[Xapian-tickets] [Xapian] #783: Remote protocol breaks

[Xapian]

#783: Remote protocol breaks
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        Reporter:  Kronuz          |      Owner:  olly
            Type:  defect          |     Status:  new
        Priority:  highest         |  Milestone:
       Component:  Backend-Remote  |    Version:
        Severity:  blocker         |   Keywords:
      Blocked By:                  |   Blocking:
Operating System:  All             |
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 Every command in the remote database '''must''' wait for a reply from the
 server, ''every single one of them''. Although some commands may look
 harmless at first sight, every command could eventually throw an
 exception. If the remote database doesn't wait for a reply, the exception
 could end up as a reply to some other command, making all further commands
 out of sync.

 A real world example folows:

 Remote database issues MSG_REPLACEDOCUMENT in line 1 doesn't wait for a
 response so some other thread or something els issues a new command
 (MSG_REOPEN, line 2) but for some reason, this time MSG_REPLACEDOCUMENT
 does return an exception!. The MSG_REPOEN in line 2 receives the exception
 and assumes it's was it's own exception, so [my] code retries a MSG_REOPEN
 (line 4); this time it gets a REPLY_DONE, actually the REPLY_DONE which
 was intended for the first MSG_REOPEN (in line 2)... a new command is
 issued MSG_POSTLIST (line 6) and this one receives REPLY_DONE, which was
 intended for the second MSG_REOPEN, hence the error in line 8... hell is
 unleashed now.

 {{{
  1  << MSG_REPLACEDOCUMENT (18)
  2  << MSG_REOPEN (12)
  3  >> REPLY_EXCEPTION (1)
  4  << MSG_REOPEN (12)
  5  >> REPLY_DONE (2)
  6  << MSG_POSTLIST (11)
  7  >> REPLY_DONE (2)
  8     ERROR: Expecting reply type REPLY_POSTLISTSTART (14) or
 REPLY_POSTLISTSTART (14), got REPLY_DONE (2)
  9  << MSG_REOPEN (12)
 10  >> REPLY_POSTLISTSTART (14)
 11     ERROR: Expecting reply type REPLY_UPDATE (0) or REPLY_DONE (2), got
 REPLY_POSTLISTSTART (14)
 12  << MSG_REOPEN (12)
 13  >> REPLY_POSTLISTITEM (15)
 14     ERROR: Expecting reply type REPLY_UPDATE (0) or REPLY_DONE (2), got
 REPLY_POSTLISTITEM (15)
 15  << MSG_REOPEN (12)
 16  >> REPLY_DONE (2)
 17  << MSG_POSTLIST (11)
 18  >> REPLY_DONE (2)
 19     ERROR: Expecting reply type REPLY_POSTLISTSTART (14) or
 REPLY_POSTLISTSTART (14), got REPLY_DONE (2)
 20  << MSG_REOPEN (12)
 21  >> REPLY_DONE (2)
 22  << MSG_POSTLIST (11)
 23  >> REPLY_DONE (2)
 }}}

 Ordering the appropriate sequence of messages, really bad things could
 happen.

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Ticket URL: <https://trac.xapian.org/ticket/783>
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