Short answer:
cd ProjectFooBarCommoneSubmodule
git checkout master
<Do your editing>
git commit --all -m "Lots of fixes"
git push submodule_origin master
cd ..
git add ProjectFooBarCommoneSubmodule
git commit -m "Bumped up the revision of ProjectFooBarCommoneSubmodule"
git push origin master
The longer one:
Git submodules are a dependency mechanism, where the main project (say A) defines a specified revision in a subproject (say B), which will be used in building project A. In order for the tool to be useful the behavior has to be predictable from A:s point of view. Dependencies cannot change, unless somebody decides to incorporate the change to project A. All kinds of nasty things could happen, were project B:s changes automatically imported, of which compile errors are probably the best ones, as A would notice the failures immediately. This is why B:s head is kept in detached state.
The state of B is store in A (check out git submodule status), and a revision change has to be done and committed in A, in order for it to have any effect. This is what happens in the example above, A changes the revision number stored in the repo, and bumps up the version to the latest one. The process will have to be repeated in the other main repo as well, so no automatic “use master” switch AFAIK.
BTW. The Git book chapter on submodules and the submodule man page contain lots of useful info about submodules, as normal usage and typical pitfalls as well. Worth checking out.
EDIT: I’ll try to explain this better
I took the liberty to create example projects on my github account. The commits are meaningless and contain junk, but the setup should be fine. Please check it out to follow.
Both ProjectFoo and ProjectBar share the code in the common submodule.
ProjectFooBarCommoneSubmodule:master is 6850e4e4c1fac49de398
In ProjectFoo:
git submodule status
-6850e4e4c1fac49de39890703f21486ca04b87a0 common
In ProjectBar:
git submodule status
-6850e4e4c1fac49de39890703f21486ca04b87a0 common
So both point to the same revision, right? The trick here is to see, that ProjectFoo and ProjectBar point to the revision (6850e4e4c1fac49de39890703f21486ca04b87a0) not the branch (master), although they are the same thing. The first one is a detached head, and the other a named branch.
If you want to do some fixing on ProjectFooBarCommoneSubmodule, you can go to the subdir in e.g. ProjectFoo, and choose the branch instead of the revision:
git checkout master
<Do your coding and pushing here>
Then go one directory up, and check git submodule status. It should tell you, that you are now out of sync. E.g
git submodule status
+e24bd2bf45d52171a63b67ac05cd4be0ac965f60 common (heads/master-1-ge24bd2b)
Now you can do a git add, to set the reference to this particular commit(ge24bd…), do a commit, and after this the submodule reference points to this revision, which also happens to be master on ProjectFooBarCommoneSubmodule.
Now you need to update the reference in ProjectBar as well. Go to ProjectBar/common, and do git fetch origin (this is a fast forward merge), do
git checkout master
cd ..
git add common
git commit -m "Bumped up the revision"
git push origin master # to publish the revision bump to everybody else
So, as with any git repository, you don’t need to work on a detached head. You can either work on master, or create a named branch. Either way, make sure that upstream contains the ProjectFooBarCommoneSubmodule changes, or you will break both ProjectFoo and ProjectBar, if they reference something that doesn’t exist. Hope this explained it better