ReplicatedPG: do not create snapdir on head eviction

Head eviction implies that no clones are present.  Also, add
an exists flag to SnapSetContext in order prevent an ssc from
a recent eviction from preventing a snap read from activating
the promotion machinery.

Fixes: #7858
Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG::do_op: use get_object_context for list-snaps

find_object_context provides some niceties which we don't need since we know
the oid of the clones.  Problematically, it also return ENOENT if the snap
requested happens to have been removed.  Even in such a case, the clone may
well still exist for other snaps.  Rather than modify find_object_context to
avoid this situation for this caller, we'll simply do it inline in do_op.

Fixes: #7858
Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG::finish_promote: add debugging assert for clone_size

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG::find_object_context: check obs.exists on clone obc before checking snaps

Fixes: #7858
Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG::_scrub: account for clone_overlap on each clone

Otherwise, we end up subtracting off clone_overlap for evicted clones
whose sizes we did not add in.

Fixes: #7964
Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: use get_clone_bytes on evict/promote

Fixes: #7964
Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG::make_writeable: check for evicted clone before adjusting for clone_overlap

Fixes: #7964
Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG::trim_object: account evicted prev clone for stats

If the previous clone is evicted, we shouldn't adjust the stats to
account for its new clone_overlap value.

Fixes: #7964
Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: do not evict head while clone is being promoted

Fixes: #8003
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1620 from ceph/wip-8003

Wip 8003

Reviewed-by: Sage Weil <sage@inktank.com>