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

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(a)redhat.com&gt; --- 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? */