git push --force-with-lease vs. --force

Solution 1:

force overwrites a remote branch with your local branch.

--force-with-lease is a safer option that will not overwrite any work on the remote branch if more commits were added to the remote branch (by another team-member or coworker or what have you). It ensures you do not overwrite someone elses work by force pushing.

I think your general idea surrounding the command is correct. If the remote branch has the same value as the remote branch on your local machine- you will overwrite remote. If it doesn't have the same value- it indicates a change that someone else made to the remote branch while you were working on your code and thus will not overwrite any code. Obviously if there are additional commits in remote then the values won't be the same.

I just think of --force-with-lease as the option to use when I want to make sure I don't overwrite any teammates code. A lot of teams at my company use --force-with-lease as the default option for a fail-safe. Its unnecessary in most circumstances but will save you lots of headache if you happen to overwrite something that another person contributed to remote.

I'm sure you looked at the docs but there might be some more wordy explanation contained in here:

https://git-scm.com/docs/git-push

Solution 2:

Looking for an answer drawing from credible and/or official sources.

The "compare and swap" mentioned by torek in the comments and in his other answer is further illustrated by the sources of Git itself.

the latter only pushes to the remote if the remote does not have commits that the local branch doesn't have?

That feature was introduced in this commit (Dec. 2013, Git v1.8.5-rc0)

--force-with-lease will protect all remote refs that are going to be updated by requiring their current value to be the same as some reasonable default, unless otherwise specified;

For now, "some reasonable default" is tentatively defined as "the value of the remote-tracking branch we have for the ref of the remote being updated", and it is an error if we do not have such a remote-tracking branch.

So "lease" means:

"force-with-lease": You assume you took the lease on the ref when you fetched to decide what the rebased history should be, and you can push back only if the lease has not been broken.

The sources still mentions "cas":

• This option was originally called "cas" (for "compare and swap"), the name which nobody liked because it was too technical.
• The second attempt called it "lockref" (because it is conceptually like pushing after taking a lock) but the word "lock" was hated because it implied that it may reject push by others, which is not the way this option works.
• This round calls it "force-with-lease".
  You assume you took the lease on the ref when you fetched to decide what the rebased history should be, and you can push back only if the lease has not been broken.

So: "git push --force-with-lease vs. --force"

As I mentioned in "push --force-with-lease by default", as Git 2.13 (Q2 2017) mentions, that the option --force-with-lease can be ignored if a background process (like the ones you find in an IDE with a Git plugin) runs git fetch origin.
In that case, --force prevails.

As Pavlus adds in the comments:

it is not ignored per se, it is just now you have identical refs for local remote head and remote head, so --force-with-lease will behave correctly -- compare these two, and if in that interval of time between fetch and push, someone updated remote, it won't behave as --force, it will still fail.

Another difference: before Git 2.29 (Q4 2020), pushing a ref whose name contains non-ASCII character with the "--force-with-lease" option did not work over smart HTTP protocol.
It would work with git push --force.

See commit cd85b44 (21 Jul 2020) by brian m. carlson (bk2204).
^{(Merged by Junio C Hamano -- gitster -- in commit c2796ac, 30 Jul 2020)}

remote-curl: make --force-with-lease work with non-ASCII ref names

^{Reported-by: Frej Bjon
Signed-off-by: brian m. carlson}

When we invoke a remote transport helper and pass an option with an argument, we quote the argument as a C-style string if necessary.
This is the case for the cas option, which implements the --force-with-lease command-line flag, when we're passing a non-ASCII refname.

However, the remote curl helper isn't designed to parse such an argument, meaning that if we try to use --force-with-lease with an HTTP push and a non-ASCII refname, we get an error like this:

error: cannot parse expected object name '0000000000000000000000000000000000000000"'  

Note the double quote, which get_oid has reminded us is not valid in an hex object ID.

Even if we had been able to parse it, we would send the wrong data to the server: we'd send an escaped ref, which would not behave as the user wanted and might accidentally result in updating or deleting a ref we hadn't intended.

Since we need to expect a quoted C-style string here, just check if the first argument is a double quote, and if so, unquote it.
Note that if the refname contains a double quote, then we will have double-quoted it already, so there is no ambiguity.

We test for this case only in the smart protocol, since the DAV-based protocol is not capable of handling this capability.
We use UTF-8 because this is nicer in our tests and friendlier to Windows, but the code should work for all non-ASCII refs.

While we're at it, since the name of the option is now well established and isn't going to change, let's inline it instead of using the #define constant.

Git 2.30 (Q1 2021) adds git push --force-if-includes

When a local branch that is based on a remote ref, has been rewound and is to be force pushed on the remote, "--force-if-includes" runs a check that ensures any updates to the remote-tracking ref that may have happened (by push from another repository) in-between the time of the last update to the local branch (via "git pull", for instance) and right before the time of push, have been integrated locally before allowing a forced update.

Solution 3:

git push --force is destructive because it unconditionally overwrites the remote repository with whatever one have locally. git's push --force is strongly discouraged as it can destroy other commits already pushed to a shared repository. One of the most common causes of force pushes is when we're forced to rebase a branch.

For example. We have a project with a feature branch that both Alice and Bob are going to work on. They both clone this repository and start work. Alice initially completes her part of the feature, and pushes this up to the main repository. This is all well and good. Bob also finishes his work, but before pushing it up he notices some changes had been merged into master. Wanting to keep a clean tree, he performs a rebase against the master branch. Of-course, when he goes to push this rebased branch it will be rejected. However not realising that Alice has already pushed her work, he performs a push --force. Unfortunately, this will erase all record of Alice's changes in the central repository.

What --force-with-lease does is refuse to update a branch unless it is the state that we expect; i.e. nobody has updated the branch upstream. In practice this works by checking that the upstream ref is what we expect, because refs are hashes, and implicitly encode the chain of parents into their value.

Here is a good post regarding git push --force and git push --force-with-lease: –force considered harmful; understanding git’s –force-with-lease

Solution 4:

Assuming any pre-receive hooks on the server accept the push, this will always succeed:

git push --force

Whereas this runs a specific client-side check before proceeding:

git push --force-with-lease

You can run the specific check yourself manually. Here's the "lease-checking" algorithm:

1. Figure out your current branch.

2. Run git for-each-ref refs/remotes. Take note of the commit-id your git client thinks corresponds to the upstream state of your current branch.

E.g., if you are on branch "foo", take note of the commit-id associated with "refs/remotes/origin/foo".

3. Determine the actual commit-id of the remote branch on the upstream git server right now.

4. Only let the "git push" proceed if the commit-ids you extracted from step 2 and step 3 agree. In other words, only proceed if your local git clone's notion of upstream agrees with actual upstream.

There's a sad implication here: since git fetch updates all refs under "refs/remotes/origin/*" to their latest versions, this combination of commands is essentially identical to git push --force:

git fetch

# The command below behaves identically to "git push --force"
# if a "git fetch" just happened!

git push --force-with-lease

To work around this inherent weakness in git push --force-with-lease I try to never run git fetch. Instead I always run git pull --rebase whenever I need to sync with upstream, since git pull only updates a single ref under refs/remotes, keeping the "lease" of --force-with-lease useful.