Pushing Squashed subtree change to gerrit

Question

While adding a subtree:

git subtree add --prefix=vendor https://example.com/repo master --squash

There are two commits created. One for the squashed subtree commits

Squashed 'vendor' changes from 15fc2b6..c6dc29b

and a merge commit

Merge commit 'SHA1' into master

When I want to push this change to gerrit, it needs a changeID. But git doesn't allow me to do a

git rebase -i HEAD~2

and rework like I do for any other commit as it is a squashed commit.

Now I can't push this change to gerrit because of this. I can't commit the changes to heads (git) directly and break stuff on the supermodule. It has to go through build and testing. Any suggestion or help is appreciated.

Answer 1

git filter-branch can do lots of things, while at the same time updating the commit pointers so the commit graph stays intact. One of these is running a command to edit all the commit messages, such as Gerrit's commit-msg hook.

git filter-branch HEAD...HEAD~1 grabs all the commits in HEAD but not HEAD~1, which for a git-subtree merge is just the merge and squash commit.

git filter-branch --msg-filter takes a command to run which gets the commit message piped in on stdin, and writes a new one to stdout. commit-msg hook scripts work on a file, so a bit of shell scripting is necessary to write the original message to a file, run the hook, and then cat the file to stdout.

I've put this all in a script to run before pushing to Gerrit:

# This command re-writes the history after doing a git subtree {pull|add}
# to add Gerrit Change-Id lines to the squash commit message.
#
# It assumes that HEAD is the merge commit merging the subtree into HEAD.
# The original HEAD commit will be backed up under refs/original, which
# is helpful if something goes wrong.

set -e
set -o pipefail

GIT_DIR=$(readlink -f "$(git rev-parse --git-dir)")
TMP_MSG="${GIT_DIR}/COMMIT_MSG_REWRITE"

git filter-branch --msg-filter \
  "cat > ${TMP_MSG} && \"${GIT_DIR}/hooks/commit-msg\" ${TMP_MSG} && \
  cat \"${TMP_MSG}\"" HEAD...HEAD~1

rm -rf "${TMP_MSG}"