fix(hub): surface silently swallowed errors in hubsync hook and replication loop

[omergk28]

Summary

Two hub code paths swallowed errors with no logging surface — the session-start hubsync hook (internal/cli/system/core/hubsync) and the cluster replication loop (internal/hub/replicate.go). Operators couldn't tell a healthy quiet sync from one that never reached the network.

This PR wires every silent return through the established warn sink with format constants in config/warn, and un-conflates the sync-error check from the empty-result check (only the error warns now). Both functions keep their signatures and never-block contracts — logging is the only behavior change.

Closes #100.

Already landed

Two of #100's sub-items were already in main before this PR (the conn.Close defer warn and the store.Append warn + keep-consuming, via CloseHubClient/HubReplicateAppend). This PR covers the rest and adds the regression tests the issue asked for — including pinning the already-landed keep-consuming behavior, which previously had no coverage.

One deliberate deviation from the issue's proposed code

The issue's sketch warns on every RecvMsg error — but io.EOF is the normal end of every sync stream, so that would emit a warning once per ReplicateInterval on perfectly healthy replication. The receive path here stays silent for eof() (the hub package's own end-of-stream helper, same as the other two RecvMsg sites) and for caller shutdown (ctx.Err() != nil), and warns on everything else. TestReplicateOnce_CleanReplicationDoesNotWarn pins this.

Tests (mapped to the issue's Tests Required)

Issue ask	Test
Warns on load error	TestSync_WarnsOnLoadError
Warns on dial error	TestSync_WarnsOnDialError — empirically, the only eager grpc.NewClient failure is a control character failing URL parse; every other malformed target (://invalid…) defers to first use. TestSync_WarnsOnPullError covers the lazy path (closed port → Sync RPC fails)
No warn on empty result (the un-conflation)	TestSync_NoWarnOnEmptyResult — real in-process hub, zero entries, asserts an empty warn buffer
Replication warns on dial error	TestReplicateOnce_WarnsOnDialError + TestReplicateOnce_WarnsOnTransportError
Keeps consuming after append error	TestReplicateOnce_KeepsConsumingAfterAppendError — read-only follower dir, asserts exactly 2 append warnings (root/Windows skip guards per the rc/require_test.go precedent)

All use the existing warn.SetSink seam; the hubsync package previously had zero tests.

Verification

• go test -race -count=2 on both packages — clean.
• Mutation checks: three deliberate regressions each fail their pinning test — (1) re-conflating syncErr with the empty check fails TestSync_NoWarnOnEmptyResult; (2) removing the EOF guard fails TestReplicateOnce_CleanReplicationDoesNotWarn; (3) dropping the dial warn fails TestSync_WarnsOnDialError.
• make audit — all checks passed.

🤖 Generated with Claude Code

[josealekhine]

The fix is correct, and the key insight is right

Some nits below:

The real bug was the conflation of "error" with "nothing new":

// internal/cli/system/core/hubsync/sync.go — before
entries, syncErr := client.Sync(context.Background(), cfg.Types, 0)
if syncErr != nil || len(entries) == 0 {
    return ""
}

The PR un-conflates them so a real error warns while a genuine empty result stays silent:

// after
if syncErr != nil {
    logWarn.Warn(cfgWarn.HubSyncPull, cfg.HubAddr, syncErr)
    return ""
}
if len(entries) == 0 {
    // Genuine empty result: not an error, no warning.
    return ""
}

Every other silent return "" now routes through the logWarn sink, and both functions keep their signatures and non-blocking contract (still return "" everywhere). The replication loop's recv suppression is the right call — silent on io.EOF (normal stream end) and ctx.Err() != nil (shutdown), warn on anything else:

// internal/hub/replicate.go
if recvErr := stream.RecvMsg(msg); recvErr != nil {
    if !eof(recvErr) && ctx.Err() == nil {
        logWarn.Warn(cfgWarn.HubReplicateRecv, masterAddr, recvErr)
    }
    return
}

…and the spec documents this as a deliberate deviation from issue #100's proposed code (which warned on every recv error and would have made clean cycles noisy).

Things I checked that hold up

• All 9 new warn format/arg arities match. go vet would not catch a mismatch here — Warn(format string, args ...any) isn't a recognized printf-wrapper — so I verified each call by hand (e.g. HubSyncWrite = "hubsync: write %d entries: %v" ← Warn(..., len(entries), writeErr)).
• No secret leakage. The token never appears in any warn format; only the hub address + error.
• Tests are genuinely excellent. Each error path asserts its warning; TestSync_NoWarnOnEmptyResult pins the un-conflation; TestReplicateOnce_CleanReplicationDoesNotWarn pins the EOF suppression (zero warnings on a real round-trip + lastSeq == 2); TestReplicateOnce_KeepsConsumingAfterAppendError pins continue-on-append-failure (chmod-0 dir → exactly 2 append warnings). The control-char ("\x00") trick to force an eager grpc.NewClient failure is a nice touch given the resolver is otherwise lazy.
• Noise is bounded. The live hook (checkhubsync.Run) is gated by check.DailyThrottled, so Sync and its warnings fire at most once per day, not every session start.
• Replication path is dormant. replicateOnce is reserved dead code today (var _ = startReplication, "called from Server.Start when a follower peer is configured"), so those warnings won't fire until cluster mode is wired.

---

Findings (all minor / adjacent — none are blockers)

1. (adjacent, pre-existing) The "must not block the session" contract isn't actually guaranteed

The doc update reaffirms the contract:

// internal/cli/system/core/hubsync/doc.go
// Every error is surfaced as a stderr warning via the warn sink,
// but never propagates: the hook must not block the session start.

But the underlying RPC has no deadline:

// sync.go — context.Background(), no timeout
entries, syncErr := client.Sync(context.Background(), cfg.Types, 0)

// internal/hub/client.go — lazy NewClient, fail-fast (not WaitForReady), no dial timeout
conn, dialErr := grpc.NewClient(addr, grpc.WithTransportCredentials(insecure.NewCredentials()), ...)

A hub that accepts the TCP connection but never responds (hung server / black-hole proxy) makes RecvMsg inside Sync block indefinitely → the session-start hook hangs, contradicting the contract this PR re-documents. Not introduced here (the context.Background() line is unchanged), and the daily throttle limits exposure to once/day — but since the PR draws attention to this exact contract, wrapping the Sync RPC in a context.WithTimeout would make "never block" actually true. This is the one finding with real teeth; worth a follow-up rather than blocking the merge.

2. (forward-looking) Replication warning cadence when wired

replicateOnce is dead code today, but when startReplication is wired (#96 territory), a persistently-down-but-reachable master would warn every ReplicateInterval (5s, per internal/config/hub) via HubReplicateRecv with no dedup/backoff — the EOF+ctx guard only suppresses normal completion and shutdown, not a recurring transport error. Consider warn-once-per-state-transition when that path goes live. Noted now because the warning constants and call sites ship in this PR.

3. (trivial) eof() uses ==, not errors.Is

// internal/hub/eof.go (pre-existing helper)
func eof(err error) bool { return err == stdio.EOF }

Correct for gRPC's unwrapped io.EOF (the clean-replication test confirms it in practice), just marginally less defensive than errors.Is(err, io.EOF) if any layer ever wraps it. Pre-existing, but this PR newly relies on it for the suppression branch.