如何在 git-subtree 添加后重建基础？

You need to use

git rebase --preserve-merges --preserve-committer --onto new_place start end

Apparently this is expected behaviour (for some perverse definition of "expected behaviour.") See: https://web.archive.org/web/20200219001959/http://git.661346.n2.nabble.com/subtree-merges-lose-prefix-after-rebase-td7332850.html.

Not that this is much help to anyone. I'd love to find a workaround for this too.

I had a similar issue: I wanted to rebase after doing a subtree add, and using --preserve-merges still left me with a merge conflict (due to conflicting .gitignore files and others).

In my case, I didn't necessarily plan on using any subtree functionality: I was simply pulling in a repo that should have been part of the superproject originally. In case it helps anyone else, here's what I ended up doing, based off of other related answers I found.

Suppose I have two projects, main_project and sub_project, in the same directory. I want to pull sub_project into a directory named sub_project within main_project, assuming neither repo has ever had a directory named sub_project:

cd main_project
git fetch ../sub_project
git checkout -b sub_project FETCH_HEAD
git filter-branch --prune-empty --tree-filter '
if [[ ! -e sub_project ]]; then
mkdir -p sub_project
git ls-tree --name-only $GIT_COMMIT | xargs -I files mv files sub_project
fi'
git checkout branch-to-merge-within
git merge sub_project
git branch -d sub_project

I'll update if I find any problems with this approach.

This isn't a solution, but its the current work around I use...

Using your initial example:

*   22c1fe6 (HEAD, master) Merge commit 'b6e698d9f4985825efa06dfdd7bba8d2930cd40e' as 'lib/mylib' -
|\
| * b6e698d Squashed 'lib/mylib/' content from commit d7dbd3d
* b99d55b Add readme
* 020e372 Initial

Rebase interactively to the 2nd commit before the subtree add:

$ git rebase -i 020e372

Delete the two subtree entries & mark edit for the prior commit:

e b99d55b Add readme

Save file/close, then when it gets to the "Add readme" commit, run the amend command:

$ git commit --amend

Then re-add your new subtree:

$ git subtree add -P lib/mylib myliborigin master

Continue the rebase:

$ git rebase --continue

Your branch should then be rebased off of master, and the subtree will be "normal" with the Squash + the Merge intact:

*   22c1fe6 (HEAD, master) Merge commit 'b6e698d9f4985825efa06dfdd7bba8d2930cd40e' as 'lib/mylib' -
|\
| * b6e698d Squashed 'lib/mylib/' content from commit d7dbd3d

This is an old question, but I just had the same problem with my repo, and I finally found a complete solution, which (hopefully) preserves all the subtree metadata.

Suppose we had this commit tree:

B   (master) Add README.md
|
A            Initial commit

and we forked a feature branch with a subtree residing in lib/:

git remote add -f githublib https://github.com/lib/lib.git
git subtree add --prefix lib/ githublib master --squash

It creates a merge commit D with two parents: our current master (B), and an unrelated commit F with the squashed history of the external repo. This commit also contains some git subtree metadata in its commit message (namely, git-subtree-dir and git-subtree-split).

   D     (feature) Merged commit 'F' as 'lib/'
/ \
/   F             Squashed 'lib/' content from GGGGGG
B        (master)  Add README.md
|
A                  Initial commit

Later, we add some commits to both branches independently.

   E     (feature) Remove .gitignore from lib/
C  |     (master)  Add LICENSE.md
|  D               Merged commit 'F' as 'lib/'
| / \
|/   F             Squashed 'lib/' content from GGGGGG
B                  Add README.md
|
A                  Initial commit

Now we want to rebase feature onto master. Here's how:

1. Cherry-pick the commits from feature one by one to create a new copy of the feature branch on top of master.

git branch -f feature C
git checkout feature
git cherry-pick D E


E'       (feature) Remove .gitignore from lib/
|
D'                 Merged commit 'F' as 'lib/'
|
|  E               Remove .gitignore from lib/
C  |     (master)  Add LICENSE.md
|  D               Merged commit 'F' as 'lib/'
| / \
|/   F             Squashed 'lib/' content from GGGGGG
B                  Add README.md
|
A                  Initial commit

Now we have the equivalent of a rebase, but we've lost all information about the external repo, required for git subtree. To restore it:

2. Add the missing parent link as a graft, and rewrite the history of feature to make it permanent.

git checkout feature
git replace --graft D' C F
git filter-branch --tag-name-filter cat -- master..

