Re: [Libguestfs] [nbdkit PATCH 1/3] server: Fix NBDKIT_ZERO_EMULATE from filters

When we turned the result of .can_zero into a tri-state for filters back in v1.16, the intention was that the backend would emulate by calling into .pwrite instead of .zero. The nozero filter already had an implementation of that algorithm, but it needs to live in backend.c to be used by all filters, rather than repeatedly recoded in each filter that wants .zero support by .pwrite emulation. Discovered because the luks filter asked for .pwrite emulation in .can_zero without providing a .zero override, resulting in: $ qemu-img create -f luks --object secret,data=LETMEPASS,id=sec0 -o key-secret=sec0 encrypted.img 100M Formatting 'encrypted.img', fmt=luks size=104857600 key-secret=sec0 $ rm -f data.img $ truncate -s 100M data.img $ nbdkit file encrypted.img --filter=luks passphrase=LETMEPASS --run 'nbdcopy data.img $nbd' nbdkit: backend.c:718: backend_zero: Assertion `c->can_zero > NBDKIT_ZERO_NONE' failed. write at offset 0 failed: Transport endpoint is not connected nbdkit: nbdkit command was killed by signal 6

As a result of moving it into the backend, the nozero filter is simplified, but the corresponding test has to change. Since --filter=log does not alter .can_zero, the emulation actually occurs earlier in the stack (prior to calling into any filter, regardless of whether log is placed before or after nozero, rather than at the point of the nozero filter), so there is now no discernable difference between sock3 and sock4 (we can eliminate the duplicate), and the old sock5a no longer shows a Zero request. [Historically, when the nozero filter was first written, we were relying on qemu to send packets, and the use of the log filter was essential to ensure that we were getting the desired NBD_CMD_WRITE_ZEROES across various versions of qemu; but now that the test uses nbdkit, we know that h.zero() is doing exactly

that, so the reduced length of the testsuite is no longer risky.] Reported-by: Richard W.M. Jones <rjones(a)redhat.com&gt; Fixes: fd361554 ("filters: Cache and change semantics of can_zero", v1.15.1) ---