Fallback

[ieQu1]

Implement a mechanism that allows classy to join nodes that don't run classy yet. This mechanism is meant to support rolling upgrades.

[Copilot]

Pull request overview

This PR adds a fallback-based bootstrapping path so a classy-aware node can join a node that does not run classy yet (to support rolling upgrades), and adjusts vote restoration to occur on run-level transition rather than supervisor start.

Changes:

• Introduces exported fallback hook registration APIs (fallback_get_site/1, fallback_get_cluster/1, fallback_get_meta/2, fallback_get_peer_nodes/1) and hookpoint macros.
• Adds a classy_node:hello/1 wrapper that attempts erpc and falls back to hook-derived bootstrap info for non-classy nodes.
• Moves vote restoration to a run-level hook and adjusts supervisor startup ordering accordingly.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/classy.erl	Exposes fallback hook registration APIs and documents fallback behavior.
src/classy_vote.erl	Adds on_run_level/2 to restore votes during run-level transition.
src/classy_sup.erl	Removes restore calls from supervisor start and reorders children to align with new restore timing.
src/classy_node.erl	Implements hello/1 with fallback path for joining non-classy nodes.
src/classy_membership.erl	Adds membership “fallback” sync-data builder used by the join fallback flow.
src/classy_internal.hrl	Adds hookpoint macros for the new fallback hookpoints.
src/classy_hook.erl	Registers the vote-restore run-level hook during hook initialization.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Suppressed comments (2)

src/classy_node.erl:445

• Only checking that PeerNodes is a list can allow non-node elements through; those will later be passed to fallback hooks and to site_info_fallback/1, potentially causing confusing failures inside hooks. Validate that the list contains only node() values (atoms) before proceeding.

        BootstrapNodeInfo = site_info_fallback(Node),
        {ok, Node, Site, Cluster, _SiteMeta} ?= BootstrapNodeInfo,
        {ok, PeerNodes} ?= classy_hook:first_match(?fallback_get_peer_nodes, [Node]),
        true ?= is_list(PeerNodes) orelse {error, {bad_peer_nodes, PeerNodes}},
        PeerInfo = [BootstrapNodeInfo | [site_info_fallback(I) || I <- PeerNodes, I =/= Node]],

src/classy_membership.erl:325

• dump_values/0 is exported but has no -spec, while nearby exported helpers (e.g., dump/0) are typed. Adding a spec helps Dialyzer and clarifies the return shape for tests/debugging.

-doc """
Format values stored in the membership CRDT to a human-readable form for tests and debugging.

Unlike @code{dump/0},
this function doesn't return internal metadata related to synchronization.
""".
dump_values() ->