Make capabilities configurable in init()

[cberner]

• FUSE_ASYNC_READ
• FUSE_POSIX_LOCKS
• FUSE_FILE_OPS
• FUSE_ATOMIC_O_TRUNC
• FUSE_EXPORT_SUPPORT
• FUSE_BIG_WRITES
• FUSE_DONT_MASK
• FUSE_SPLICE_WRITE
• FUSE_SPLICE_MOVE
• FUSE_SPLICE_READ
• FUSE_FLOCK_LOCKS
• FUSE_HAS_IOCTL_DIR
• FUSE_AUTO_INVAL_DATA
• FUSE_DO_READDIRPLUS
• FUSE_READDIRPLUS_AUTO
• FUSE_ASYNC_DIO
• FUSE_WRITEBACK_CACHE
• FUSE_NO_OPEN_SUPPORT
• FUSE_PARALLEL_DIROPS
• FUSE_HANDLE_KILLPRIV
• FUSE_POSIX_ACL
• FUSE_ABORT_ERROR
• FUSE_MAX_PAGES
• FUSE_CACHE_SYMLINKS
• FUSE_NO_OPENDIR_SUPPORT
• FUSE_EXPLICIT_INVAL_DATA

[wmanley]

Regarding init: I've got some plans to use the typestate pattern to simplify this. The idea is that Init is then no longer a callback, but is a function instead. That way the filesystem callback no longer needs to contain state like is_initialised or do any synchronisation around that. So we'd have a type like:

pub struct ChannelNeedsInit{
    chan: Channel,
    init_msg: op::Init
};
impl ChannelNeedsInit {
    fn init(self, capabilities: u32, ...) -> Channel {
        ...
    }
}

Usage:

let (mount, chan_needs_init) = create_mount("/my/mountpoint");
if chan_needs_init.abi_version() > (7, 19) {
    // Blah blah
}
let chan = chan_needs_init.init(my_capabilities, abi_version, ...);
serve_sync(chan, handler);

I've not yet started implementation of this though. I guess it would serve some of the same purpose as Session, but it would not be parametrized on the Filesystem/Handler type, nor own the Mount.