[GRASS-dev] git: how to switch between branches?

[Panagiotis Mavrogiorgos]

Hello Markus

I don't have real-world experience with this, but I am not sure if changing
branches and
distcleaning/recompiling is the most convenient workflow. I think I would
prefer to have
multiple local repositories with different branches checked out. Thankfully
git makes it
rather easy to have this.  E.g.:

1. You create two root directories. One for Python2 and one for Python3:

    mkdir /home/user/git/grass-p{2,3}

2. Create a virtualenv inside grass-p2 and grass-p3. E.g.:

    cd /home/user/git/grass-p2
    virtualenv -p python2 venv

    cd /home/user/git/grass-p2
    python3 -m venv venv

3. Clone the remote GRASS repo and name the local repo `dev`. You will only
do this for `grass-p3`:

    cd /home/user/git/grass-p3
    git clone https://github.com/neteler/grass dev

5. For python 2 and for each release branch you will make a local git
clone. This way
   you won't be wasting disk space. Read more about this
   [here](
https://stackoverflow.com/questions/7187088/git-how-to-create-local-repo-with-symbolic-links-to-the-git-main-repo
).

    cd /home/user/git/grass-p2
    git clone ../grass-p3/dev dev
    git clone ../grass-p3/dev r72
    git clone ../grass-p3/dev r74
    git clone ../grass-p3/dev r76

6. The dir structure should look like this:

    $ tree grass*

    grass-p2
    ├── dev
    ├── r72
    ├── r74
    ├── r76
    └── venv
    grass-p3
    ├── dev
    └── venv

7. On each release clone you need to checkout the respective branch:

    cd /home/user/git/grass-p2/r72 && git checkout releasebranch_7_2
    cd /home/user/git/grass-p2/r74 && git checkout releasebranch_7_4
    cd /home/user/git/grass-p2/r76 && git checkout releasebranch_7_6

7. Each directory is a full blown git repo. It is only by convention that
7.6 backports
   will happen in `r76` etc.  If you want to directly pull/push from/to
github from the release
   directories, you will probably need to setup remotes. Regardless,
setting up remotes
   etc should only be done once.

8. Obviously, when 7.8 gets released, two more directories will need to be
added (one
   for python2  and one for python 3).

The benefit of this approach is that at least for trivial backports, you
will not need
to recompile everything and, perhaps more importantly, you will not need to
recompile
"master" in order to test a fix in 7.2.

all the best,
Panos

PS1. There are other ways to achieve something like this, e.g.
[`--singlebranch`](https://stackoverflow.com/a/1911126/592289) but I don't
see much
benefit.

PS2. You can optionally use something like [direnv](
https://github.com/direnv/direnv) to
automatically activate the virtualenvs when you cd into the corresponding
directory.
This way there is no confusion WRT which python is active. I use this and
it works
marvellously.
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