cluster/dht: Fix races to avoid deletion of linkto
 file

Explanation of Race between rebalance processes:
https://bugzilla.redhat.com/show_bug.cgi?id=1110694#c4

scenario-1:
===========

STATE 1:                          BRICK-1
only one brick                   Cached File
in the system

STATE 2:
Add brick-2                       BRICK-1                BRICK-2

STATE 3:                                       Lookup of File on brick-2
                                               by this node's rebalance
                                               will fail because hashed
                                               file is not created yet.
                                               So dht_lookup_everywhere is
                                               about to get called.

STATE 4:                         As part of lookup
                                 link file at brick-2
                                 will be created.

STATE 5:                         getxattr to check that
                                 cached file belongs to
                                 this node is done

STATE 6:

                                            dht_lookup_everywhere_cbk detects
                                            the link created by rebalance-1.
                                            It will unlink it.

STATE 7:                        getxattr at the link
                                file with "pathinfo" key
                                will be called will fail
                                as the link file is deleted
                                by rebalance on node-2

Fix:
So in the STATE 6, we should avoid the deletion of link file. Every time
dht_lookup_everywhere gets called, lookup will be performed on all the nodes.
So to avoid STATE 6, if linkto file is found, it is not deleted until valid
case is found in dht_lookup_everywhere_done.

Case 1: if linkto file points to cached node, and cached file exists,
        uwind with success.

Case 2: if linkto does not point to current cached node, and cached file
        exists:
        a) Unlink stale link file
        b) Create new link file

Case 3: Only linkto file exists:
        Delete linkto file

Case 4: Only cached file
        Create link file (Handled event without patch)

Case 5: Neither cached nor hashed file is present
        Return with ENOENT (handled even without patch)

Reviewed-on: http://review.gluster.org/8231

******************************************************************************

scenario-2:
===========
cluster/dht: Modified logic of linkto file deletion on non-hashed

Currently whenever dht_lookup_everywhere gets called, if in
dht_lookup_everywhere_cbk, a linkto file is found on non-hashed
subvolume, file is unlinked. But there are cases when this file
is under migration. Under such condition, we should avoid deletion
of file.

When  some other rebalance process changes the layout of parent
such that dst_file (w.r.t. migration) falls on non-hashed node,
then may be lookup could have found it as linkto file but just
before unlink, file  is under migration or already migrated
In such cased unlink can be avoided.

Race:
-------
If we have two bricks (brick-1 and brick-2) with initial file "a"
under BaseDir which is hashed as well as cached on (brick-1).

Assume "a"  hashing gives 44.

                              Brick-1              Brick-2

Initial Setup:               BaseDir/a             BaseDir
                             [1-50]                [51-100]

Now add new-brick Brick-3.

1. Rebalance-1 on node Node-1 (Brick-1 node) will reset
the BaseDir Layout.

2. After that it will perform
a)  Create linkto file on  new-hashed (brick-2)
b)  Perform file migration.

1.Rebalance-1 Fixes the base-layout:
                 Brick-1             Brick-2           Brick-3
                 ---------         ----------         ------------
                 BaseDir/a            BaseDir           BaseDir
                  [1-33]              [34-66]           [67-100]

2. Only a) is     BaseDir/a          BaseDir/a(linkto)   BaseDir
   performed                         Create linktofile

Now rebalance 2 on node-2 jumped in and it will perform
step 1 and 2-a.

After (rebal-2, step-1), it changes the layout of the BaseDir.
                    BaseDir/a     BaseDir/a(link)    BaseDir
                    [67-100]           [1-33]        [34-66]

For  (rebale-2, step-2), It will perform lookup at Brick-3 as w.r.t new
layout 44 falls for brick-3. But lookup will fail.
So  dht_lookup_everywhere gets called.

NOTE: On brick-2 by rebalance-1, a linkto file was created.

Currently that linkto files gets deleted by rebalance-2 lookup as it
is considered as stale linkto file.  But  with patch if rebalance is
already in progress or rebalance is over,  linkto file will not be
unlinked. If rebalance is in progress fd will be  open and if rebalance
is over then linkto file wont be set.

Reviewed-on: http://review.gluster.org/8345

*******************************************************************************

scenario-3:
===========

cluster/dht: Added keys in dht_lookup_everywhere_done

Case where both cached  (C1)  and hashed file are found,
but hash does not point to above cached node (C1), then
dont unlink if either fd-is-open on hashed or
linkto-xattr is not found.

Reviewed-on: http://review.gluster.org/8429
BUG: 1139995
Signed-off-by: Venkatesh Somyajulu <vsomyaju@redhat.com>
Change-Id: I86d0a21d4c0501c45d837101ced4f96d6fedc5b9
Signed-off-by: Venkatesh Somyajulu <vsomyaju@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: susant palai <spalai@redhat.com>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/8674
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>