bug: Range loop modification during time-backwards handling [MEDIUM]

[CybotTM]

Upstream Reference

Addresses robfig#480 - PR implementing time backwards handling.

Summary

The time-backwards handling code iterates over entries with a range loop while calling Update(), which modifies the underlying slice ordering.

Location

cron.go:277-284

for _, e := range c.entries {
    if !e.Prev.IsZero() && e.Prev.After(now) {
        e.Next = e.Schedule.Next(now)
        c.entries.Update(e)  // Calls heap.Fix which reorders elements
        c.logger.Info("reschedule", ...)
    }
}

Problem

1. range iterates with original indices
2. Update() → heap.Fix() → Swap() reorders elements
3. After swaps, some entries may be visited twice or skipped

Impact

• Severity: Medium
• Only affects rare "time moved backwards" scenario (NTP correction, VM restore)
• Some entries may miss rescheduling during correction
• Heap remains structurally valid afterward

Fix Options

1. Iterate over a copy of the entries
2. Use index-based iteration accounting for swaps
3. Collect entries to update first, then update in separate loop

// Option 1: Copy entries
entriesCopy := make([]*Entry, len(c.entries))
copy(entriesCopy, c.entries)
for _, e := range entriesCopy {
    // ... update logic
}

Validation

• Consensus: Disputed initially, resolved as valid bug with medium severity