Make new masters inherit replication offsets.

Currently replication offsets could be used into a limited way in order
to understand, out of a set of slaves, what is the one with the most
updated data. For example this comparison is possible of N slaves
were replicating all with the same master.

However the replication offset was not transferred from master to slaves
(that are later promoted as masters) in any way, so for instance if
there were three instances A, B, C, with A master and B and C
replication from A, the following could happen:

C disconnects from A.
B is turned into master.
A is switched to master of B.
B receives some write.

In this context there was no way to compare the offset of A and C,
because B would use its own local master replication offset as
replication offset to initialize the replication with A.

With this commit what happens is that when B is turned into master it
inherits the replication offset from A, making A and C comparable.
In the above case assuming no inconsistencies are created during the
disconnection and failover process, A will show to have a replication
offset greater than C.

Note that this does not mean offsets are always comparable to understand
what is, in a set of instances, since in more complex examples the
replica with the higher replication offset could be partitioned away
when picking the instance to elect as new master. However this in
general improves the ability of a system to try to pick a good replica
to promote to master.

1 files changed, 12 insertions, 1 deletions

diff --git a/src/replication.c b/src/replication.c
index b4bd9fe69..b1d08c774 100644
--- a/src/replication.c
+++ b/src/replication.c
@@ -1242,6 +1242,7 @@ void replicationSetMaster(char *ip, int port) {
     freeReplicationBacklog(); /* Don't allow our chained slaves to PSYNC. */
     cancelReplicationHandshake();
     server.repl_state = REDIS_REPL_CONNECT;
+    server.master_repl_offset = 0;
 }
 
 /* Cancel replication, setting the instance as a master itself. */
@@ -1249,7 +1250,17 @@ void replicationUnsetMaster(void) {
     if (server.masterhost == NULL) return; /* Nothing to do. */
     sdsfree(server.masterhost);
     server.masterhost = NULL;
-    if (server.master) freeClient(server.master);
+    if (server.master) {
+        if (listLength(server.slaves) == 0) {
+            /* If this instance is turned into a master and there are no
+             * slaves, it inherits the replication offset from the master.
+             * Under certain conditions this makes replicas comparable by
+             * replication offset to understand what is the most updated. */
+            server.master_repl_offset = server.master->reploff;
+            freeReplicationBacklog();
+        }
+        freeClient(server.master);
+    }
     replicationDiscardCachedMaster();
     cancelReplicationHandshake();
     server.repl_state = REDIS_REPL_NONE;