ubik: update epoch as soon as sync-site is elected

ubik: update epoch as soon as sync-site is elected

The ubik_epochTime represents the time at which the coordinator first
received its coordinator mandate. However, this global is currently not
updated at the moment when a new sync-site is elected. Instead,
ubik_epochTime is only updated at the very end of the first write
transaction, when a new database label is written (in udisk_commit).
This causes at least 2 different issues:

For one, this means that we change ubik_epochTime while a remote
transaction is in progress. If VOTE_Beacon is called after
ubik_epochTime is updated, but before the remote transaction ends, the
remote sites will detect that the transaction id in ubik_currentTrans is
wrong (via urecovery_CheckTid(), since the epoch doesn't match), and
they will abort the transaction. This means the transaction will fail,
and it may cause a loss of quorum until another election is completed.

Another issue is that ubik_epochTime can be 0 at the beginning of a
write transaction, if this is the first election that this site has won.
Since ubik_epochTime is used to construct transaction ids, this means
that we can have different transactions that originate from different
sites at different times, but they have the same epoch in their tid.
For example, say a write transaction starts with epoch 0, but the
originating site is killed/interrupted before finishing. That write
transaction will linger on remote sites in ubik_currentTrans with an
epoch of 0 (since the originating site will never call
DISK_ReleaseLocks, or DISK_Abort, etc). Normally the sync site will kill
such a lingering transaction via urecovery_CheckTid, but since the epoch
is 0, and the election winner's epoch is also 0, the transaction looks
valid and may never be killed. If that transaction is holding a lock on
the database, this means that the database will forever remain locked,
effectively preventing any access to the db on that site.

To fix both of these issues, update ubik_epochTime with the current
time as soon as we win the election. This ensures that the epoch is not
updated in the middle of a transaction, and it ensures that all
transactions are created with a unique epoch: the epoch of the election
that we won.

Note that with this commit, we do not ever set ubik_epochTime to the
magic value of '2' during database init. The special '2' epoch only
needs to be set in the database itself, and it is never an actual epoch
that represents a real quorum that went through the election process.
The database will be labelled with a 'real' epoch after the first write,
like normal.

[kaduk@mit.edu: comment the locking strategy in ubeacon_Interact()]

Change-Id: I6cdcf5a73c1ea564622bfc8ab7024d9901af4bc8
Reviewed-on: https://gerrit.openafs.org/12609
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>