[libvirt] [PATCH 5/5] virsh: Add snapshot-list --topological

[Eric Blake]

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 <eblake at 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