Proposed Changes

In a multi-node cluster after a rolling upgrade from below 4.2 to 4.2
supervisor rabbit_federation_exchange_link_sup_sup crashed because
rabbit_federation_link_sup:start_link had arity 1 until 4.1.x. PR
mirrored supervisor preserves the child definitions which still
include a call with arity 1 (without the link module).

To keep old child specs valid, add back a start_link/1 function in rabbit_federation_link_sup.

Fixes #15239

Run the test with

SECONDARY_DIST=$(PWD)/secondary/rabbitmq_server-4.1.7 make -C  deps/rabbitmq_exchange_federation ct-exchange t=rolling_upgrade:child_id_format

Without the patch the test case rolling_upgrade:child_id_format fails with:

=== Location: [{erpc,call,1366},
              {exchange_SUITE,'-child_id_format/1-fun-5-',675},
              {lists,foreach_1,2310},
              {exchange_SUITE,child_id_format,670},
              {test_server,ts_tc,1794},
              {test_server,run_test_case_eval1,1303},
              {test_server,run_test_case_eval,1235}]
=== === Reason: {exception,
                     {noproc,
                         {gen_server,call,
                             [rabbit_federation_exchange_link_sup_sup,
                              which_children,infinity]}}}

Types of Changes

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• Bug fix (non-breaking change which fixes issue #NNNN)
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