#!/bin/bash
#
# Checkout a branch in a submodule whose latest commit matches the commit id
# recorded in the parent repository submodule config.
#
# Intended to run only in development environments. In production, always use
# "git submodule update" only.
#
# There are a git-submodule feature to help tracking subdmoule branches:
#
#   Git submodules: Specify a branch/tag - Stack Overflow
#   https://stackoverflow.com/questions/1777854/git-submodules-specify-a-branch-tag
#
# But note that using "git submodule add -b" coupled with "git submodule update --remote"
# does not leverage the additional security of using the commit ID of the submodule's
# commit recoreded in the parent commit.
#
# Given that the benefits of using git-submodule is both tracking sub-repository state
# and ensuring a basic integrity check on it's contents, the following implementation
# is different from the sollution given by the article above by ensuring we only checkout
# to the branch if it's latest commit is the one having the revision recorded by the parent
# repository.

# Parameters
BASENAME="`basename $0`"

# Checkout the branch containing a commit
function checkout_branch {
  # Ensure we have the right ssh command
  GIT_SSH_COMMAND="`git config core.sshCommand`"

  # Fetch from all repositories
  if [ ! -z "$GIT_SSH_COMMAND" ]; then
    GIT_SSH_COMMAND="$GIT_SSH_COMMAND" git fetch --all
  else
    git fetch --all
  fi

  # Check if we are in a detached HEAD
  if git branch | grep -q '* (HEAD detached'; then
    # Determine the commit we're in
    local commit="`git log -n 1 | head -1 | cut -d ' ' -f 2`"

    # Get all branches were the commit occurs
    local branches="`git branch -r --contains $commit 2> /dev/null | grep -v 'HEAD'`"
   
    # Get the first branch whose last commit is our commit
    if [ ! -z "$branches" ]; then
      for branch in $branches; do
        branch_commit="`git log $branch -1 | head -1 | cut -d ' ' -f 2`"

        # In the future some criteria might be stablished to determine how to decide
        # if the comment is present in more than one branch. Which one to prioritize?
        #
        # - A branch recorded in `config -f $toplevel/.gitmodules submodule.$name.branch`?
        # - A topic branch in the form of "feature/"?
        # - The "develop" branch?
        #
        # This whole business is getting too complicated!
        if [ "$commit" == "$branch_commit" ]; then
          # Remove an eventual remote name from branch name
          local_branch="`echo $branch | sed -e 's|^[^/]*/||'`"

          # Get the commit of the local branch for the case the matching branch is a remote one
          if git branch | grep -q " $local_branch$"; then
            local_commit="`git log $local_branch -1 | head -1 | cut -d ' ' -f 2`"
          else
            # Branch does not exist
            local_commit="$branch_commit"
          fi

          # Checkout to the given commit
          #
          # Note that there's space for a race condition here during this
          # checkout and the merge from the next statement block.
          #
          # So be careful and use this script just in development.
          git checkout $local_branch

          # Update the local branch if needed
          if [ "$branch_commit" != "$local_commit" ]; then
            git merge $branch
          fi

          # Done
          break
        fi
      done
    else
      echo "$BASENAME: no such branch containing $commit as it's latest commit"
    fi
  fi
}

# Dispatch
checkout_branch