Backport of Move sidecar port assignment logic to locked stage of add flow into release/1.12.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #14057 to be assessed for backporting due to the inclusion of the label backport/1.12.

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The below text is copied from the body of the original PR.

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Closes #8254

Description

This relocates automatic sidecar port assignment logic to a locked method immediately before registration, thereby preventing collisions when handling simultaneous requests.

Testing & Reproduction steps

Manual verification:

• Submit many simultaneous requests for registration with sidecar proxy and no port specified: for name in $(shuf -i 1-10000); do curl -d '{"name": "'$name'", "connect": {"sidecar_service": {}}}' -X PUT localhost:8500/v1/agent/service/register &; echo registered $name; done
• See how many duplicate ports have been registered: curl localhost:8500/v1/agent/services | jq -r ' . | to_entries | map(select(.key | match("sidecar";"i"))) | map(.value) | .[] | .Port' | sort | uniq -d
• Before this change: there should be several duplicate ports
• After this change: no duplicate ports

Links

GH Issue Pre-req PR 1 Pre-req PR 2

PR Checklist

• [ ] updated test coverage
• [N/A] external facing docs updated
• [N/A] not a security concern

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Overview of commits

• 3a8999a98dd812c8eeb0591fbb8b98e143d70931

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