WIP / RFC: Define IO reading interface

[jakobnissen]

This is a work in progress proposal to define the interface of the abstract type IO, and provide a robust, low-level API that makes it possible to implement efficient, generic IO operations.

Discussion is very welcome, see https://hackmd.io/UgeBwtIkTTipkgZSQnXc8w#Discussion for more details and discussion of design decisions.

Having used this API for some of my own work, I must say I really like it (well, most of it).

This PR will, in time, have several parts:

1. Define and document the core IO interface, and also fallback definitions of derived functions.
2. Use Base.IOBuffer as a test case for the new interface, to see if a) the API is pleasant, b) the performance is expected, and most importantly, c) this is non-breaking.
3. Add a set of generic IO objects in Test and test the generic definitions using those, to make sure the generic fallbacks work for buffered IOs, unbuffered IOs, and IOs that do not implement this new interface at all (i.e. previously existing IOs)

TODO

• Implement fallback definitions for all functions that do not currently have a fallback definition
• Implement IOBuffer using these new abstractions as a test case for this PR, and fix what breaks
• Add generic IO types in Test and test all main IO methods
• Figure out what to do about the current system's dependence on pointer APIs.
• Add documentation, including a manual section
• Add NEWS

Decisions

See https://hackmd.io/UgeBwtIkTTipkgZSQnXc8w for points of discussion

Timeline

I hope to finish this before the feature freeze of 1.13, but I'd rather have this be done well than done soon.

Things left out of this PR

• Cancellation: The original IO discussion included a discussion of a cancellation API. This is somewhat orthogonal to the IO interface, and IOBuffer, being purely in-memory, does not need to work with cancellation. Therefore, this can be left to a future PR.
• Writing interface: To start with, I'll only implement the reading part of the interface. If people are happy with this, I'll move on to the writing part in a later PR.
• Deserialization / serialization: Methods like read(::IO, ::Int) are one of the main uses of IO. However, this is a different problem with a different design space.

Closes #55835
Closes #47771

[nhz2]

This is tricky because there is an existing fallback method that uses only read(io, UInt8).

If I understand correctly, fillbuffer can fallback to return 0 and getbuffer can fallback to return an empty vector for IO without an underlying buffer. This would hit the isempty(buf) && iszero(nfilled) case.

Currently, you have this throw an EOFError, but you could instead do:

unsafe_store!(dst, read(io, UInt8)::UInt8)
dst += 1
nbytes -= 1

[jakobnissen]

fillbuffer can fall back to 0, but getbuffer should only be implemented if the IO is buffered. And having it buffered implies that when the buffer is empty, and fillbuffer returns 0, the IO is EOF. I'll clarify the documentation.

W.r.t the fallback calling read(io, UInt8) you're right. I just drew out the call graph for the current generic IO functions, and it's not too complex, actually. All reading functions fall back to read(io, UInt8).
I'm thinking of circumventing this by adding a check similar to this:

if readbuffering(typeof(io)) == NotBuffered() || hasmethod(getbuffer, Tuple{typeof(io)})
    # use methods relying on new interface
else
    # use method relying only on read(io, UInt8)
end

This will work, because, since IOs are buffered by default, IOs that have either opted out of buffering, or implemented getbuffer must be aware of the new interface. It's an ugly solution, but it'll work.

Another consideration for this function specifically, is whether we can write a fast, generic fallback for unsafe_read(::IO, src::Any, ::UInt). To do this, we need to be able to dispatch on whether we can write to src using a pointer, which we don't currently have any abstractions for. This is a hobby horse of mine, but for a reason; you really run into it again and again. I hope to be able to address this in this PR.

[nhz2]

Ideally, this would somehow fall back to readbytes! to work well with existing IO types. For example, you could check isnothing(buffer) && !eof(io) (which should only happen for legacy IO types) and then fallback to readbytes! in that case.

[jakobnissen]

Edit: This is a new function, so I think it would actually be a boon if it failed for old IOs. That will push people to implement this new API for old IO types, without breaking any existing code.
It will also simplify the implementation because we don't have to do hacky workarounds to support IOs which don't adhere to the (new) interface.