afr: add arbitration support

Add logic in afr to work in conjunction with the arbiter xlator when a
replica 3 arbiter volume is created. More specifically, this patch:

* Enables full locks for afr data transaction for such volumes.
* Removes the upfront marking of pending xattrs at the time of pre-op
  and defer it to post-op. (This is an arbiter independent change and is made for all afr transactions.)
* After pre-op stage, check if we can proceed with the fop stage without
  ending up in split-brain by examining the changelog xattrs.
* Unwinds the fop with failure if only one source was available at the
   time of pre-op and the fop happened to fail on particular source brick.
* Skips data self-heal if arbiter brick is the only source available.
* Adds the arbiter-count option to the shd graph.

This patch is a part of the arbiter logic implementation for 3 way AFR
details of which can be found at http://review.gluster.org/#/c/9656/

Change-Id: I9603db9d04de5626eb2f4d8d959ef5b46113561d
BUG: 1199985
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/10258
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/xlators/cluster/afr/src/afr-self-heal-data.c b/xlators/cluster/afr/src/afr-self-heal-data.c
index a8a7326e4ec..7567fe9f851 100644
--- a/xlators/cluster/afr/src/afr-self-heal-data.c
+++ b/xlators/cluster/afr/src/afr-self-heal-data.c
@@ -684,6 +684,12 @@ __afr_selfheal_data (call_frame_t *frame, xlator_t *this, fd_t *fd,
 
 		source = ret;
 
+                if (priv->arbiter_count == 1 && source == ARBITER_BRICK_INDEX &&
+                    AFR_COUNT (sources, priv->child_count) == 1) {
+                        did_sh = _gf_false;
+                        goto unlock;
+                }
+
 		ret = __afr_selfheal_truncate_sinks (frame, this, fd, healed_sinks,
 						     locked_replies,
 						     locked_replies[source].poststat.ia_size);