buffer_get_token_sa_pred copies data from b to sa until predicate(sa) returns 1 or -1.
If predicate returns 1 once a '\n' was read, that new-line character is still appended to sa -- use
stralloc_chop or stralloc_chomp to get rid of it. predicate can also return 0 (indicating further input
is required to complete the token) or -1 (abort and return -1; use this if predicate wants to enfore a
maximum message size or does timeout handling or detects a malformed message).
If reading from the buffer or allocating memory fails, buffer_get_new_token_sa_pred returns -1 and sets
errno appropriately. At that point sa may already contain a partial token.
On success, buffer_get_new_token_sa_pred returns 0.
If you want to read from a non-blocking socket, use buffer_get_token_sa_pred instead.