Constant ZX_ERR_SHOULD_WAIT 

pub const ZX_ERR_SHOULD_WAIT: i32 = -22;

The operation cannot be performed currently but potentially could succeed if the caller waits for a prerequisite to be satisfied, like waiting for a handle to be readable or writable.

Example: Attempting to read from a channel that has no messages waiting but has an open remote will return ZX_ERR_SHOULD_WAIT. In contrast, attempting to read from a channel that has no messages waiting and has a closed remote end will return ZX_ERR_PEER_CLOSED.