FUSE passthrough after opening?

[Josh Triplett]

I'm really happy to see the FUSE passthrough support merged!

I have a use case for which it'd be nice to start FUSE passthrough
some time after the initial open: I have a network-backed filesystem,
and in some cases I'd like to serve an initial request before retrieving
the full file. For instance, for a library .so file, I'd love to service
the initial 832-byte read while still downloading the full file.

Would it be possible to add support for transitioning an open FUSE file
to being backed by passthrough *after* the initial open? Could that work
with the current passthrough architecture? (I realize that this might be
more complex, as the initial open is a good time to directly substitute
a file from the underlying filesystem.)

Thanks,
Josh Triplett