git-graph-dag

#!/usr/bin/env sh
# # Draw a graphviz diagram of the Git DAG
#
# Labels consist of the short SHA1 and any refs.
# Unreachable commits (ignoring the reflog) will be drawn with dashed lines.
#
# ## Usage
#
# `git graph-dag [<flags>] [<git refs>] (-- [git rev-list args])`
#
# Flag | Description
# ---- | -----------
# -h   | Show this screen.
# -c   | Also output commit message (must be short!).
# -w   | Watch mode (see below).
# --   | Pass any args through to git rev-list.
#
# With no rev arguments, the entire repo is rendered (equivalent to passing
# `-- --reflog --all`), matching what watch mode does on each refresh.
#
# ## Examples
#
#     git graph-dag
#     git graph-dag HEAD~10..
#     git graph-dag -c HEAD~10..
#     git graph-dag HEAD~10.. abcdefa
#     git graph-dag HEAD~10.. abcdefa abcdefb
#     git graph-dag -c -- --reflog --all
#     git graph-dag -w
#
#     # Linux & Windows (WSL) w/ ImageMagick
#     git graph-dag HEAD~10.. | dot -Tpng | display -antialias
#
#     # OS X
#     git graph-dag HEAD~10.. | dot -Tpng | open -a Preview.app -f
#
# ## Watch mode
#
# Install (then remove) a Git hook that will update a PNG image on every ref
# change and start the `display` ImageMagick viewer that will watch that file
# for changes. This is an easy and simple way to view updates to the DAG in
# real-time as they happen (useful for live demos when teaching Git classes).
# E.g.:
#
# Terminal 1:
#
#     git init /path/to/foo
#     cd /path/to/foo
#     git graph-dag -w
#
# Terminal 2:
#
#     cd /path/to/foo
#     touch A; git add A; git commit -m 'Add-A'
#     touch B; git add B; git commit -m 'Add-B'
#     git reset --hard HEAD~1
#     touch C; git add C; git commit -m 'Add-C'
#     git commit --amend -o -m 'Add-C-2'

HOOKSCRIPT='.git/hooks/reference-transaction'

GITDIR="$(git rev-parse --absolute-git-dir)"
test $? -eq 0 || exit 1

IMG="${GITDIR}/graph-dag.png"

run_watch() {
    command -v dot 1>/dev/null || \
        { printf 'Graphviz required.\n' 1>&2; exit 1; }

    if [ -e "$HOOKSCRIPT" ]; then
        printf 'Existing hook found. Not overwriting!\n' 1>&2
        exit 1
    fi

    # shellcheck disable=SC2317
    cleanup() {
        excode=$?
        trap - EXIT
        rm -rf -- "$HOOKSCRIPT" "${IMG}.tmp"
        exit "$excode"
    }
    trap cleanup INT TERM EXIT

    # The reference-transaction hook fires multiple times per ref update
    # (prepared / committed / aborted), so short-circuit on anything but
    # "committed" to avoid redundant rewrites and AppleScript activations.
    # Write to a sibling .tmp and rename into place so watchers never see a
    # half-written file (atomic rename on the same filesystem).
    cat >> "$HOOKSCRIPT" <<-EOS
#!/usr/bin/env sh
[ "\$1" = committed ] || exit 0
git graph-dag -c -- --reflog --all | dot -Tpng > "${IMG}.tmp" && mv "${IMG}.tmp" "$IMG"
EOS
    chmod +x "$HOOKSCRIPT"

    # Look for existing commits:
    if git rev-list --quiet --all -n 1 HEAD 2>/dev/null; then
        git graph-dag -c -- --reflog --all | dot -Tpng > "${IMG}.tmp" && mv "${IMG}.tmp" "$IMG"
    else
        # Make blank 200x200 PNG placeholder.
        cat <<-'EOF' | base64 -d | gzip -d > "$IMG"
H4sICHoQAWQAA2dyYXBoLWRhZy5wbmcA6wzwc+flkuJiYGDg9fRwCQLSJ0CYgw1Iro1YN4+BgbHV
08UxpOLW28uMvAwMHMwKb+ffvMukVnCh0U+NdU6UgAALC6OjA4JC57MIogswovMdGQjwHQQI8cmz
l6BDyLEYwyFk2Ysi0Ngvti1Qxu3VLD1tYLwweLr6uaxzSmgCAISVGHO+AQAA
EOF
    fi

    if uname -s | grep -q Darwin; then
        # Preview only redraws when brought to the foreground, so append an
        # AppleScript call to the hook that activates Preview after each
        # rewrite (then refocuses iTerm). Combined with the atomic .tmp +
        # rename above, this avoids the old "file changed under me" crashes
        # we used to hit. If Preview ever gets flaky again, the previous
        # iteration of this script supported a browser-based viewer (HTML
        # wrapper that polls the PNG with a cache-busting query param) as a
        # fallback. See git history.
        printf 'osascript %s\n' \
            '/Users/michael.sims/.local/bin/update-image.scpt' \
            >> "$HOOKSCRIPT"
        open -W -g -a Preview.app "$IMG"
    else
        command -v display 1>/dev/null || \
            { printf 'ImageMagick required.\n' 1>&2; exit 1; }

        display -update 1 -antialias "$IMG"
    fi
    exit
}

main() {
    while getopts chw opt; do
        case $opt in
        c) show_msg=1;;
        h) show_help=1;;
        w) run_watch;;
        *) show_help=1;;
        esac
    done
    shift $(( OPTIND - 1 ))

    if [ -n "$show_help" ] ; then
        awk 'NR == 1 { next } /^$/ { exit } { print substr($0, 3) }' "$0"
        exit 1
    fi

    # No revs given: render the entire repo, matching watch mode's behavior.
    if [ $# -eq 0 ]; then
        set -- --reflog --all
    fi

    {
        git fsck --unreachable --no-reflogs --no-progress \
            | awk '/commit/ { print $3 }'

        printf 'XXX\n'

        git rev-list --no-commit-header --pretty=format:'%H|%h|%p|%D|%f' "$@" ;
    } |
    awk -v show_msg="$show_msg" '

    BEGIN {
        FS="|"
        print "digraph lattice {"
    }

    /^XXX$/ { revlist = 1; next }

    !revlist { unreachable[$1] = 1 }

    revlist {
        hashfull = $1
        hash = $2
        split($3, parents, " ")
        refs = $4
        msg = $5

        sub(" -> ", " -\\&gt; ", refs)
        unreachable[hashfull] == 1 ? isunr = 1 : isunr = 0

        for (p in parents) {
            printf("x%s -> x%s\n", hash, parents[p])
        }

        printf("x%s [shape=Mrecord, style=%s, label=<%s%s%s>]\n",
            hash, \
            isunr == 1 ? "dashed" : "filled", \
            "<b>" hash "</b>",
            show_msg == 1 ? " <font color=\"gray20\">" msg "</font>" : "", \
            refs == "" ? "" : "<br/><font color=\"gray20\" point-size=\"10.0\">" refs "</font>")
    }

    END { printf("}\n") }
    '
}

main "$@"