A custom input port needs to buffer at least one item, to support peek-* operation.  

get-position handler must return whatever position marker of the source (that is, the position the next read! will occur).  it may or may not corresponds to port-position returns. It doesn't have to know what port-position returns.

A custom textual input port must have a slot to keep an underlying position value returned by get-position.

peek-char on a custom port must call get-position first, and remember its value, then call read! to read just one character.   Other read operations clear underlying position value.

port-position returns an implementation-specific value embedding the underlying position value.  If there's a value in underlying position value, then it returns a position value embedding it.  Otherwise, it calls get-position to obtain the current position.