@ -229,4 +229,33 @@ test_expect_success 'new branch already exists' '
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)
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'
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+test_expect_success 'background updates of REMOTE can be mitigated with a non-updated REMOTE-push' '
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+ rm -rf src dst &&
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+ git init --bare src.bare &&
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+ test_when_finished "rm -rf src.bare" &&
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+ git clone --no-local src.bare dst &&
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+ test_when_finished "rm -rf dst" &&
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+ (
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+ cd dst &&
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+ test_commit G &&
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+ git remote add origin-push ../src.bare &&
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+ git push origin-push master:master
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+ ) &&
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+ git clone --no-local src.bare dst2 &&
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+ test_when_finished "rm -rf dst2" &&
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+ (
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+ cd dst2 &&
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+ test_commit H &&
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+ git push
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+ ) &&
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+ (
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+ cd dst &&
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+ test_commit I &&
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+ git fetch origin &&
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+ test_must_fail git push --force-with-lease origin-push &&
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+ git fetch origin-push &&
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+ git push --force-with-lease origin-push
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+ )
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+'
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+
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test_done