error in skipchars(BufferedInputStream)

[denius]

Hello!
I'm trying to parse a buffered file, but

skipchars(isspace, BufferedInputStream(IOBuffer(" test")))

yields ERROR: ArgumentError: n must be non-negative in skip(::BufferedInputStream, n) due to
https://github.com/BioJulia/BufferedStreams.jl/blob/b85ff82efcd95d3a4d72a4021f0833b579554bec/src/bufferedinputstream.jl#L126 which is different from Julia/base/iobuffer.jl

[JeffBezanson]

It would be very useful to implement negative skip amounts, since that allows using a buffered stream as a drop-in replacement in more cases. For example in JuliaLang/julia#34195 this would be needed for BufferedStreams to work with SeisIO.

[stevengj]

It's not clear to me how to implement negative skip amounts if the underlying stream is not seekable? Actually we already seem to support seek to arbitrary positions if the underlying stream is seekable, so we should be able to do the same thing with skip I guess.

In the meantime you can use mark and reset for a lot of cases where you might otherwise use negative skips.

In the meantime we should simply implement skipchars, since this can be done more efficiently for buffered streams than for generic streams.