tree 69890462bdc195307ff46c07c80d48108f76c7b3
parent 9af36d7844501206e1480eebc4c9a80e875c52b3
author Neels Hofmeyr <neels@hofmeyr.de> 1516321415 +0100
committer Neels Hofmeyr <neels@hofmeyr.de> 1516374195 +0100

HO: fix recovery from failed handover

Do not instruct the MGW to move the RTP to the new lchan before we have
received a HANDOVER DETECT.

Before:

  Chan Activ
  Chan Activ Ack
  IPACC-CRCX
   -ACK
  IPACC-MDCX
   -ACK
  MGCP MDCX --> MGW
  ...
  HANDOVER DETECT
  Call continues on new lchan

In above sequence, if the HANDOVER DETECT times out, the MGW has moved to the
new lchan which never becomes used and is released. Furthermore, from the IPACC
MDCX until the HANDOVER DETECT, the RTP stream would break off momentarily.

After:

  Chan Activ
  Chan Activ Ack
  IPACC-CRCX
   -ACK
  IPACC-MDCX
   -ACK
  ...
  HANDOVER DETECT
  MGCP MDCX --> MGW
  Call continues on new lchan

If the HANDOVER DETECT times out, the call happily continues on the old lchan.

This change is inspired by Ivan Kluchnikov's HO work, who implemented a similar
fix in the openbsc.git codebase (branch fairwaves/master-rebase): his patch
moves ipacc_mdcx() to connect RTP to the new lchan from switch_for_handover()
(which triggered on S_ABISIP_CRCX_ACK, i.e. creation of the new lchan) to later
on in ho_detect() a.k.a. the S_LCHAN_HANDOVER_DETECT signal handler:
http://git.osmocom.org/openbsc/commit/?h=fairwaves/master-rebase&id=9507a7a1ea627e07370c9d264816bb190b3b91b8

This patch does essentially the same: remove the mgcp_handover() call from the
MDCX-ACK handling (creation of the new lchan), and add a signal handler for
S_LCHAN_HANDOVER_DETECT to osmo_bsc_mgcp.c to effect the MGW switchover.

Note, it would have been possible to call mgcp_handover() directly from rx of
the HANDOVER DETECT message, but that produces linking fallout in some utils/
projects, which then need to link the mgcp code as well. That is because those
aren't properly separated from the more complex parts of libbsc. Using the
signal is a bit bloaty, but saves the linking hell for now. I've faced a
similar problem twice recently, it would pay off to separate out the simpler
utils/ and ipaccess/ tools so that they don't need to link all of libbsc and
osmo-bsc, at some point (TM).

Change-Id: Iec58c5fcc5697f1775da7ec0111135108ed1fc8f