[The1TrueJoe]

d075095 ("fs: keep the bytes a short read already delivered") clears the stream's indicators after read(<length>), but read("line"), read("all") and read(<delimiter>) still leave the end-of-file indicator set once they reach the end. stdio keeps it set (musl, glibc since 2.28, the BSDs), so every later read of those kinds on the handle returns nothing, even after more data has arrived: a file that grew, a tty in non-canonical mode with VMIN 0, a FIFO that got a new writer.

let r = fs.open('/tmp/f', 'r'), w = fs.open('/tmp/f', 'a');
r.read('line');            // "first\n"
r.read('line');            // null, at the end of the file
w.write('second\n');
w.flush();
r.read('line');            // null, where "second\n" is expected

This clears the indicators on entry to uc_fs_read_common(), so no read starts from a state an earlier one left behind. tests/custom/99_bugs/65_fs_read_sticky_eof covers all four modes; it fails without the change and passes with it.

Related: #514 handles an interrupted read("all") and clears the indicators in that branch. The two touch nearby lines but apply cleanly in either order.