feat: Delete Progress.PendingSnapshot and improve snapshot handling

[soma00333]

Summary

This PR addresses issue #124 by cleaning up and improving snapshot handling in the Raft library. The main objectives include removing the PendingSnapshot, improving the handling of ReportSnapshot() and MsgSnapStatus, and cleaning up related state transitions to enhance clarity and robustness in the Raft snapshot management logic.

Change

Removal of PendingSnapshot

• Eliminated Progress.PendingSnapshot, which is no longer necessary after changes in #110.
• Simplified state transitions related to snapshot handling.

Enhancements to ReportSnapshot() and MsgSnapStatus

• Updated ReportSnapshot() and MsgSnapStatus to accept the index of the applied snapshot.
• This change allows Raft to handle cases where the snapshot’s applied index differs from the requested index, as seen in complex systems like CockroachDB.
• Logic now directly updates Match and transitions to StateReplicate when applicable.

Additional Improvements

• Improved comments for snapshot-related state handling.
• Added test cases to ensure correctness for scenarios involving differing snapshot indices.

Breaking Changes

Public API Changes

• ReportSnapshot() and MsgSnapStatus now require the applied snapshot’s index as a parameter.
• These changes are not backward-compatible and necessitate updates in all downstream implementations.
• The changes are designed for inclusion in version 4.0.0 to prevent breaking compatibility with existing 3.x users.

References

• Issue: Clean up and improve snapshot handling #124
• PR: Snapshot handling improvements in #110
• Issue: CockroachDB’s snapshot logic #87583

How I tested

$ make test
PASSES="unit" ./scripts/test.sh 
Starting at: Wed Nov 27 22:33:49 JST 2024

'unit' started at Wed Nov 27 22:33:49 JST 2024
ok      go.etcd.io/raft/v3      0.634s
ok      go.etcd.io/raft/v3/confchange   0.626s
ok      go.etcd.io/raft/v3/quorum       (cached)
ok      go.etcd.io/raft/v3/raftpb       (cached)
ok      go.etcd.io/raft/v3/rafttest     1.544s
ok      go.etcd.io/raft/v3/tracker      0.772s
'unit' completed at Wed Nov 27 22:33:52 JST 2024
SUCCESS

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

Hi @soma00333. Thanks for your PR.

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[soma00333]

@ahrtr
Hello, I hope you are doing good.
I handled one of the tickets in the v4.0.0 milestone.
Please take a look after you prepare the branch
Thank you

[serathius]

cc @joshuazh-x
Can you help with review?

[soma00333]

Thank you for checking this
I appreciate your help

[soma00333]

/retest

[k8s-ci-robot]

@soma00333: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest

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