Replication slots for cascading replication.

Hi,

I encountered an issue this weeked and was wondering if there is a simple
solution to avoid this specific problem.

A simplified database setup here i three servers, which uses streaming
replication with replication slot on pg10.1
M: a "master" server
C: a hot standby that cascades to ...
S: a slave.

So data data flow is
M -> C -> S.

There is a replication slot for C on M - and a slot for S on C.

S was taken down for mainenance - and during this window C suffered a
hardware failure which redered the machine dead.

Switching S to restore from M instead of C lead to missing WAL files, since
the lsn for the C slot on M had already passed to lsn on S.

So - is there a way to pass the cascaded info upstream? E.g. is there a way
to non-manually make sure the WAL files on M are present for both C and S?

(Hopefully this understandable)