Confidence: It's likely to be this, but I can't be certain.
Notes: This is actually a chrysalis. It would normally be attatched to the glass by a silk strand around the middle as well as a silk pad on it's tail. It looks to me that the silk strand from the middle has become broken (where the two white dots are on the glass) and so the chrysalis is dangling down. Difficult to tell the size, so it might be a Small White butterfly chrysalis (hence my doubt). But I am almost certain it is a Large White butterfly chrysalis. Definitely not a snail of any sort.