Attempt to create a new file in a snapshot crashes the MDS

["Adam C. Emerson" <aemerson@linuxbox.com>]

I know it's not supported, but I tried it out to see what kind of
error behaviour I would get.

The following were preformed on a simple ceph filesystem (one monitor,
one mds, one osd all on the same machine) running current unstable
with the filesystem mounted via cfuse:

/mnt# mkdir foo
/mnt# cd foo
/mnt/foo# cat > something
These are the contents of the file.
/mnt/foo# cd .snap
/mnt/foo/.snap# mkdir old
/mnt/foo/.snap# cd ..
/mnt/foo# cat > something
These are different contents of the file.
/mnt/foo# cd .snap
/mnt/foo/.snap# ls
old/
/mnt/foo/.snap# cd old 
root@seasalt:/mnt/foo/.snap/old# ls
something
/mnt/foo/.snap/old# cat something 
These are the contents of the file.
/mnt/foo/.snap/old# cat > something
zsh: invalid argument: something
/mnt/foo/.snap/old# cat > somthingelse
These are divergent contents.

This last command hangs, and in the MDS log I find:
mds/CDir.cc: In function 'CDentry* CDir::add_null_dentry(const std::string&, snapid_t, snapid_t)':
mds/CDir.cc:225: FAILED assert(lookup_exact_snap(dname, last) == 0)

Thank you very much.