A caller can pass Free an offset that is misaligned, out of range, or simply not the start of a real allocation. A good allocator reports the mistake instead of scribbling on memory or crashing. Today you validate every offset.
Reject frees of offsets that are not valid payload starts, without panicking.
Callers make mistakes: they free a pointer into the middle of a block, a stale offset, or plain garbage. A real allocator that trusts these blindly reads a bogus header and corrupts itself; a good one validates first. An offset is only a legal payload if it is aligned, inside the arena, and sits exactly 8 bytes past the start of a genuine block - which you confirm by walking the implicit list and matching the block start.
Reject anything else with an error and leave the heap untouched - never panic on
bad input. A misaligned offset, an out-of-range offset, and an offset that lands
inside a block instead of at a payload boundary are all caught. Combined with the
double-free guard, Free is now robust against the ordinary ways a caller can
misuse it, which is what makes the allocator safe to actually hand to someone. All
that remains is to prove the whole thing works end to end.
func (a *Allocator) valid(payload int) bool {if payload < 8 || payload >= len(a.buf) || payload%8 != 0 { return false }// the payload must sit exactly 8 bytes past a real block startfor off := 0; off < len(a.buf); {size, _ := a.blockAt(off)if off+8 == payload { return true }off += size}return false}