Re: [Libguestfs] [nbdkit PATCH 3/3] tests: Add regression test for NBDKIT_EMULATE_CACHE fix

On Fri, Jun 10, 2022 at 05:21:19PM +0100, Richard W.M. Jones wrote: > > Series looks good: > > Acked-by: Richard W.M. Jones <rjones(a)redhat.com&gt; > > I think nbdkit-ext2-filter is still wrong (although at least it should > no longer corrupt disk images) because it unnecessarily calls > next->can_zero, so it might be worth dropping those two lines. ext2 does not have an interface (yet?) for emulating fallocate() operations (or other bulk-zeroing operation) against a file; if it _did_ have one, then that's what ext2's .zero should do. But we _do_ want to allow the compressed transmission effects of NBD_CMD_WRITE_ZEROES over the network, so having .can_zero return NBDKIT_ZERO_EMULATE is right. On the other hand, ext2 DOES have a way for the file system itself to request bulk-zeroing of a portion of the underlying disk, so we _do_ call into the plugin's next->zero(), which means we _do_ need to check next->can_zero() up front (see the io.c callback io_zeroout). I don't see any bugs in this area, once this series is in.

> > Regression test for the LUKS case might be useful. But note that the > test should not only check that zeroes work, but also that the result > is still a LUKS image. The reason is that a naive fix for the earlier > problem can (as I discovered) corrupt the disk image, because it will > forward zeroes (eg to offset 0) straight through to the plugin. If > this is too much, then I can write this test case tomorrow. I'll let you handle that one, then. These three patches pushed as: 09a0e4dc..6ced998b