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 -- GPG Fingerprint: ED31 791B 2C5C 1613 AF38 8B8A D113 FCAC 3C4E 599F »Time flies like an arrow, fruit flies like a Banana.«
