[fuse] writeback cache triggers read() for O_WRONLY files - bug?

[Nikolaus Rath <Nikolaus@rath.org>]

Hello,

When enabling writeback cache for SSHFS, appending to files overwrites
data at a different position (cf. https://github.com/libfuse/sshfs/issues/72).

When trying to track this issue down, I noticed that the libfuse
passthrough_ll example also has problems with appending: calling
fuse_reply_data gives a "Bad File Descriptior" error.

This in turn I traced this down to the fact that when writeback caching
is enabled, and userspace calls open(name, O_WRONLY|O_APPEND) the kernel
passes the O_WRONLY flag on to libfuse. But when userspace then writes
data, the kernel issues a read() request to libfuse (presumably to fill
the write cache) - for a file that has been opened write-only.

In the passthrough_ll example, this then fails because the underlying
file has also been opened O_WRONLY and the strace read then fails.

I am not sure what to make of this. Is this behavior of the kernel
correct? Should libfuse be prepared to accept READ requests for files
that have been opened write-only? Or should the kernel never open files
write-only when writeback caching is enabled?

(I am not sure if something like this is also the cause of the
dataloss problem in SSHFS, but it seems worth addressing in any case)

Thanks,
-Nikolaus

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