Re: [Libguestfs] [nbdkit PATCH 3/6] retry: Check next_ops->can_FOO on retry

After a retry, if the second connection has fewer permissions than the first, but we blindly call next_ops->FOO, we end up triggering an assertion failure in backend.c. This is particularly noticeable when the force_readonly flag is in effect, as that makes it much easier for there to be fewer permissions than before. Add testsuite coverage of pwrite to demonstrate. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- filters/retry/retry.c | 58 ++++++++++++++++++++++++++++++++++++++----- 1 file changed, 52 insertions(+), 6 deletions(-)

@@ -307,7 +348,12 @@ retry_cache (struct nbdkit_next_ops *next_ops, void *nxdata, int r; again: - r = next_ops->cache (nxdata, count, offset, flags, err); + if (next_ops->can_cache (nxdata) != 1) {