Constant SYNC_PRESENT 

pub const SYNC_PRESENT: u32 = _; // 1u32

[TensorDesc::sync] carries a real handle. When clear, sync is meaningless and must not be waited on.

A flag rather than a sentinel because every plausible sentinel is a legal handle somewhere: 0 is a valid GLsync-adjacent value and a valid fd number is never negative, so -1 and 0 cannot both be spare across the kinds this field spans.