Git smart http: parsing commit messages in git-receive-pack

Dear All,

we implemented a java servlet around the git-http-backend. This servlet intercepts the requests sent by the git client when pushing. One thing I want to achieve is parsing the commit messages in the pre push phase (request param service==git-receive-pack) and after checking if the commit messages contain a given string reject/accept the request. The problem is that I couldn't find a way of parsing the commit messages in git-receive-pack (I can do this in the post  push phase but in that case I cannot reject the push because the changes are already on the server). Is there a way of getting the commit messages before they're actually on the server so that I can reject the push?

Thanks in advance, Ákos Tajti