Re: [Libguestfs] [PATCH nbdkit] xz: Do not pass can_write through to the plugin.

Using ‘nbdkit --filter=xz file disk.xz’ and loop mounting eventually results in this error: nbdkit: file[1]: debug: xz: pwrite count=4096 offset=1048576 flags=0x0 nbdkit: file[1]: debug: pwrite count=4096 offset=1048576 fua=0 nbdkit: file[1]: error: pwrite: Bad file descriptor nbdkit: file[1]: debug: sending error reply: Bad file descriptor Since the filter did not intercept can_write, it was passed through to the layer below (the file plugin). The filter opens the file plugin passing readonly=1, but in the case where there are no filters, nbdkit core would never call can_write() (assuming that because we opened the server readonly, it would return false). However with the filter, can_write is called and returns the default (true). This caused writes to be generated which were also passed through to the plugin which generates "Bad file descriptor" because: EBADF fd is not a valid file descriptor or is not open for writing. This change also disables zero and trim because the server logic disables those when the top backend->can_write returns false.

--- filters/xz/xz.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/filters/xz/xz.c b/filters/xz/xz.c index 28a6a81..5ff791e 100644 --- a/filters/xz/xz.c +++ b/filters/xz/xz.c @@ -173,6 +173,13 @@ xz_get_size (struct nbdkit_next_ops *next_ops, void *nxdata, void *handle) return xzfile_get_size (h->xz); } +static int +xz_can_write (struct nbdkit_next_ops *next_ops, void *nxdata, + void *handle)

+{ + return 0; +} + /* Read data from the file. */ static int xz_pread (struct nbdkit_next_ops *next_ops, void *nxdata, @@ -225,6 +232,7 @@ static struct nbdkit_filter filter = { .close = xz_close, .prepare = xz_prepare, .get_size = xz_get_size, + .can_write = xz_can_write, .pread = xz_pread, };