https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236340
Bug ID: 236340
Summary: [FUSE] fuse(4) should pass through O_APPEND to the
filesystem daemon
Product: Base System
Version: CURRENT
Hardware: Any
OS: Any
Status: New
Severity: Affects Many People
Priority: ---
Component: kern
Assignee: b...@freebsd.org
Reporter: asom...@freebsd.org
Currently when you open a fuse-backed file with O_APPEND, the kernel hides the
O_APPEND flag from the fuse filesystem and computes write offsets in
fuse_write_directbackend. However, that behavior doesn't work for networked
fuse filesystems. The problem is that two clients both writing to the same
file with O_APPEND wouldn't cooperate to update the file size. Or to put it
another way, implementing O_APPEND in-kernel requires cacheing the file size
attribute, and cacheing attributes is not generally allowed for networked file
systems.
The libfuse documentation makes it explicit that filesystems must handle the
O_APPEND flag as of protocol 7.23. Implicitly, filesystems should handle it on
older protocols as well.
There are two possible approaches we could take:
1) Pass O_APPEND to FUSE_OPEN and set the offset of all FUSE_WRITE operations
to 0
2) Pass O_APPEND to FUSE_OPEN but still compute the offset of all FUSE_WRITE
operations as we do currently, expecting that compliant fuse filesystems will
ignore it.
I don't know how many fuse filesystems correctly handle O_APPEND, but I bet
it's less than all of them.
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