I expect any good program should do all their reads and writes in a loop until all data written/read without relying that write will write everything (even with regular files). Am I right?

Implemented simple FUSE filesystem which only allows reading and writing with small buffers, very often returning that it is written less bytes that in a buffer (using -o direct_io). Some programs work, some not (notably mountlo). Are them buggy or programs should not expect truncated writes and reads from the regular files?

In general, are seekable file descriptors expected to truncate data like sockets and pipes?