client: fix crash in trim_caps

In a75af4c2, procedure was added to invalidate root's dentries
if the trimming failed to free enough caps.  This would sometimes
crash because root->dir wasn't necessarily open.

Fix by only doing it if root dir is open, though I suspect this
may not be the end of it...

Signed-off-by: John Spray <john.spray@redhat.com>
client: more precise cap trimming

Two fixes:
 * Client would unlink everything it could, instead of just
   meeting its goal, because caps.size() doesn't change until
   dentries are cleaned up later.  Take account of the trimmed
   count in the while() condition to fix that.
 * Don't count the root ino as trimmed, as although it has no
   dentries (of course), we will never give up the cap.

With this change, the client will now precisely achieve the number
of caps requested in CEPH_SESSION_RECALL_STATE messages.

Signed-off-by: John Spray <john.spray@redhat.com>
client: fix potentially invalid read in trim_caps

trim_dentry can potentially free an inode, so get/put
it around the block where we use the inode's dn_set.

Signed-off-by: John Spray <john.spray@redhat.com>
client: failure injection for cap release

Used for simulating a buggy client that trips
the error detection in #9282 (warn clients
which aren't revoking caps)

Signed-off-by: John Spray <john.spray@redhat.com>
client: fix trim_caps for inodes in root

Previously client would fail to release caps for files
in the root directory in response to CEPH_SESSION_RECALL_STATE
messages.

Signed-off-by: John Spray <john.spray@redhat.com>
xlist: implement const_iterator

Signed-off-by: John Spray <john.spray@redhat.com>
mds: add state for tracking RECALL progress

To be used later for generating health metrics
for clients which are failing to promptly service
CEPH_SESSION_RECALL_STATE messages.

Signed-off-by: John Spray <john.spray@redhat.com>
mds: add a health metric for failure to recall caps

Fixes: #9284

Signed-off-by: John Spray <john.spray@redhat.com>
mon: trigger transaction on MDS health changes

I think this was previously only working as a side effect
of other MDS map changes.

Signed-off-by: John Spray <john.spray@redhat.com>
mds: health metric for late releasing caps

Follow up on Yan Zheng's "mds: warn clients which
aren't revoking cap" to include a health metric
for this condition as well as the clog messages.

Signed-off-by: John Spray <john.spray@redhat.com>
xlist: implement copy constructor

...so that I can have a std::map of them.

Signed-off-by: John Spray <john.spray@redhat.com>
mds: keep per-client revoking caps list

...to avoid doing an O(caps) scan to find out
which clients are responsible for any late-revoking
caps during health checks.

Signed-off-by: John Spray <john.spray@redhat.com>
mds: limit number of caps inspected in caps_tick

This is to avoid hitting an O(caps) loop in the worst
cast scenario.  This mechanism is a little crude but
should be superceded at some point by admin socket
functionality to inspect session caps so that we
don't need to spit out this level of detail in logs.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2492 from ceph/wip-9284

#9284 - fix client RECALL handling and add health metrics

Reviewed-by: Greg Farnum <greg@inktank.com>