When read() encounters an error or end-of-file condition after successfully reading n > 0 bytes, it returns the number of bytes read. It may return the (non-nil) error from the same call or return the error (and n == 0) from a subsequent call. An instance of this general case is that a Reader returning a non-zero number of bytes at the end of the input stream may return either err == EOF or err == null. The next read() should return 0, EOF.

Callers should always process the n > 0 bytes returned before considering the EOF. Doing so correctly handles I/O errors that happen after reading some bytes and also both of the allowed EOF behaviors.

Implementations of read() are discouraged from returning a zero byte count with a null error, except when p.byteLength == 0. Callers should treat a return of 0 and null as indicating that nothing happened; in particular it does not indicate EOF.

Implementations must not retain p.