Constant GD_AT_END 

pub const GD_AT_END: u64 = 0xffff_ffff_ffff_ffff;

Sentinel gdOffset in the primary header of a streamOptimized extent.

When gdOffset == GD_AT_END the real GD location is in the footer header appended to the end of the file: SparseExtentHeader at file_end − 1024, followed by an EOS marker at file_end − 512 (VDF 1.1 §4.6).