Re: (ITS#8444) Out-of-sync issue with memberOf overlay, Delta-syncrepl MMR and >2 nodes

[okuznik@symas.com]

A more self-contained log of the same issue, available at
ftp://ftp.openldap.org/incoming/its8444.log

(line numbers below are against current master, commit
91f4d3a6b75e73bf4ea498e83e2e4cb4e7a320e0)

There are some things that occur in all the failures I have seen so far:
- the server that received the operation (#1) sends the accesslog entry
  with no CSN in the cookie, then another provider (#2) picks up this
  message and relays it to its consumers, this one with a CSN in the
  cookie
- a consumer picks up these two in short succession, in the log above,
  processing of the one from #2 is finished first (they are being
  processed concurrently)

Usually, once one of them gets processed, the new CSN should be noted
and the other threads should just skip it (syncrepl.c:943 and onwards).
In this one, having no CSN in the cookie seems to allow both to process
so far as to run syncrepl_message_to_op(), and one of them will then
inevitably fail to add the entry.

I don't understand yet why server #1 sends the operations without a CSN
in the cookie and (especially if I reorder the overlays to set up
memberof last), the race goes the other way around and the operation to
fail is the one from server #2.

My take on it was that in a delta-sync environment all entries would be
passed with a new CSN and that should end up in the cookie, allowing
syncrepl.c:986 to do its job.

--=20
Ond=C5=99ej Kuzn=C3=ADk
Senior Software Engineer
Symas Corporation                       http://www.symas.com
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