And now we get a picture exactly equivalent to the one with started with. The old commits D and E are still out there, but they can be garbage-collected later.

E'       (feature) Remove .gitignore from lib/
|
D'                 Merged commit 'F' as 'lib/'
|\
| \
C  \     (master)  Add LICENSE.md
|   \
|    \
|     F            Squashed 'lib/' content from GGGGGG
B                  Add README.md
|
A                  Initial commit

Warning: This rewrites the history of feature, so be wary of publishing it if anybody else collaborates with you on this branch. However, since you wanted to make a rebase in the first place, you are probably aware of that :-)

This works in simple cases:

git rebase --preserve-merges master

Thanks to @Techlive Zheng in the comments.

You may see

fatal: refusing to merge unrelated histories
Error redoing merge a95986e...

Which means that git failed to automatically apply your subtree. This puts you in the situation @ericpeters described in his answer. Solution:

Re-add your subtree (use the same command you originally used):

git subtree add -P lib lib-origin master

Continue the rebase:

git rebase --continue

And you're all set!

If you're wondering if it worked successfully, you can compare with your original version after rebasing to make sure you didn't change anything:

git diff <ref-before-rebase> <ref-after-rebase> -- .

(the -- . at the end instructs git to only diff the files in your current directory).

If all else fails and you don't care about preserving the commits themselves, you can simply git cherry-pick the original subtree commit.

The commit message will look something like Add 'lib/' from commit '9767e6...' -- that's the one you want.

Git 2.24.0 (released 2019-11-04) added support for git rebase --rebase-merges --strategy [strategy]. So now if you run git rebase --rebase-merges --strategy subtree [branch] when your current branch contains a subtree merge, it will Just Work now.

For my project, I've decided I might not use git subtree add, but instead throw away the second parent of the merge commit using git replace --edit. I've also used the Git book v1's obsolete "subtree" tutorial, which does the same thing but is tedious.

An alternative way is to git rebase -i master, then substitute the merged squashed commit with b (break here), so git rebase would stop at the time of the original subtree add commit. Run git subtree add ... again at that moment, then git rebase --continue. You would get the same history but rebased onto the latest master.

You can edit the interactive-rebase contents manually to re-add the subtree from the original squashed commit, using the exec command, without stopping and doing any manual work.

Start the rebase by using --rebase-merges:

git rebase -i --rebase-merges <commit>

• Commit graph at the start:

  *   22c1fe6 (HEAD, master) Merge commit 'b6e698d9f4985825efa06dfdd7bba8d2930cd40e' as 'lib/mylib' -
  |\
  | * b6e698d Squashed 'lib/mylib/' content from commit d7dbd3d
  * b99d55b Add readme
  * 020e372 Initial
  

• Auto generated interactive rebase contents:

  reset [new root]
  pick b6e698d Squashed 'lib/mylib/' content from commit d7dbd3d
  label b6e698d9f4985825efa06dfdd7bba8d2930cd40e-2
  
  
  reset onto
  pick 020e372 Initial
  pick b99d55b Add readme
  merge -C 22c1fe6 b6e698d9f4985825efa06dfdd7bba8d2930cd40e-2 # Merge commit 'b6e698d9f4985825efa06dfdd7bba8d2930cd40e' as 'lib/mylib'
  

• New contents:

  reset [new root]
  pick b6e698d Squashed 'lib/mylib/' content from commit d7dbd3d
  label b6e698d9f4985825efa06dfdd7bba8d2930cd40e-2
  
  
  reset onto
  pick 020e372 Initial
  pick b99d55b Add readme
  # We have to remove the subtree folder first, otherwise `git subtree` will complain about its existence.
  exec rm -r lib/mylib
  # Re-add the subtree, but from the existing commit.
  # NOTE: `--squash` is important, otherwise a duplicate squash commit will be created.
  exec git subtree add --prefix=lib/mylib b6e698d --squash
  # We can remove the last `merge` action as we are manually adding the subtree
  

Caveats

If you've pulled or merged after adding the subtree you will have to apply this procedure to those commits too but changing the command to the corresponding one (merge/pull/etc).