Merge Upstream into Fork

Merge Upstream

Sync the current fork branch with upstream/<branch> using a merge commit by default. Preserve local commit hashes, keep push non-destructive, and never rewrite history.

Usage

Treat these as equivalent triggers:

/merge-upstream [--base=<branch>] [--ff-allow]
merge upstream [--base=<branch>] [--ff-allow]
sync fork with upstream

Options:

• --base=<branch>: use that upstream branch instead of auto-detecting the upstream default branch.
• --ff-allow: allow git merge --ff-only when the current branch has no unique local commits. Without this, use git merge --no-ff to leave an explicit sync commit.

Invariants

• Do not run git rebase.
• Do not run git push --force or git push --force-with-lease.
• Do not bypass hooks or signing with --no-verify or --no-gpg-sign.
• Ask before pushing.

Workflow

1. Validate the repository:

   git rev-parse --is-inside-work-tree
   git branch --show-current
   git remote get-url upstream
   git remote get-url origin

   Abort on detached HEAD or missing upstream. If origin is missing, continue locally and skip push.

2. Require a clean worktree:

   worktree_status=$(git status --porcelain) || exit 1
   test -z "$worktree_status"

   If dirty, stop and ask the user to clean or commit the changes, or use a clean task worktree.

3. Detect the upstream target branch:

   • If --base=<branch> is provided, fetch and verify upstream/<branch>.
   • Otherwise run git remote set-head upstream -a, then read refs/remotes/upstream/HEAD.
   • If detection fails, ask the user for --base=<branch>.

4. Fetch enough history for a reliable merge base:

   git rev-parse --is-shallow-repository
   git fetch --tags upstream "+refs/heads/${upstream_branch}:refs/remotes/upstream/${upstream_branch}"
   origin_branch_exists=false
   if git remote get-url origin >/dev/null 2>&1; then
     if ! origin_branch=$(git ls-remote --heads origin "refs/heads/${current_branch}"); then
       echo "failed to inspect origin/${current_branch}" >&2
       exit 1
     elif [ -n "$origin_branch" ]; then
       git fetch origin "+refs/heads/${current_branch}:refs/remotes/origin/${current_branch}" || exit 1
       origin_branch_exists=true
     else
       echo "origin/${current_branch} does not exist; current branch is unpublished"
     fi
   fi
   git merge-base HEAD "upstream/${upstream_branch}"

   If the repository is shallow, unshallow origin first when available, then upstream only if still shallow.

   Always use the fetched remote-tracking ref as the target; do not decide from a previously cached upstream/<branch> tip.

5. Record the exact refs and report divergence:

   current_head=$(git rev-parse HEAD)
   upstream_tip=$(git rev-parse "upstream/${upstream_branch}")
   git rev-list --count "upstream/${upstream_branch}..HEAD"
   git rev-list --count "HEAD..upstream/${upstream_branch}"
   GIT_PAGER=cat git log --oneline "HEAD..upstream/${upstream_branch}"
   GIT_PAGER=cat git log --first-parent --oneline "upstream/${upstream_branch}..HEAD"

   Report HEAD and upstream/${upstream_branch} with their full SHAs.

6. Merge:

   • Behind 0 means the fetched upstream_tip is already an ancestor of current_head. Confirm that state with both checks:

     git merge-base --is-ancestor "$upstream_tip" "$current_head"
     upstream_range=$(git rev-list "$current_head..$upstream_tip") || exit 1
     test -z "$upstream_range"

     This completes upstream integration as a successful no-op. Report the exact refs, SHAs, ancestry result, and empty range; skip the merge, release, and other change-dependent gates. Do not create an empty commit or pull request, or publish a branch solely to represent the sync. If an independent request explicitly approves pushing existing local commits, use step 9's non-destructive push semantics.

   • If behind is greater than 0 and --ff-allow is set with ahead 0, run:

     git merge --ff-only "upstream/${upstream_branch}"

   • Otherwise capture previous_head and upstream_tip, then run:

     git merge --no-ff "upstream/${upstream_branch}" -m "merge: sync ${current_branch} with upstream/${upstream_branch}"

7. Resolve conflicts locally when they occur:

   git diff --name-only --diff-filter=U

   Read each conflicted file. Auto-resolve only mechanically obvious conflicts such as non-overlapping additions, import unions, formatting-only differences, or generated lockfile refreshes. For semantic conflicts, present the specific conflict and ask whether to keep ours, keep theirs, manually edit, or abort. After resolution:

   git add <file>
   git -c core.editor=true merge --continue

8. Verify:

   git rev-parse --git-path MERGE_HEAD
   git rev-parse --git-path rebase-merge
   git rev-parse --git-path rebase-apply
   git show -s --format=%P HEAD
   git rev-list --left-right --count "upstream/${upstream_branch}...HEAD"
   if [ "$origin_branch_exists" = true ]; then
     git merge-base --is-ancestor "origin/${current_branch}" HEAD
   fi

   In default mode, verify HEAD has two parents: previous_head as first parent and upstream_tip as second parent. In --ff-allow fast-forward mode, verify HEAD equals upstream_tip.

9. Push only with explicit approval:

   GIT_PAGER=cat git log --oneline --graph --decorate -10
   git push origin "${current_branch}"

   If the remote branch does not exist, use git push -u origin "${current_branch}". If push is rejected as non-fast-forward, re-fetch and offer only non-destructive options: merge origin/<branch> into HEAD and retry, or stop.