Potential fix for code scanning alert no. 3: Size computation for allocation may overflow

[vinodvx]

Potential fix for https://github.com/agenticenv/durable-go/security/code-scanning/3

Add an overflow guard before the allocation in wrapSidecarMAC in journal.go.

General approach:

• Before computing len(body)+len(mac) for allocation, verify it cannot overflow int.
• If it would overflow, fail safely instead of allocating.

Best fix here (minimal functional impact):

• len(mac) is fixed (sha256.Size) when MAC is enabled, but using generic guard is cleaner.
• Change wrapSidecarMAC to return ([]byte, error) and guard with:
  • if len(body) > math.MaxInt-len(mac) { return nil, fmt.Errorf(...) }
• Update call site in saveMeta to handle the returned error.
• This preserves existing behavior for valid sizes and prevents panic/undersized allocation on overflow.

Files/regions to change:

• journal.go
  • import block: add math
  • saveMeta: handle error from wrapSidecarMAC
  • wrapSidecarMAC: change signature and add overflow check

No changes are required in task.go; its alert variant flows into the same vulnerable allocation, so fixing wrapSidecarMAC addresses both.

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