worktree

#!/usr/bin/env bash
set -euo pipefail

usage() {
    cat <<'EOF'
Usage: ./worktree --check-main | --update-main

Checks whether this checkout contains the latest origin/main, or safely
fast-forwards a checkout that is directly behind it.

Options:
  --check-main   Fetch origin/main and warn when HEAD does not contain it.
                 Never changes HEAD or the working tree; a fetch failure warns
                 without blocking worktree setup.
  --update-main  Fetch origin/main and fast-forward only when HEAD is its
                 ancestor. Refuses divergent history.
  -h, --help     Show this help.
EOF
}

if [ "$#" -ne 1 ]; then
    usage >&2
    exit 1
fi

mode="$1"
main_ref="refs/remotes/origin/main"

fetch_main() {
    git fetch --quiet origin '+refs/heads/main:refs/remotes/origin/main'
}

short_commit() {
    git rev-parse --short=8 "$1"
}

case "$mode" in
    --check-main)
        if ! fetch_main; then
            echo "warning: could not fetch origin/main; main freshness is unknown" >&2
            exit 0
        fi

        if git merge-base --is-ancestor "$main_ref" HEAD; then
            echo "==> Checkout contains latest origin/main ($(short_commit "$main_ref"))"
        else
            echo "warning: checkout $(short_commit HEAD) does not contain latest origin/main ($(short_commit "$main_ref"))" >&2
            echo "warning: run the 'Update to latest main' action if this checkout should be a current main checkout" >&2
        fi
        ;;
    --update-main)
        echo "==> Fetching origin/main"
        fetch_main

        if git merge-base --is-ancestor "$main_ref" HEAD; then
            echo "==> Checkout already contains latest origin/main ($(short_commit "$main_ref"))"
            exit 0
        fi

        if ! git merge-base --is-ancestor HEAD "$main_ref"; then
            echo "error: checkout $(short_commit HEAD) has diverged from origin/main ($(short_commit "$main_ref")); refusing to rewrite or merge feature history" >&2
            echo "error: merge or rebase origin/main explicitly instead" >&2
            exit 1
        fi

        git merge --ff-only "$main_ref"
        echo "==> Fast-forwarded checkout to latest origin/main ($(short_commit HEAD))"
        ;;
    -h|--help)
        usage
        ;;
    *)
        echo "error: unknown option '$mode'" >&2
        usage >&2
        exit 1
        ;;
esac