Fix log divergence bug (#508)

* Fix log divergence bug.

When a follower with uncommitted entries in a prior term when it
was leader receives certain append entry rpc combinations it would
incorrectly increment and evalaute the leader's commit index and
in some cases apply uncommitted entries.

This commit ensures that the commit index is only incremented and
evaluated _if_ the local log's last index is validated against
the leaders's log.

Also handle the case where a snapshot with an index lower than
the follower's last applied index would cause the replication
to this follower to stop.

* reset open segment counter on init

* Update src/ra_server.erl

Co-authored-by: Michael Davis <[email protected]>

* Update src/ra_server.erl

Co-authored-by: Michael Davis <[email protected]>

* Update src/ra_server.erl

Co-authored-by: Michael Davis <[email protected]>

---------

Co-authored-by: Michael Davis <[email protected]>

Author: kjnilsson

src/ra_log_reader.erl

@@ -87,6 +87,7 @@ init(UId, Dir, MaxOpen, AccessPattern, SegRefs0, #{}, Counter)
                       undefined
               end,
     SegRefsRev = lists:reverse(SegRefs),
+    reset_counter(Cfg, ?C_RA_LOG_OPEN_SEGMENTS),
     #?STATE{cfg = Cfg,
             open_segments = ra_flru:new(MaxOpen, FlruHandler),
             range = Range,
@@ -434,6 +435,12 @@ limit(LimitIdx, [{PrevRange, PrevFn} | PrevRem]) ->
             [{NewPrevRange, PrevFn} | PrevRem]
     end.
 
+reset_counter(#cfg{counter = Cnt}, Ix)
+  when Cnt =/= undefined ->
+    counters:put(Cnt, Ix, 0);
+reset_counter(#cfg{counter = undefined}, _) ->
+    ok.
+
 incr_counter(#cfg{counter = Cnt}, Ix, N) when Cnt =/= undefined ->
     counters:add(Cnt, Ix, N);
 incr_counter(#cfg{counter = undefined}, _, _) ->