[hardikkaurani]

Summary

This PR fixes a critical iterator invalidation and potential use-after-free defect in raft_server::handle_append_entries_resp when handling append entries responses during server step-down/removal timeout (fixes #653).

Problem Description

In handle_append_entries_resp, peers_.find(resp.get_src()) was executed prior to check_srv_to_leave_timeout():

peer_itor it = peers_.find(resp.get_src());
...
check_srv_to_leave_timeout();
...
ptr<peer> p = it->second;

When a leaving node (srv_to_leave_) exceeds its timeout window during catch-up:

1. check_srv_to_leave_timeout() triggers remove_peer_from_peers(srv_to_leave_).
2. This erases the peer entry from peers_, which immediately invalidates iterator it if the response being handled belongs to the leaving peer.
3. Accessing it->second subsequently results in dereferencing an invalidated iterator, causing segmentation faults or undefined behavior in production environments.

In comparison, other handlers such as handle_hb_timeout (src/handle_timeout.cxx) and handle_join_leave (src/handle_join_leave.cxx) consistently invoke check_srv_to_leave_timeout() before performing peer lookups.

Key Changes

1. Reorder check before iterator lookup: Moved check_srv_to_leave_timeout() to the start of handle_append_entries_resp() before peers_.find(resp.get_src()). If the leaving peer timed out and was removed, peers_.find() safely returns peers_.end(), logging and returning cleanly without crashes.
2. Immediate pointer extraction: Extracted ptr<peer> p = it->second; immediately upon confirming it != peers_.end(), preventing any subsequent dangling iterator accesses.
3. Regression Test: Added remove_srv_timeout_append_entries_resp_test in tests/unit/raft_server_test.cxx and registered it in the test suite to verify late response handling for removed/timed-out servers.

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[hardikkaurani]

Hi @greensky00,

I opened this pull request to resolve the iterator invalidation issue reported in #653 within raft_server::handle_append_entries_resp.

Summary of changes:

1. Lookup Ordering: Moved check_srv_to_leave_timeout() to execute prior to peers_.find(resp.get_src()). When a leaving server times out during catch-up, remove_peer_from_peers() erases it from peers_. Calling this check before lookup ensures peers_.find() returns peers_.end() cleanly and avoids dereferencing an invalidated iterator. This also aligns with the ordering already used in handle_hb_timeout and handle_join_leave.
2. Safe Pointer Extraction: Captured ptr<peer> p = it->second; immediately following iterator validation to guard against dangling iterator references.
3. Regression Test: Added remove_srv_timeout_append_entries_resp_test in tests/unit/raft_server_test.cxx verifying graceful handling when responses arrive after node removal/timeout.

The change is structured across 4 atomic signed-off commits. Whenever convenient, please approve the workflow run and let me know if any adjustments are needed. Thank you!