sgsn: Integrate the GSUP client into the SGSN

This commit adds GSUP client configuration (via VTY), connection set
up, and real message sending.

The following configuration commands are added:

 - gsup remote-ip A.B.C.D            set server IP address
 - gsup remote-port PORT             set server TCP port

Sponsored-by: On-Waves ehf
openbsc: Ignore generated coverage analysis files

This patch adds *.gcda and *.gcno to .gitignore to skip files that
are generated by gcc --coverage and related tools.

Sponsored-by: On-Waves ehf
sgsn: Replace subscr.authenticate by global require_authentication flag

Currently the flag 'authenticate' is managed per subscriber.

This patch replaces that flag by a global cfg.require_authentication
flag that enables/disables the use of the Auth & Ciph procedure for
every subscriber. The flag is set by the VTY, if and only if the
authorization policy is 'remote'.

The VTY command

  - update-subscriber imsi IMSI insert authenticate <0-1>

is removed.

Sponsored-by: On-Waves ehf
sgsn: Add global require_update_location flag

This flag is used to determine, whether the Update Location procedure
shall be invoked. This is currently only set, when the 'remote'
authorization policy is set. When the flag is set, sgsn_auth_update
will not never be called directly by sgsn_auth_request, if an Attach
Request procedure is pending, even if the remote connection fails for
some reason.

Sponsored-by: On-Waves ehf
sgsn: Log requested state change in sgsn_auth_update

Sponsored-by: On-Waves ehf
sgsn/test: Add wrapper for gprs_subscr_rx_gsup_message

Currently, several lines of boiler plate are needed around a call to
gprs_subscr_rx_gsup_message.

This patch puts all of this (including the call to
gprs_subscr_rx_gsup_message) into a separate function.

Sponsored-by: On-Waves ehf
sgsn/test: Add GMM test for a GSUP based attach procedure

This test calls test_gmm_attach() where the mocked subscriber
functions insert GSUP messages instead of manipulating the
subscriber structure directly.

Sponsored-by: On-Waves ehf
sgsn/test: Extend tests to simulate lost GSUP requests

This patch extends test_gmm_attach to optionally resend Attach
Requests until the SGSN responds to it instead of calling
OSMO_ASSERT at a few places. The test_gmm_attach_subscr_gsup_auth
test optionally uses this feature. It is called once in either mode.

Sponsored-by: On-Waves ehf
sgsn: Add automatic re-connect if the GSUP connection is down

Currently the GSUP connection to a server is not restarted if the
connection cannot be established or is terminated during operation.

This commit adds a timer based connection mechanism, basically
consisting of a timer callback that calls gsup_client_connect. The
timer is eventually triggered (up == 0) or cleared (up != 0) by
gsup_client_updown_cb. It adds calls to osmo_timer_del() to
gsup_client_connect and gprs_gsup_client_destroy. The latter is now
called instead of talloc_free in gprs_gsup_client_create on error to
be on the safe side.

Sponsored-by: On-Waves ehf
sgsn: Avoid sending stale GSUP requests after reconnect

Currently, messages are added to the tx queue even if the connection
is down for some reason and all of these messages are eventually sent
after a re-connect.  The MS has probably sent several Attach Requests
while the connection was down and will continue doing so. Therefore
these stored messages could be dropped.

This patch clears the queue before re-connecting and also extends
gprs_gsup_client_send to return immediately, when the connection is
not established instead of calling ipa_client_conn_send.

Sponsored-by: On-Waves ehf
sgsn: Handle return code of ipa_client_conn_open correctly

The ipa_client_conn_open function does not distinguish between a
connection being already established or waiting for establishment.
In either case, the application gets informed about the connection
state via the updown_cb. The 'up' parameter is only set, if
poll/select consider the socket as writable.

This patch handles both cases equally and fully relies on the
updown_cb to adjust the gsupc obejct state.

Sponsored-by: On-Waves ehf
sgsn: Handle incoming IPA CCM message in gsup_client_read_cb

Currently the IPA CCM messages are not handled by the GSUP client.
This means, that the client doesn't answer to PING and ID_GET and
logs notices when receiving PONG or ID_ACK. At least the PING
functionality (remotely originated PING) shall be supported.

This patch extends gsup_client_read_cb by a call to
ipaccess_bts_handle_ccm. Only when the return code is 0, the message
is processed further and checked for an OSMO/GSUP message. ID_GET
messages are answered by a dummy description, where only the
unit_name is set to 'SGSN'.

Sponsored-by: On-Waves ehf
sgsn: Send PING and eventually reconnect

Currently, the reconnect mechanism relies on gsup_client_updown_cb
which in turn gets called based on the OS' view of connection state.

This patch adds a timer based PING mechanism that regularly sends
PING messages and forces a reconnect if a PONG message won't be
received until the next PING message is scheduled. The current ping
interval is 20s.

Sponsored-by: On-Waves ehf
sgsn: Show GSUP client info on 'show sgsn'

This commit adds a line to the output of 'show sgsn' if the GSUP
client has been initialized:

  - Remote authorization: [not] connected to HOST:PORT via GSUP

Sponsored-by: On-Waves ehf
sgsn: Add logging to connect_timer_cb (TODO)

TODO:
  - LOGLevel ?

Sponsored-by: On-Waves ehf
sgsn: Add PDP info to subscriber data (TODO)

TODO:
  - commit message
  - raw binary representation ???
  - tests

Sponsored-by: On-Waves ehf
sgsn: Do not die in _bssgp_tx_dl_ud on TLLI mismatch (TODO)

Currently an OSMO_ASSERT fails, if the message's and the mmctx's
TLLIs do not match.

This commit turns the assertion into an ERROR log message and uses
the default values for IMSI, DRX, and RA CAP instead of the MM
context values in this case.

TODO:
  - find the real source for the problem
  - add test cases derived from what is being described in the ticket

Ticket: OW#1322
Sponsored-by: On-Waves ehf
sgsn: Added TODOs (TODO)

TODO:
  - fix and remove TODOs
  - add test cases for Detach(reattach)
  - add test cases for PDP context deletion (OW#1311)
  - add test cases for Cancel pending timer in sgsn_mm_ctx_free
  - remove this commit

Sponsored-by: On-Waves ehf