git-subtree conflict when pulling from central repo

Question

I have several projects that depend on the same library, for which I'd like to maintain a separate git repository to be managed with git-subtree within each project. So for example, within each project I can do:

project1$  git subtree add --prefix=lib1 /path/to/lib1.git master
project2$  git subtree add --prefix=lib1 /path/to/lib1.git master

Now in the course of working on project1, I make some changes to lib1, say lib1/file1.c, and push this back to the central repo:

project1$  git add lib1/file1.c
project1$  git commit -m "updates to lib1"
project1$  git subtree push --prefix=lib1 /path/to/lib1.git master

So far, so good. But now I'd like to update project2's copy of lib1. So I try:

project2$  git subtree pull --prefix=lib1 /path/to/lib1.git master
Auto-merging lib1/file1.c
CONFLICT (content): Merge conflict in lib1/file1.c
Automatic merge failed; fix conflicts and then commit the result.

What's going on? I know for certain that no changes were made to any of the lib1 files under project2, so why should there be a conflict here?

The conflicts are half-empty, like those reported in this question. Everything is being pulled/pushed within a single system (OS X), so I know there's no issue with line endings as suggested there.

Surely this is a common use case for git-subtree, and has a simple answer I just can't see. Please help!

EDIT: I found an unsatisfying workaround: immediately after pushing changes to the subtree, I need to re-run subtree pull:

project1$  git subtree push --prefix=lib1 /path/to/lib1.git master
project1$  git subtree pull --prefix=lib1 /path/to/lib1.git master

Even though there were no changes, it will find something, and do a merge commit. Then, after making some changes elsewhere, the conflict won't happen the second time I pull from the central repo. But if I forget to run pull immediately after pushing, the next pull will get this conflict.

So now my question is, why does this work? Is there a bug in the way git-subtree tracks pushes, or am I missing something?

Answer 1

I actually found the proper way to do this through some trial and error.

After this command:

project1$  git subtree push --prefix=lib1 /path/to/lib1.git master

execute a fetch command:

project2$  git fetch /path/to/lib1.git master

and then do your pull:

project2$  git subtree pull --prefix=lib1 /path/to/lib1.git master