Fix DeactPDPCtxAcc when UE goes PMM ENABLED but lost its PDP context

Scenario: UE activates a PDP context, then after a while goes PMM IDLE
(Iu conn is destroyed but PDP is kept).
When UE connects through Iu again, it sends eg. RAU or ServiceRequest
with pdp_status bitmask statis the active NSAPIs.
If some NSAPI (PDP context) is enabled at SGSN but doesn't show up in
the bitmask, SGSN will destroy the PDP context with GGSN
(DeletePDPContextReq) towards GGSN prior to re-creating it.
When SGSN receives the DeletePDPContextResp, it would forward a
DeactivatePDPContextReq to the UE for a PDP context which was not known
by the UE anymore, this is wrong.

With this patch, the state of the NSAPI/PDP at the UE side is tracked,
and used to know whether when the PDP gets deleted on the GGSN side then
it needs to also be deleted on the Iu side.

Change-Id: I0ccd9228d71c29248b5f510356dbfdb09565dc30

include/osmocom/sgsn/pdpctx.h

@@ -68,6 +68,7 @@ struct sgsn_pdp_ctx {
 	//uint32_t		qos_profile_neg;
 	uint8_t			radio_prio;
 	//uint32_t		charging_id;
+	bool			ue_pdp_active; /* PDP Context is active for this NSAPI?  */
 
 	struct osmo_timer_list	timer;
 	unsigned int		T;		/* Txxxx number */

src/sgsn/gprs_gmm.c

@@ -1591,6 +1591,7 @@ static void process_ms_ctx_status(struct sgsn_mm_ctx *mmctx,
 		LOGMMCTXP(LOGL_NOTICE, mmctx, "Dropping PDP context for NSAPI=%u "
 			"due to PDP CTX STATUS IE=0x%02x%02x\n",
 			pdp->nsapi, pdp_status[1], pdp_status[0]);
+		pdp->ue_pdp_active = false;
 		if (pdp->ggsn)
 			sgsn_delete_pdp_ctx(pdp);
 		else /* GTP side already detached, freeing */

src/sgsn/sgsn_libgtp.c

@@ -378,6 +378,7 @@ int send_act_pdp_cont_acc(struct sgsn_pdp_ctx *pctx)
 	rc = gsm48_tx_gsm_act_pdp_acc(pctx);
 	if (rc < 0)
 		return rc;
+	pctx->ue_pdp_active = true;
 
 	if (pctx->mm->ran_type == MM_CTX_T_GERAN_Gb) {
 		/* Send SNDCP XID to MS */
@@ -567,9 +568,11 @@ static int delete_pdp_conf(struct pdp_t *pdp, void *cbp, int cause)
 			return -ENOTSUP;
 #endif
 		}
-
-		/* Confirm deactivation of PDP context to MS */
-		rc = gsm48_tx_gsm_deact_pdp_acc(pctx);
+		if (pctx->ue_pdp_active) {
+			/* Confirm deactivation of PDP context to MS */
+			rc = gsm48_tx_gsm_deact_pdp_acc(pctx);
+			pctx->ue_pdp_active = false;
+		}
 	} else {
 		LOGPDPCTXP(LOGL_NOTICE, pctx,
 			   "Not deactivating SNDCP layer since the MM context "