Load without seek support.

[rmolives]

It will appear when using stream transmission:

rmolives@debian:~$ scheme --script <(echo "(display (+ 2 3))")
Exception in port-position: failed on #<binary input port /dev/fd/63>: illegal seek

Related issues are #901

So we changed some codes to make it support the same operation as Python.

[mflatt]

Thanks for the PR, and this is certainly a problem worth solving, but I'm doubtful of the approach of catching an exception and then reading differently. I can't point to something that is both likely and that would go wrong, but catching an error and assuming that it means a non-seekable file seems indirect, and indirect things can have unexpected consequences. I also worry about reading in a potentially large input before parsing.

I think it's probably better to just avoid port-position by ensuring that unget-u8 can work to put back up to three bytes. In fact, I think unget-u8 will already work here if the file-buffer length is parameterized to be at least 3, even though putting back multiple bytes is not allowed by the specification of unget-u8. That it turns out to work is internal implementation detail, but it would be fair to rely on that internal detail in this internal function.

[mflatt]

Well, I see that $source-file-descriptor is also an obstacle. Maybe there's not going to be an approach that avoids reading all of the input.