Re: [libvirt] [PATCH v3] qemu: read backing chain names from qemu

https://bugzilla.redhat.com/show_bug.cgi?id=1199182 documents that after a series of disk snapshots into existing destination images, followed by active commits of the top image, it is possible for qemu 2.2 and earlier to end up tracking a different name for the image than what it would have had when opening the chain afresh. That is, when starting with the chain 'a <- b <- c', the name associated with 'b' is how it was spelled in the metadata of 'c', but when starting with 'a', taking two snapshots into 'a <- b <- c', then committing 'c' back into 'b', the name associated with 'b' is now the name used when taking the first snapshot. Sadly, older qemu doesn't know how to treat different spellings of the same filename as identical files (it uses strcmp() instead of checking for the same inode), which means libvirt's attempt to commit an image using solely the names learned from qcow2 metadata fails with a cryptic: error: internal error: unable to execute QEMU command 'block-commit': Top image file /tmp/images/c/../b/b not found even though the file exists. Trying to teach libvirt the rules on which name qemu will expect is not worth the effort (besides, we'd have to remember it across libvirtd restarts, and track whether a file was opened via metadata or via snapshot creation for a given qemu process); it is easier to just always directly ask qemu what string it expects to see in the first place. As a safety valve, we validate that any name returned by qemu still maps to the same local file as we have tracked it, so that a compromised qemu cannot accidentally cause us to act on an incorrect file. * src/qemu/qemu_monitor.h (qemuMonitorDiskNameLookup): New prototype. * src/qemu/qemu_monitor_json.h (qemuMonitorJSONDiskNameLookup): Likewise. * src/qemu/qemu_monitor.c (qemuMonitorDiskNameLookup): New function. * src/qemu/qemu_monitor_json.c (qemuMonitorJSONDiskNameLookup) (qemuMonitorJSONDiskNameLookupOne): Likewise. * src/qemu/qemu_driver.c (qemuDomainBlockCommit) (qemuDomainBlockJobImpl): Use it. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- v3: better sanity checks in qemu_monitor_json, rebase and retested atop Peter's fixes for interlocking block jobs src/qemu/qemu_driver.c | 28 ++++++------ src/qemu/qemu_monitor.c | 20 ++++++++- src/qemu/qemu_monitor.h | 8 +++- src/qemu/qemu_monitor_json.c | 102 ++++++++++++++++++++++++++++++++++++++++++- src/qemu/qemu_monitor_json.h | 9 +++- 5 files changed, 149 insertions(+), 18 deletions(-)