Invalidations are triggered mainly by two codepaths - upcall and
write-behind unwinding a cached write with zeroed out stat. For the
case of upcall, following race can happen:
* stat s1 is fetched from brick
* invalidation is detected on brick
* invalidation is propagated to md-cache and cache is invalidated
* s1 updates md-cache with a stale state

For the case of write-behind, imagine following sequence of operations,
* A stat s1 was issued from application thread t1 when size of file
  was s1
* stat s1 completes on brick stack, but yet to reach md-cache
* A write w1 from application thread t2 extends file to size s2 is
  cached in write-behind and response is unwound with zeroed out stat
* md-cache while handling write-cbk, invalidates cache
* md-cache receives response for s1, updates cache with stale stat
  with size of s1 overwriting invalidation state

Fix is to remember when s1 was incident on md-cache and update cache
with results of s1 only if the it was incident after invalidation of
cache.

This patch identified some bugs in regression tests which is tracked
in https://bugzilla.redhat.com/show_bug.cgi?id=1608158. As a stop gap
measure I am marking following tests as bad
        basic/afr/split-brain-resolution.t
        bugs/bug-1368312.t
        bugs/replicate/bug-1238398-split-brain-resolution.t
        bugs/replicate/bug-1417522-block-split-brain-resolution.t
        bugs/replicate/bug-1438255-do-not-mark-self-accusing-xattrs.t

Change-Id: Ia4bb9dd36494944e2d91e9e71a79b5a3974a8c77
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Updates: bz#1512691

During lookup and discover, currently read_subvol is based
only on data_readable. read_subvol should be decided based
on both data_readable and metadata_readable.

Credits to Ravishankar N for the logic of afr_first_up_child
from http://review.gluster.org/10905/ .

Change-Id: I98580b23c278172ee2902be08eeaafb6722e830c
BUG: 1240244
Signed-off-by: Anuradha Talur <atalur@redhat.com>
Reviewed-on: http://review.gluster.org/11551
Reviewed-by: Ravishankar N <ravishankar@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>