Re: [libvirt] [RFC PATCH] snapshot: better virsh handling of missing current, parent

Previously, virsh 'snapshot-parent' and 'snapshot-current' were completely silent in the case where the code conclusively proved there was no parent or current snapshot, but differed in exit status; this silence caused some confusion on whether the commands worked. Furthermore, commit d1be48f introduced a regression where snapshot-parent would leak output about an unknown function, but only on the first attempt, when talking to an older server that lacks virDomainSnapshotGetParent. This changes things to consistenly report an error message and exit with status 1 when no snapshot exists, and to avoid leaking unknown function warnings when using fallbacks. * tools/virsh.c (vshGetSnapshotParent): Alter signature, to distinguish between real error and missing parent. Don't pollute last_error on success. (cmdSnapshotParent): Adjust caller. Always output message on failure. (vshGetSnapshotParent): Adjust caller. (cmdSnapshotCurrent): Always output message on failure. --- This patch is an RFC because of backwards-compatibility concerns: Currently, snapshot-current outputs nothing but has status 0 if there is no current snapshot; but snapshot-parent outputs nothing but has status 1 if there is no parent snapshot. Either way, we have an inconsistency that needs to be fixed, and gaining consistency means breaking backwards compatibility with at least one command. Approach 1 (this patch): change snapshot-parent and snapshot-current to always output something, whether to stdout on success or to stderr on failure, with lack of a snapshot being considered a failure (where snapshot-current used to treat it as success). Approach 2 (not written) since snapshot-current existed much longer, makesnapshot-parent consistent by giving status 0 when it is silent. Preferences? (I guess mine is approach 1, by evidence of this patch).