How to detect a forced update

Question

When a branch history is changed on the remote, you typically get

o git@git.server.com:XXXXX/Project.git
 + efe2e8b...cda0ee7 HEAD -> Ant_Config_processin         


        

Answer 1

You can use git-merge-base command, it finds nearest common ancestor for two commits.

For fast-forward update, common ancestor for oldrev and newrev must point to oldrev. Sample code to be put into pre-receive hook to block non fast-forward:

mergebase=`git merge-base $oldrev $newrev`
if [ "$oldrev" != "$mergebase" ]; then
  echo "Non fast-forward update not allowed for $refname, from ${oldrev:0:16} to ${newrev:0:16} merge base ${mergebase:0:16}"
  exit 1
fi