Share a working tree with git submodules

Question

Suppose I have a library Common that may be used stand-alone, and is used by projects P1 and P2, so the tree I want looks like

/Common/.git
        ...
/P1/.git
    .gitmodules  # points to remote server
    Common/
    ...
/P2/.git
    .gitmodules  # points to remote server
    Common/
    ...

When I make a change in /Common, I would like to be able to test it using P1 and P2 before committing. With the usual git submodule command set, I would have to commit from /Common, push to remote, then pull from both /P1/Common and /P2/Common. If the commit breaks something, it cannot be amended because the bad change has already been published. Alternatively, I could git remote add quicktest /Common from /P?/Common to be able to pull without touching the remote server. But this has lots of opportunities for inconsistency and it is dirty to strip broken commits from /P?/Common so that they can be amended in /Common.

I would rather that, during development, the working tree from /Common be used by P1 and P2, but I can't make /P1/Common a symlink to /Common because git submodule recognizes the symlink as different from a directory. Hardlinking directories is not allowed by most file systems. I can hardlink all the files using

rm -rf /P1/Common
cp -rl /Common /P1/Common

which works quite well until a new file is added to /Common in which case this process needs to be repeated. Is there an elegant way to both

1. keep git clone --recursive git://remote/P1.git working for the end user, and
2. allow me to easily test that changes in /Common work with P1 and P2?

Answer 1

Try git worktree feature since git 2.5.

1. Delete /P1/Common
2. cd /Common
3. Create P1 branch for the coming /P1/Common working tree
4. run git worktree add ../P1/Common P1

Do the same thing on /P2.

Then, /P1/Common, /P2/Common and /Common working trees share the same repository /Common/.git.

And you can easily just checkout any commit which is commit in another working tree.

P.S. you can still use git submodule command for daily work with this git worktree feature.