Add isseekable to Stream/IO interface?

[ssfrr]

If I'm writing some code that takes an IO object externally, I might want to do different things based on whether the given stream is seekable, but as of now there isn't a standard way to check for it.

Here's a mock use-case that matches some real code I'm writing right now, where there's a leading length field in the file format that's should be written, but it's not the end of the world if its not:

struct AudioEncoder{T}
    io::T
    headerpos::Int64
    filelen::Int64
end

function AudioEncoder(io::IO)
    headerpos = position(io)
    write_len_header(io, 0xffffffffffffffff)
    
    AudioEncoder(io, headerpos, 0)
end

function AudioEncoder(f, io::IO)
    ae = AudioEncoder(io)
    try
        f(ae)
    finally
        close(ae)
    end
end

function write(ae::AudioEncoder, buf::SampleBuf)
    io.filelen += write_encoded_samples(ae.io, buf)
    length(buf)
end

function close(ae)
    if isseekable(ae.io) #this doesn't exist
        seek(ae.io, ae.headerpos)
        write_len_header(ae.io, ae.filelen)
    else
        # too bad, couldn't write the correct length, but the receiver can figure it out
    end
end

Which the user might use like so:

open("somefile", "w") do io
    AudioEncoder(io) do enc
        for _ in 1:1000
            write(enc, synthesize_audio_block())
        end
    end
end

At first it seems like this is a use-case for the mark API, but that requires you to just keep buffering the whole file, which could be gigabytes.