[Bug 236378] [FUSE] fuse must set FUSE_WRITE_CACHE when writing from the page cache

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236378

            Bug ID: 236378
           Summary: [FUSE] fuse must set FUSE_WRITE_CACHE when writing
                    from the page cache
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: asomers at FreeBSD.org

FUSE operations all record the pid, uid, and gid of the requesting process. 
Many FUSE filesystem daemons use that to authenticate accesses.  Ideally, they
would only do it on FUSE_OPEN.  But may daemons are internally stateless, which
requires them to authenticate operations on every operation, including
FUSE_WRITE.

However, writes that come from the page cache (such as when a fuse-backed file
is mmap()ed or when using the writeback caching strategy introduced in protocol
7.23) cannot set these fields correctly, because multiple processes may have
written to the same file.  For such cases, FUSE_WRITE defines the
FUSE_WRITE_CACHE flag, which indicates to the filesystem daemon that these
fields are not valid.

fuse(4) must set this flag when issueing writes from the cache.

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