wrong PFCP control flow during HSS Initiated Subscribed QoS Modification

[RoadRunnr]

HSS Initiated Subscribed QoS Modification is defined in TS 23.401 clause 5.4.2.2 and updated for CUPS in TS 23.214 clause 6.3.1.7.

The resulting control flow should invoke a PFCP session modification only after the Update Bearer Response (step in 23.401) has been received.

We currently trigger a PFCP usage query on receiving the modification command and trigger another PFCP usage query when getting the Update Bearer Response. Since QoS is currently not supported, updating the bearers in the UPF is not needed, but the first usage query is not needed at all and the second query is only needed when the QoS settings have indeed been changed.

A race between the session termination when the update bearer procedure returns a failure and the first usage query causes random test case failures in pgw_suite.modify_bearer_command_congestion.

Failure is update bearer requests are rare, so this bug has only medium priority.