Re: [libvirt] [PATCH v4 1/5] blockjob: allow omitted arguments to QMP block-commit

We are about to turn on support for active block commit. Although qemu 2.0 was the first version to mostly support it, that version mis-handles 0-length files, and doesn't have anything available for easy probing. But qemu 2.1 fixed bugs, and made life simpler by letting the 'top' argument be optional. Unless someone begs for active commit with qemu 2.0, for now we are just going to enable it only by probing for qemu 2.1 behavior (anyone backporting active commit can also backport the optional argument behavior). Although all our actual uses of block-commit supply arguments for both base and top, we can omit both arguments and use a bogus device string to trigger an interesting behavior in qemu. All QMP commands first do argument validation, failing with GenericError if a mandatory argument is missing. Once that passes, the code in the specific command gets to do further checking, and the qemu developers made sure that if device is the only supplied argument, then the block-commit code will look up the device first, with a failure of DeviceNotFound, before attempting any further argument validation (most other validations fail with GenericError). Thus, the category of error class can reliably be used to decipher whether the top argument was optional, which in turn implies a working active commit. Since we expect our bogus device string to trigger an error either way, the code is written to return a distinct return value without spamming the logs. * src/qemu/qemu_monitor.h (qemuMonitorSupportsActiveCommit): New prototype. * src/qemu/qemu_monitor.c (qemuMonitorSupportsActiveCommit): Implement it. * src/qemu/qemu_monitor_json.h (qemuMonitorJSONBlockCommit): Allow NULL for top and base, for probing purposes. * src/qemu/qemu_monitor_json.c (qemuMonitorJSONBlockCommit): Likewise, implementing the probe. * tests/qemumonitorjsontest.c (mymain): Enable... (testQemuMonitorJSONqemuMonitorSupportsActiveCommit): ...a new test. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- src/qemu/qemu_monitor.c | 12 +++++++++++ src/qemu/qemu_monitor.h | 1 + src/qemu/qemu_monitor_json.c | 29 ++++++++++++++++++++++++--- src/qemu/qemu_monitor_json.h | 3 +-- tests/qemumonitorjsontest.c | 47 ++++++++++++++++++++++++++++++++++++++++++++ 5 files changed, 87 insertions(+), 5 deletions(-)