Setting up jjvcs collaboration between local machines via ssh

Introduxion

This blog provides instruxions for using jjvcs to collaborate between two laptops (host and guest) over ssh, without relying on github or similar hosting services. Essentially this approach should apply to any number of local machines.

Note that this blog is targeted at macos. It relies on mDNS to announces a machine's hostname and ip via multicast dns on every network it joins, so other machines can automatically resolve .local hostnames without manual dns configuration.

We use host/guest to indicate the commands in this section shall be run on the host/guest laptop.

The ssh part

Ensure the ssh server is running (host)

# check if ssh is enabled
sudo systemsetup -getremotelogin
# if not enabled, enable it
# sudo systemsetup -setremotelogin on

Sudo set hostname (host)

The formation of this section shall allow us to use hostname.local addresses that automatically resolve on any local network without manual configuration, so we needn't reconfigure ip addresses on new networks.

# verify your local hostname
scutil --get LocalHostName

This returns something like MacBook (without the .local suffix).

We can set a memorable hostname:

sudo scutil --set LocalHostName host

This machine will now be reachable at host.local on any local network.

Verify mDNS is working (guest)

ping host.local

Generate an ssh key (guest)

ssh-keygen -t ed25519 -C "your_email@example.com"

Copy guest's public key to host (guest)

# substitute username with the output of `whoami`
ssh-copy-id username@host.local

Test the connection (guest)

ssh username@host.local

Note that you don't need to ssh into the host in the following steps.

The jj part

Initialize a jj repo (host)

Let's assume that we put all of our projects under ~/dev. We create a new project jjssh here:

cd ~/dev && mkdir jjssh && jj git init --colocate

We make some dummy changes:

echo "initial content" >readme

And commit them:

jj ci -m "first commit" && jj b s main -r @-

So far so good. But unfortunately, we can't directly use this repo for ssh-based collaboration. Instead, we need a bare git repo.

Create a bare repo (host)

We need to create a bare repo that will serve as the remote. We will put all our bare repos under ~/remotes.

mkdir -p ~/remotes && cd ~/remotes && git init --bare jjssh.git

Push jj repo to bare repo (host)

We can now add the bare repo as origin push the bookmark to it:

cd ~/dev/jjssh && jj git remote add origin ~/remotes/jjssh.git
jj b t main --remote origin && jj git push -b main --remote origin

The workflow should henceforth be analogous to that in normal jj.

Clone the bare repo (guest)

cd ~/dev && jj git clone --colocate ssh://username@host.local/~/remotes/jjssh.git jjssh

Make changes and push the bookmark (guest)

cd ~/dev/jjssh && jj new main@origin
echo "changes from guest" >>readme && jj ci -m "changes from guest"
jj b s guest -r @- && jj b t guest --remote origin && jj git push -b guest --remote origin

Fetch commits (host)

cd ~/dev/jjssh && jj git fetch --remote origin

My shortcut, a jjinit function to automate the project setup

function jjinit
    set project_name $argv[1]
    set project_dir ~/dev/$project_name
    set remote_dir ~/remotes/$project_name.git

    mkdir -p $project_dir
    cd $project_dir

    jj git init --colocate

    mkdir -p ~/remotes
    git init --bare $remote_dir

    jj git remote add origin $remote_dir
end