Constant MAXIMUM_ALLOWED_WINDOW_SIZE 

pub const MAXIMUM_ALLOWED_WINDOW_SIZE: u64 = _ /* 134_217_728u64 */;

Largest window a frame may declare before a decoder that holds the window itself refuses it, unless FrameDecoder::set_max_window_size raised the ceiling.

The bound is what makes decoding untrusted input safe: a frame header can ask for a window far larger than the data behind it, and honouring that would let a few bytes of input demand gigabytes of memory. The decoder’s default window ceiling (128 MiB = 1 << 27), upstream zstd’s default ZSTD_d_windowLogMax (ZSTD_WINDOWLOG_LIMIT_DEFAULT = 27). On the paths where the decoder holds the window itself, a frame advertising a larger one is refused to bound that allocation on untrusted input, until the caller raises the ceiling (FrameDecoder::set_max_window_size). Every frame our encoder emits by default (up to window_log 27 at level 22) fits it.