[Tarantool-patches] [PATCH] limbo: introduce request processing hooks

[Cyrill Gorcunov]

Guys, there are some moments even in current code structure which
looks somehow strange so I would like to discuss them. Lets consider
the following case: one replic sends us a promote request (assume
we're sitting in term 2 and max term is 2).

applier 1
---------
applier_apply_tx
  (promote term = 3
   current max term = 2)
  applier_synchro_filter_tx
  apply_synchro_row
    journal_write
      (sleeping)

at this moment another applier comes in with obsolete
data and term 2
                              applier 2
                              ---------
                              applier_apply_tx
                                (term 2)
                                applier_synchro_filter_tx
                                  txn_limbo_is_replica_outdated -> false
                                journal_write (sleep)

applier 1
---------
journal wakes up
  apply_synchro_row_cb
    set max term to 3
return to applier read and
applier 2 could finish its write
and wake up too

at this moment the data from applier 2 is actually queued
for write as valid but we just wrote the term 3, so if we would
had been updating terms map earlier (before jornal write) the data
from applier 2 should be NOPified. I think there is some problem
because due to journal write lag the data is not as it could be
if terms map updated early. Serge, Vlad, am I right? Which consequences
can be here? IOW, should not we track terms earlier even without
my filter series?