Don't rollback uncommittable indices on become_follower

[cjen1-msft]

Closes #7618

The hypothesis is that a backup had previously acked a regular entry, but was missing any subsequent signature, was partitioned, and became a pre-vote candidate.
When the partition was removed it received an append-entries message and became a follower again, in doing so truncating the previously acked regular entry.

From this point onwards, the backup would keep nacking the append-entries, but the leader who's match_idx for that replica is now higher than the last index in the ledger would never reduce the sent_idx past the match_idx, and so keep sending messages which could not be ack'd by the backup.

The solution is to remove the rollback of uncommittable entries during become_follower.
This leaves the recv_append_entries function fixup path as the only place backups truncate entries, and only until a non-fixup append-entries is received.

Additionally I've moved the last_ack_timeout to after the checks for a valid append_entries_response, as if a leader is unhealthy it should step-down via check-quorum, rather than keeping on being a faulty leader.

Finally I cleaned up a line number in the documentation, and made the invalid view check more explicit.

[Copilot]

Pull request overview

This PR fixes issue #7618 where a backup node could get into a perpetual disagreement with the leader after a network partition. The issue occurred when a backup node acked an entry, became a pre-vote candidate during a partition, and then truncated its log when becoming a follower again. This left the leader's match_idx higher than the backup's actual ledger, causing an infinite nack loop.

Changes:

• Removed rollback of uncommittable entries from become_follower() to prevent premature log truncation
• Moved last_ack_timeout reset to occur only after successful append-entries validation
• Improved code clarity by using ccf::VIEW_UNKNOWN constant instead of literal 0

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/raft_scenarios/follower_rollback_match_index	New test scenario that reproduces the partition scenario from issue #7618
src/consensus/aft/raft.h	Core fix: removed rollback from become_follower, moved last_ack_timeout reset, and improved code clarity with VIEW_UNKNOWN constant

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated no new comments.