sys/socket: accept interface index in Ipv6MembershipRequest::new
#2688
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`Ipv6MembershipRequest::new` hardcoded `ipv6mr_interface` to 0, which maps to the “default” multicast interface. That works for some scopes, but for link-local groups (ff02::/16) and many practical deployments an explicit ifindex is required. Because the wrapper’s inner `ipv6_mreq` is private, callers had no way to override the interface. This change updates `new()` to take an additional `c_uint` argument for the interface index and forwards it to `ipv6mr_interface`. Passing 0 preserves the previous behavior.
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What does this PR do
Summary
This PR changes Ipv6MembershipRequest::new so that it accepts an interface index (c_uint) in addition to the multicast group address. The interface index is then passed to ipv6mr_interface instead of being hardcoded to 0.
Passing 0 preserves the previous behavior (use the default multicast interface). Supplying a non-zero ifindex allows explicit interface selection.
Motivation
IPv6 multicast membership is maintained per interface. Hardcoding ipv6mr_interface = 0 works for some scopes, but it prevents joining link-local groups (e.g. ff02::/16) or joining on a specific interface. Because the wrapper hides the underlying ipv6_mreq fields, there was no way for users to set the ifindex themselves.
Allowing the caller to pass an interface index makes the API consistent with the semantics of ipv6_mreq on Linux and *BSD, and covers real-world cases where explicit interface selection is required.
Alternatives
Add a separate constructor, e.g. with_ifindex(group, ifindex), and keep new(group) as-is (non-breaking).
If maintainers prefer a non-breaking approach, I can submit a separate PR adding with_ifindex instead of modifying new.
Checklist:
CONTRIBUTING.md