git-neck: a tool for splitting commits

git-neck

#!/bin/sh
# https://opensource.org/licenses/MIT (c) 2018
# Oren Tirosh <[email protected]>
#
# See and change what your repository looks like from the neck down.
#
# Under git-neck, the HEAD is HEAD^ and worktree is HEAD.
# If you make any changes to the neck, your real HEAD commit is adjusted
# so that its content remains unmodified with the exact same tree hash.
# If you look at it as the difference from its (now modified) parent it
# will appear different.
#
# Examples:
#
# Split a commit:
#   git neck status     # see contents of commits
#   git neck add file   # add specific file for new commit
#   git neck add -p     # add chosen patches to new commit
#   git neck commit
#
# The added changes will form a new commit just above your original
# HEAD. You may wish to git commit --amend to update the description
# of HEAD to reflect whatever was removed from it.

# bash only:
trap 'ERRL2=$ERRL1; ERRL1=" at line $LINENO"' DEBUG 2>/dev/null

set -e
trap 'echo Unexpected error$ERRL2' EXIT

exitwith() {
    if test "$2" != ""; then echo "${0##*/}: $2" >&2; fi
    trap EXIT; exit $1
}

fail() {
    exitwith 1 "$*"
}

REL_GITD="$(git rev-parse --git-dir)" || fail
HEAD_HASH=$(git rev-parse --verify HEAD) || fail

git rev-parse --quiet --verify HEAD^1 >/dev/null ||
    fail 'Must have at least two commits (HEAD + "NECK")'

! git rev-parse --quiet --verify HEAD^2 >/dev/null ||
    fail 'HEAD is a merge commit. We have more than one neck.'

GITD="$(cd "$REL_GITD" && pwd)"
test "$REL_GITD" -ef "$GITD"

NECK_ORIG_HEAD=$(git --git-dir=$GITD/neck/.git rev-parse ORIG_HEAD 2>/dev/null || true)

ngit() { git --git-dir="$NECKD/.git" --work-tree="$NECKD/" "$@"; }

if test "$NECK_ORIG_HEAD" != "$HEAD_HASH"; then
    rm -rf "$GITD/neck" "$GITD/neck.tmp"
    NECKD="$GITD/neck.tmp"
    mkdir -p "$NECKD"
    git init --template= -q "$NECKD"
    rm -rf "$NECKD/.git/refs" "$NECKD/.git/objects"
    ln -s "$GITD/objects" "$NECKD/.git/objects"
    ln -s "$GITD/refs"    "$NECKD/.git/refs"
    ngit update-ref --no-deref HEAD $HEAD_HASH
    ngit reset -q HEAD
    # Lightweight worktree: only modified files actually checked out
    ngit ls-files -z |
        ngit update-index --skip-worktree -z --stdin
    ngit diff --diff-filter=AMT --name-only --cached HEAD^ -z |
        ngit update-index --no-skip-worktree -z --stdin
    ngit reset -q --hard HEAD
    ngit reset -q HEAD^
    mv "$GITD/neck.tmp" "$GITD/neck"
fi

NECKD="$GITD/neck"

# Run user provided command in temporary repo:
ngit "${@:-status}" ||
    exitwith $?

NEW_NECK=$(ngit rev-parse HEAD)
ORIG_NECK=$(ngit rev-parse ORIG_HEAD^)
if test "$ORIG_NECK" != "$NEW_NECK"; then
    echo "NECK has been modified, updating HEAD" >&2

    HEADCOMMIT=$(git cat-file commit HEAD)
    TREELINE="${HEADCOMMIT%%parent*}"
    NEWPARENT="parent ${NEW_NECK}"
    REMAINDER="${HEADCOMMIT##*parent ????????????????????????????????????????}"
    NEWHEAD=$(
        echo "$TREELINE$NEWPARENT$REMAINDER" |
            git hash-object -t commit -w --stdin
    )
    git update-ref -m "git-neck modified" HEAD $NEWHEAD
    rm -rf "$NECKD"
fi

exitwith 0