[libvirt] [PATCH 1/5] snapshots: Add flag to guarantee topological sort

[Ján Tomko]

On Fri, Mar 08, 2019 at 12:05:08AM -0600, Eric Blake wrote:
>When doing REDEFINE on multiple snapshot metadata XML descriptions, we
>require that a child cannot be redefined before its parent.  Since
>libvirt already tracks a DAG, it is more convenient if we can ensure
>that virDomainListAllSnapshots() and friends have a way to return data
>in an order that we can directly reuse, rather than having to
>post-process the data ourselves to reconstruct the DAG.
>
>Add VIR_DOMAIN_SNAPSHOT_LIST_TOPOLOGICAL as our new guarantee (well, a
>guarantee at the time of the API call conclusion; there's always a
>possible TOCTTOU race where someone redefining snapshots in between
>the API results and the client actually using the list might render
>the list out-of-date). Four listing APIs are directly benefitted by
>the new flag; additionally, since we document that the older racy
>ListNames interfaces should be sized by using the same flags on their
>Num counterparts, the Num interfaces must document when they accept
>(and ignore) the flag.
>

I'd rather deal with that by not introducing the new flag to the old
APIs. The ListAll APIs were introduced back in 2012 and are even older
than the minimum QEMU version we support.

>Signed-off-by: Eric Blake <eblake at redhat.com>
>---
> include/libvirt/libvirt-domain-snapshot.h |  4 +++
> src/libvirt-domain-snapshot.c             | 42 ++++++++++++++++++++++-
> 2 files changed, 45 insertions(+), 1 deletion(-)
>
>diff --git a/include/libvirt/libvirt-domain-snapshot.h b/include/libvirt/libvirt-domain-snapshot.h
>index d9b689abbd..602e5def59 100644
>--- a/include/libvirt/libvirt-domain-snapshot.h
>+++ b/include/libvirt/libvirt-domain-snapshot.h
>@@ -135,6 +135,10 @@ typedef enum {
>     VIR_DOMAIN_SNAPSHOT_LIST_EXTERNAL    = (1 << 9), /* Filter by snapshots
>                                                         that use files external
>                                                         to disk images */
>+
>+    VIR_DOMAIN_SNAPSHOT_LIST_TOPOLOGICAL = (1 << 10), /* Ensure parents occur
>+                                                         before children in
>+                                                         the resulting list */
> } virDomainSnapshotListFlags;
>
> /* Return the number of snapshots for this domain */
>diff --git a/src/libvirt-domain-snapshot.c b/src/libvirt-domain-snapshot.c
>index 11d84289f8..d133c84933 100644
>--- a/src/libvirt-domain-snapshot.c
>+++ b/src/libvirt-domain-snapshot.c
>@@ -1,7 +1,7 @@
> /*
>  * libvirt-domain-snapshot.c: entry points for virDomainSnapshotPtr APIs
>  *
>- * Copyright (C) 2006-2014 Red Hat, Inc.
>+ * Copyright (C) 2006-2019 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

So only these two hunks would be needed:

>@@ -457,6 +469,14 @@ virDomainSnapshotListNames(virDomainPtr domain, char **names, int nameslen,
>  * an array to store those objects.  This API solves the race inherent in
>  * virDomainSnapshotListNames().
>  *
>+ * If @flags contains VIR_DOMAIN_SNAPSHOT_LIST_TOPOLOGICAL and @snaps
>+ * is non-NULL, and no other connection is modifying snapshots, then
>+ * it is guaranteed that for any snapshot in the resulting list, no
>+ * snapshots later in the list can be reached by a sequence of
>+ * virDomainSnapshotGetParent() starting from that snapshot;
>+ * otherwise, the order of snapshots in the resulting list is
>+ * unspecified.
>+ *
>  * By default, this command covers all snapshots; it is also possible to
>  * limit things to just snapshots with no parents, when @flags includes
>  * VIR_DOMAIN_SNAPSHOT_LIST_ROOTS.  Additional filters are provided in

>@@ -697,6 +729,14 @@ virDomainSnapshotListChildrenNames(virDomainSnapshotPtr snapshot,
>  * snapshot, and allocate an array to store those objects.  This API solves
>  * the race inherent in virDomainSnapshotListChildrenNames().
>  *
>+ * If @flags lacks VIR_DOMAIN_SNAPSHOT_LIST_DESCENDANTS or contains
>+ * VIR_DOMAIN_SNAPSHOT_LIST_TOPOLOGICAL, @snaps is non-NULL, and no
>+ * other connection is modifying snapshots, then it is guaranteed that
>+ * for any snapshot in the resulting list, no snapshots later in the
>+ * list can be reached by a sequence of virDomainSnapshotGetParent()
>+ * starting from that snapshot; otherwise, the order of snapshots in
>+ * the resulting list is unspecified.
>+ *
>  * By default, this command covers only direct children; it is also possible
>  * to expand things to cover all descendants, when @flags includes
>  * VIR_DOMAIN_SNAPSHOT_LIST_DESCENDANTS.  Also, some filters are provided in

Reviewed-by: Ján Tomko <jtomko at redhat.com>

Jano
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