Enable dynamic change of connector cost

[gabordozsa]

Enable dynamic change of the cost of inter-router connector and its existing connections.

Fixes #1857

The major steps of connector cost change are as follows.

Connector side:

1. Mgmt thread calls update on ConnectorEntity
2. Update function (in C) changes the connector cost, then it posts a core action for each existing connections.
3. Core thread updates qdr_connection_t->inter_router_cost, then it posts a general work item to update the peer cost in the local link state.
4. Peer cost in local link state (in Python) is updated, ls_seq is bumped

Listener side:

5. Llistener side receives LSU from connector side with updated cost. It posts a core action to update the cost of the connection.
6. Listener side core thread updates the qdr_connection_t->inter_router_cost, then it posts a general work item to update the peer cost in the local link state.

Notes:

• The same core function (qdr_core_update_connection_cost_CT) handles both steps 3 and 6.
• The remote cost is saved in qdr_connection_t at connection open time. It is used at the connector side to find the maximum cost of connector and listener when the dynamic update happens. This approach works as long as listener cost is immutable.

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Codecov Report

❌ Patch coverage is 6.34921% with 118 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.3%. Comparing base (60cd6ec) to head (39ad457).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1867     +/-   ##
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- Coverage   70.5%   70.3%   -0.3%     
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  Files        209     211      +2     
  Lines      47838   49537   +1699     
  Branches    5156    5302    +146     
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+ Hits       33765   34855   +1090     
- Misses     11608   12185    +577     
- Partials    2465    2497     +32     

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Components	Coverage Δ
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[c-kruse]

Interested to hear what the upgrade experience is in mixed router version networks. I.e. if a connector is updated and the listener is on a current router release what happens/what should happen.

That said, hooked it up with a control plane experiment and it went very smoothly!

[gabordozsa]

Interested to hear what the upgrade experience is in mixed router version networks. I.e. if a connector is updated and the listener is on a current router release what happens/what should happen.

The cost of existing inter-router connection will still be updated correctly at the connector side. The connection cost at listener side will not change, resulting asymmetric peer costs at connector and listener sides. That should not be a problem from functional point of view, as far as I understand it.

For new connections, everything would work as expected after connector cost update.

That said, hooked it up with a control plane experiment and it went very smoothly!

Thank you for giving it a try!