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virsh: Add snapshot-list --topological

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For snapshots, 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, especially since the server can give us a
topological sorting with less effort than our naive client
reconstruction.

[1] The XXX comment in virshSnapshotListCollect() about --tree being
O(n^3) is telling; https://en.wikipedia.org/wiki/Topological_sorting
is an interesting resource describing Kahn's algorithm and other
approaches for O(n) topological sorting for anyone motivated to use a
more elegant algorithm than brute force - but that doesn't affect this
patch.

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 code we use for --tree could be repurposed to be such a fallback,
whether or not we keep it naive or improve it to be faster - but
again, no one should spend time on a fallback without evidence that we
need it.)

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 any flags, 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, so --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 is then
possible (but not guaranteed) that adding the --topological flag
changes the --tree output (the client-side --tree algorithm breaks
ties based on alphabetical sorting between two nodes that share the
same parent, while the --topological sort skips the client-side
alphabetical sort and ends up exposing the server's internal order for
siblings, whether that be historical creation order or dependent on a
random hash seed).  But even if the results differ, they will still be
topologically correct.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
This commit is contained in:
Eric Blake
2019-03-07 22:12:01 -06:00
parent e3e8fa1fb4
commit c615c14246
2 changed files with 17 additions and 4 deletions
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14
tools/virsh-snapshot.c
View File

@@ -1,7 +1,7 @@
/*
* virsh-snapshot.c: Commands to manage domain snapshot
*
* Copyright (C) 2005, 2007-2016 Red Hat, Inc.
* Copyright (C) 2005-2019 Red Hat, Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
@@ -1351,8 +1351,9 @@ 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 +1452,10 @@ 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 +1517,9 @@ 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;