n_possible variable goes unused in channel_get_for_extend()
$ grep n_possible *.c channel.c: int n_noncanonical = 0, n_possible = 0; channel.c: ++n_possible;
All we do is start it off at 0, and increment it sometimes.
Maybe it had a purpose once, but it doesn't anymore.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information