4-way-diff

#!/bin/bash
#
# Perform a 4-way diff between:
#   - local feature/topic branch + whether it is clean (fully committed)
#   - remote feature/topic branch
#   - remote main branch
#   - local main branch
#
# Give the user a good message about which is not in sync
# and what needs to be done to make it so.
#
# pragma to protect from some of my githooks trying to execute this:
#   pre-commit-exec-exempt
#
# Author: Ariel Faigon, 2021
#

# Default name to use for 'origin'
# Some call it 'remote' or 'upstream'- adapt to taste
UPSTREAM=${UPSTREAM:-origin}

# Default names to try (guess) 'main/master' branch name - adapt to taste
MAIN_BRANCH_NAMES=( 'dev' 'main' 'master' )

#
# $TB branch is throwaway, temporary to do the sync/merge work
# in a way that is totally non-destructive to any other branches
# We remove it after the merge with $MB is successful.
# We can call it whatever we want. It should never exist.
#
TB='temp-branch'

function out() {
    echo "$0: $*"
}

function err() {
    out "$@" >&2
}

function die() {
    err "$@"
    exit 1
}

function current-branch() {
    git rev-parse --abbrev-ref HEAD
}

function reflog-last-checkout() {
    local CB
    CB="$(current-branch)"
    git reflog |
        grep -Eo ": moving from .* to $CB\$" |
            perl -ne '
                if (/^: moving from (.*) to (.*)$/) {
                    my ($from, $to) = ($1, $2);
                    # print STDERR "perl: matched from=$from to=$to\n";
                    if ($from ne "'"$TB"'") {
                        print $from, "\n";
                        exit 0;
                    }
                }
            '
}

function local-branch-exists() {
    local branchname="$1"
    git show-ref --verify --quiet "refs/heads/$branchname"
    # $? == 0 means local branch <branchname> exists.
}

function get-main-branch-name() {
    local arg="$1"       # preferred branch (optional)
    local lc
    lc="$(reflog-last-checkout)"

    # $arg is explicitly passed by user, others are guesses
    for branch in "$arg" "${MAIN_BRANCH_NAMES[@]}" "$lc"; do
        if [[ -z "$branch" ]]; then
            continue
        fi
        if local-branch-exists "$branch"; then
            echo "$branch"
            return
        fi
    done
}

function remote-branch-exists() {
    git ls-remote --heads "$UPSTREAM" | grep -q "refs/heads/$1\$"
}

#
# -- main
#
CB="$(current-branch)"
MB="$(get-main-branch-name "$1")"

if git diff --exit-code; then
    echo "(1) $CB branch is clean, cool"
else
    die "(1) $CB branch not clean. Need to commit (or stash) locally"
fi

if git diff --exit-code "$UPSTREAM/$CB" "$CB"; then
    echo "(2) $CB branch: local == remote, no need to push, cool"
else
    die "(2) $CB branch: local != remote. Need to push"
fi

if remote-branch-exists "$CB"; then
    if git diff --exit-code "$UPSTREAM/$MB" "$CB"; then
        echo "(3) local $CB == remote $MB, no need to remote-merge"
    else
        die "(3) local $CB != remote $MB. Need to remote-merge"
    fi
else
    echo "(3) remote $CB doesn't exist (already merged?)"
fi

if git diff --exit-code "$MB"; then
    echo "(4) local $CB == local $MB, no need to pull in $MB, cool"
else
    err "(4) local $CB != local $MB. Need to pull in local $MB"
    printf "To fix:\n\tgit checkout %s && git pull && git checkout %s\n" "$MB" "$CB"
fi