Combining Git and Mercurial using a custom login shell

scm.sh

#!/bin/sh
# This script is part of the Uplink project. It is used as login-shell for
# SSH users git@ and hg@ in development, allowing for integration of both Git
# and Mercurial workflows, and for the user to push into any repository that
# is identified by a valid name, irregardless whether it exists already.
basename="`basename \"$0\"`"

# Login shells are meant to be executed with option -c followed by a command
if [ $# -ne 2 -o "x-c" != "x$1" ]; then
  echo "$basename: Invalid or malformed login shell command: $@" >&2
  exit 1
fi

# Environment variable $SSH_ORIGINAL_COMMAND is required by hg-ssh(8) only,
# but made available to the entire process, including any Git triggers:
SSH_ORIGINAL_COMMAND="$2"
export SSH_ORIGINAL_COMMAND

# Expand the parameter list according to the given command:
set -e -- $2

# Determine which SCM the client is using:
case "$USER" in

  *hg*|*mercurial*) # https://www.mercurial-scm.org/wiki/SharedSSH#hg-ssh

    if [ $# -eq 5 -a "# $1 $2 ... $4 $5" = "# hg -R ... serve --stdio" ]; then
      backend="hg-ssh"
      convert="push"
      repository="$3"
      set -- "$repository"
    else
      echo "$basename: Malformed or unrecognized Mercurial command: $@" >&2
      exit 1
    fi;;

  *git*) # https://git-scm.com/docs/git-shell

    if [ $# -eq 2 -a "# ${1%%-*}-command ..." = "# git-command ..." ]; then
      backend="git-shell"
      convert="pull"
      repository=${2%"'"}; repository=${repository#"'"}
      set -- -c "$1 '$repository/.git'"
    else
      echo "$basename: Malformed or unrecognized Git command: $@" >&2
      exit 1
    fi;;

  *)  echo "$basename: Try Git or Mercurial instead of: $@" >&2
      exit 1
    ;;

esac

# Ensure the repository name is neither empty nor contains any path fragments:
case "$repository" in */*|*.*|'')
  echo "$basename: Invalid repository name: '$repository'" >&2
  exit 1;;
esac

# The directory associated with the repository hosts not only the Mercurial
# and Git resources, but also the lock file for remote operations, hence it
# must exist even before the repositories are crated on-demand (see below)
mkdir -p "$repository"

# All interaction with the repository is done in transaction scope: After
# aquiring a lock on the repository, execution is delegated to either the
# Git or Mercurial handler, before finally synchronizing the repositories
# and releasing the lock:
( if ! flock -n 9; then
    echo "$basename: Encountered lock on \"$repository\"" >&2
    exit 1
  fi

  # Ensure the Mercurial repository has been initialized
  test -d "$repository/.hg" || hg init -q "$repository" >&2
  # Ensure the Git repository has been initialized
  test -d "$repository/.git" || git init -q --bare "$repository/.git" >&2

  # Invoke the "original" SCM shell handler:
  $backend "$@"

  # Perform the subsequent synchronization via hg-git pull or push:
  hg -R "$repository" $convert -q --force "$repository/.git" >&2

) 9>"$repository/.lock"