fusefs: avoid cache corruption with buggy fuse servers

[asomers]

Summary

fusefs: avoid cache corruption with buggy fuse servers

The FUSE protocol allows the client (kernel) to cache a file's size, if the
server (userspace daemon) allows it. A well-behaved daemon obviously should
not change a file's size while a client has it cached. But a buggy daemon
might. If the kernel ever detects that that has happened, then it should
invalidate the entire cache for that file. Previously, we would not only
cache stale data, but in the case of a file extension while we had the size
cached, we accidentally extended the cache with zeros.

Reported by: Ben RUBSON <[e-mail]>

Test Plan

Tests added

[cem]

I'm not familiar with GoogleTest/C++ and didn't look at that bit.

sys/fs/fuse/fuse_internal.c

914

I would probably be even more explicit "this fuse fileserver is buggy and must be mounted with -o direct_io" or something like that. We're very much papering over possible corruption here and it is easy for it to go undetected if a file is not extended or truncated.

sys/fs/fuse/fuse_node.h

137–138

style(9) static inline bool on its own line.

[asomers]

Make a warning message scarier, and fix a style nit

[asomers]

Thanks for such a speedy review!