To some extent, virsh already has a (shockingly naive [1]) client-side topological sorter with the --tree option. But as a series of REDEFINE calls must be presented in topological order, it's worth letting the server do the work for us.
[1] The XXX comment about O(n^3) in virshSnapshotListCollect() is telling; https://en.wikipedia.org/wiki/Topological_sorting is an interesting resource for anyone motivated to use a more elegant algorithm than brute force. For now, I am purposefully NOT implementing virsh fallback code to provide a topological sort when the flag was rejected as unsupported; we can worry about that down the road if users actually demonstrate that they use new virsh but old libvirt to even need the fallback. The test driver makes it easy to test: $ virsh -c test:///default ' snapshot-create-as test a snapshot-create-as test c snapshot-create-as test b snapshot-list test snapshot-list test --topological snapshot-list test --descendants a snapshot-list test --descendants a --topological snapshot-list test --tree snapshot-list test --tree --topological ' Without --topological, virsh does client-side sorting alphabetically, and lists 'b' before 'c' (even though 'c' is the parent of 'b'); with the flag, virsh skips sorting, and you can now see that the server handed back data in a correct ordering. As shown here with a simple linear chain, there isn't any other possible ordering, and --tree mode doesn't seem to care whether --topological is used. But it is possible to compose more complicated DAGs with multiple children to a parent (representing reverting back to a snapshot then creating more snapshots along those divergent execution timelines), where it may become possible to observe non-deterministic behavior when --topological is in use, but even so, the result will still be topologically correct. Signed-off-by: Eric Blake <ebl...@redhat.com> --- tools/virsh-snapshot.c | 16 +++++++++++++--- tools/virsh.pod | 7 ++++++- 2 files changed, 19 insertions(+), 4 deletions(-) diff --git a/tools/virsh-snapshot.c b/tools/virsh-snapshot.c index 025321c58e..31153f5b10 100644 --- a/tools/virsh-snapshot.c +++ b/tools/virsh-snapshot.c @@ -1272,7 +1272,9 @@ virshSnapshotListCollect(vshControl *ctl, virDomainPtr dom, * still in list. We mark known descendants by clearing * snaps[i].parents. Sorry, this is O(n^3) - hope your * hierarchy isn't huge. XXX Is it worth making O(n^2 log n) - * by using qsort and bsearch? */ + * by using qsort and bsearch? Or even a linear topological + * sort such as Kahn's algorithm? Should we emulate + * --topological for older libvirt that lacked the flag? */ if (start_index < 0) { vshError(ctl, _("snapshot %s disappeared from list"), fromname); goto cleanup; @@ -1351,8 +1353,9 @@ virshSnapshotListCollect(vshControl *ctl, virDomainPtr dom, } } } - qsort(snaplist->snaps, snaplist->nsnaps, sizeof(*snaplist->snaps), - virshSnapSorter); + if (!(orig_flags & VIR_DOMAIN_SNAPSHOT_LIST_TOPOLOGICAL)) + qsort(snaplist->snaps, snaplist->nsnaps, sizeof(*snaplist->snaps), + virshSnapSorter); snaplist->nsnaps -= deleted; VIR_STEAL_PTR(ret, snaplist); @@ -1451,6 +1454,10 @@ static const vshCmdOptDef opts_snapshot_list[] = { .type = VSH_OT_BOOL, .help = N_("list snapshot names only") }, + {.name = "topological", + .type = VSH_OT_BOOL, + .help = N_("sort list topologically rather than by name"), + }, {.name = NULL} }; @@ -1512,6 +1519,9 @@ cmdSnapshotList(vshControl *ctl, const vshCmd *cmd) FILTER("external", EXTERNAL); #undef FILTER + if (vshCommandOptBool(cmd, "topological")) + flags |= VIR_DOMAIN_SNAPSHOT_LIST_TOPOLOGICAL; + if (roots) flags |= VIR_DOMAIN_SNAPSHOT_LIST_ROOTS; diff --git a/tools/virsh.pod b/tools/virsh.pod index 5759a396d4..66e2bf24ec 100644 --- a/tools/virsh.pod +++ b/tools/virsh.pod @@ -4726,7 +4726,7 @@ Output basic information about a named <snapshot>, or the current snapshot with I<--current>. =item B<snapshot-list> I<domain> [I<--metadata>] [I<--no-metadata>] -[{I<--parent> | I<--roots> | [{I<--tree> | I<--name>}]}] +[{I<--parent> | I<--roots> | [{I<--tree> | I<--name>}]}] [I<--topological>] [{[I<--from>] B<snapshot> | I<--current>} [I<--descendants>]] [I<--leaves>] [I<--no-leaves>] [I<--inactive>] [I<--active>] [I<--disk-only>] [I<--internal>] [I<--external>] @@ -4734,6 +4734,11 @@ with I<--current>. List all of the available snapshots for the given domain, defaulting to show columns for the snapshot name, creation time, and domain state. +Normally, table form output is sorted by snapshot name; using +I<--topological> instead sorts so that no child is listed before its +ancestors (although there may be more than one possible ordering with +this property). + If I<--parent> is specified, add a column to the output table giving the name of the parent of each snapshot. If I<--roots> is specified, the list will be filtered to just snapshots that have no parents. -- 2.20.1 -- libvir-list mailing list libvir-list@redhat.com https://www.redhat.com/mailman/listinfo/libvir-list
