Re: [libvirt PATCH v5 26/32] qemu: try to connect to nbdkit early to detect errors

When using nbdkit to serve a network disk source, the nbdkit process will start and wait for an nbd connection before actually attempting to connect to the (remote) disk location. Because of this, nbdkit will not report an error until after qemu is launched and tries to read from the disk. This results in a fairly user-unfriendly error saying that qemu was unable to start because "Requested export not available". Ideally we'd like to be able to tell the user *why* the export is not available, but this sort of information is only available to nbdkit, not qemu. It could be because the url was incorrect, or because of an authentication failure, or one of many other possibilities. To make this friendlier for users and easier to detect misconfigurations, try to connect to nbdkit immediately after starting nbdkit and before we try to start qemu. This requires adding a dependency on libnbd. If an error occurs when connecting to nbdkit, read back from the nbdkit error log and provide that information in the error report from qemuNbdkitProcessStart(). User-visible change demonstrated below: Previous error: $ virsh start nbdkit-test 2023-01-18 19:47:45.778+0000: 30895: error : virNetClientProgramDispatchError:172 : internal error: process exited while connecting to monitor: 2023-01-18T19:47:45.704658Z qemu-system-x86_64: -blockdev {"driver":"nbd","server":{"type":"unix", "path":"/var/lib/libvirt/qemu/domain-1-nbdkit-test/nbdkit-libvirt-1-storage.socket"}, "node-name":"libvirt-1-storage","auto-read-only":true,"discard":"unmap"}: Requested export not available error: Failed to start domain 'nbdkit-test' error: internal error: process exited while connecting to monitor: 2023-01-18T19:47:45.704658Z qemu-system-x86_64: -blockdev {"driver":"nbd","server":{"type":"unix", "path":"/var/lib/libvirt/qemu/domain-1-nbdkit-test/nbdkit-libvirt-1-storage.socket"}, "node-name":"libvirt-1-storage","auto-read-only":true,"discard":"unmap"}: Requested export not available After this change: $ virsh start nbdkit-test 2023-01-18 19:44:36.242+0000: 30895: error : virNetClientProgramDispatchError:172 : internal error: Failed to connect to nbdkit for 'http://localhost:8888/nonexistent.iso': nbdkit: curl[1]: error: problem doing HEAD request to fetch size of URL [http://localhost:8888/nonexistent.iso]: HTTP response code said error: The requested URL returned error: 404 error: Failed to start domain 'nbdkit-test' error: internal error: Failed to connect to nbdkit for 'http://localhost:8888/nonexistent.iso]: error: problem doing HEAD request to fetch size of URL [http://localhost:8888/nonexistent.iso]: HTTP response code said error: The requested URL returned error: 404 Signed-off-by: Jonathon Jongsma <jjongsma(a)redhat.com&gt; --- meson.build | 7 +++++++ meson_options.txt | 1 + src/qemu/meson.build | 1 + src/qemu/qemu_nbdkit.c | 24 ++++++++++++++++++++++++ 4 files changed, 33 insertions(+)