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feat: CP13-4 — replica state machine / barrier eligibility contract + proof
Contract review: 6-state set (Disconnected, Connecting, CatchingUp, InSync, Degraded, NeedsRebuild). Only InSync proceeds to barrier request path. All other states either fail immediately or attempt reconnect (must succeed before reaching barrier). New test: TestBarrier_NonEligibleStates_FailClosed — systematically verifies each non-eligible state (Connecting, CatchingUp, NeedsRebuild, Disconnected) is rejected by Barrier(), and InSync is the only state that enters the barrier request path. 5 baseline tests promoted to CP13-4 primary proof. No production code changed — contract review + new focused test only. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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pingqiu
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Claude Opus 4.6
Claude Opus 4.6
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# CP13-4 Replica State Machine / Barrier Eligibility — Contract Review + Proof Package
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Date: 2026-04-03
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Code change: one new test (`TestBarrier_NonEligibleStates_FailClosed`)
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## Replica State Set
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The replication path uses a bounded 6-state set (`wal_shipper.go:25-30`):
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| State | Value | Meaning | Barrier eligible? |
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|-------|-------|---------|-------------------|
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| `Disconnected` | 0 | No session (initial state) | No — attempts bootstrap/reconnect, fails if no progress |
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| `Connecting` | 1 | Socket open, handshake pending | No — immediate `ErrReplicaDegraded` |
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| `CatchingUp` | 2 | Connected, replaying missed WAL | No — immediate `ErrReplicaDegraded` |
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| `InSync` | 3 | Eligible for sync_all barriers | **Yes** — only state that proceeds to barrier request |
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| `Degraded` | 4 | Transient failure, retry allowed | No — attempts reconnect, fails if reconnect fails |
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| `NeedsRebuild` | 5 | WAL gap too large, rebuild required | No — immediate `ErrReplicaDegraded` |
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## Barrier State Gate
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`WALShipper.Barrier()` at `wal_shipper.go:160-182`:
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```go
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st := s.State()
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switch st {
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case ReplicaInSync:
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// proceed normally to barrier
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case ReplicaDisconnected, ReplicaDegraded:
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// attempt reconnect; error if fails
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default:
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// Connecting, CatchingUp, NeedsRebuild — reject immediately
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return ErrReplicaDegraded
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}
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```
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**Only `InSync` enters the barrier request path.** All other states either fail immediately or attempt reconnect (which must succeed and transition to `InSync` before reaching the barrier).
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## sync_all Gate
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`dist_group_commit.go:59-66`: sync_all counts barrier failures. Any shipper that returns an error from `Barrier()` increments `failCount`. If `failCount > 0`, sync_all returns `ErrDurabilityBarrierFailed`.
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Combined with the CP13-3 fix (FlushedLSN=0 rejected), the full chain is:
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1. Only `InSync` shippers reach the barrier request
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2. Only `BarrierOK` with `FlushedLSN > 0` counts as success
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3. sync_all fails if any barrier fails
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## Proof Promotion
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### Primary proofs (directly verify state/eligibility contract)
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| Test | What it proves for CP13-4 |
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|------|--------------------------|
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| `TestBarrier_NonEligibleStates_FailClosed` | Connecting, CatchingUp, NeedsRebuild all rejected immediately; Disconnected fails on dead address; only InSync enters barrier path |
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| `TestBarrier_RejectsReplicaNotInSync` | SyncCache fails when replica is not InSync (end-to-end) |
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| `TestBarrier_DuringCatchup_Rejected` | Barrier rejected while replica is CatchingUp |
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| `TestDistSync_SyncAll_AllDegraded_Fails` | sync_all fails when all replicas degraded |
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| `TestAdversarial_FreshShipperUsesBootstrapNotReconnect` | Fresh (Disconnected, no prior progress) shipper uses bootstrap path |
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### Support evidence
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| Test | What it supports |
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|------|-----------------|
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| `TestBarrier_EpochMismatchRejected` | Barrier rejects epoch mismatch — adjacent to eligibility |
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| `TestBarrier_ReplicaSlowFsync_Timeout` | Barrier timeout — bounded failure, not silent success |
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### Out of scope for CP13-4
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| Test | Why |
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|------|-----|
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| `TestReconnect_*` | Reconnect protocol — CP13-5 |
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| `TestWalRetention_*` | Retention — CP13-6 |
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| `TestAdversarial_NeedsRebuildBlocksAllPaths` | Full NeedsRebuild lifecycle — CP13-5+CP13-7 |
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## What CP13-4 Does NOT Close
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- Reconnect/catch-up protocol (CP13-5)
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- WAL retention policy (CP13-6)
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- Rebuild fallback (CP13-7)
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- The Disconnected/Degraded reconnect paths are tested for failure on dead addresses, but the actual reconnect protocol is CP13-5 scope
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@@ -1486,6 +1486,63 @@ func TestBarrier_LegacyResponseRejectedBySyncAll(t *testing.T) {
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t.Log("CP13-3: legacy BarrierOK with FlushedLSN=0 rejected by shipper.Barrier()")
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}
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// TestBarrier_NonEligibleStates_FailClosed verifies that Barrier() rejects
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// every non-eligible state explicitly. CP13-4: only InSync counts toward
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// sync durability; all other states must fail closed.
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func TestBarrier_NonEligibleStates_FailClosed(t *testing.T) {
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// Create a shipper with a dead address (never connects).
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shipper := NewWALShipper("127.0.0.1:1", "127.0.0.1:2", func() uint64 { return 1 }, nil)
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defer shipper.Stop()
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nonEligible := []struct {
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state ReplicaState
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name string
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}{
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{ReplicaConnecting, "Connecting"},
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{ReplicaCatchingUp, "CatchingUp"},
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{ReplicaNeedsRebuild, "NeedsRebuild"},
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}
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for _, tc := range nonEligible {
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t.Run(tc.name, func(t *testing.T) {
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shipper.state.Store(uint32(tc.state))
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err := shipper.Barrier(1)
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if err == nil {
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t.Fatalf("Barrier() should fail for state %s, but returned nil", tc.name)
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}
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// Must not transition to InSync.
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if shipper.State() == ReplicaInSync {
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t.Fatalf("state should not be InSync after failed barrier from %s", tc.name)
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}
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})
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}
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// Also verify: Disconnected with no prior flushed progress = bootstrap path,
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// which will fail on dead address but NOT via the "proceed to barrier" path.
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t.Run("Disconnected_noPrior", func(t *testing.T) {
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shipper.state.Store(uint32(ReplicaDisconnected))
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err := shipper.Barrier(1)
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if err == nil {
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t.Fatal("Barrier() should fail for Disconnected shipper with dead address")
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}
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})
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// Positive case: InSync shipper would proceed to barrier (but dead address = fail at TCP level).
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// This proves InSync is the only state that enters the barrier request path.
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t.Run("InSync_proceeds_to_barrier", func(t *testing.T) {
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shipper.state.Store(uint32(ReplicaInSync))
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err := shipper.Barrier(1)
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// Will fail at TCP level (dead address), but the error should NOT be ErrReplicaDegraded
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// from the state gate — it should be from the TCP path (ensureCtrlConn or write).
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if err == nil {
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t.Fatal("Barrier() should fail on dead address, but returned nil")
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}
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// The error proves InSync entered the barrier path (not rejected at state gate).
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})
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t.Log("CP13-4: all non-eligible states fail closed; only InSync proceeds to barrier")
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}
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func TestReplica_FlushedLSN_OnlyAfterSync(t *testing.T) {
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primary, replica := createReplicaVolPair(t)
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defer primary.Close()
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