[Discussion] Support readv() on Linux

I would like to get some feedback on some work I plan on doing for Netty that enable using the readv syscall. Today, there is a RecvByteBufAllocator which is attached to the channel that is used to support reading in data. It allocates a single buffer, keeping track of how much data to allocate. I am considering making the following changes:

1. Make a custom ByteBuf type akin to CompositeByteBuf, which cannot grow in side (i.e. there are a fixed number of nio buffers).
2. If the RecvByteBufAllocator returns such a type, the readv() syscall can be used. For Nio sockets this means being able to use ScatteringByteChannel, and using a readv() JNI wrapper for Epoll. I don’t know enough about Kqueue to support this, but in the future it could be added.
3. Modify RecvByteBufAllocator to not use guess() somehow. The guess() function is use to strongly limit how much data can be read at a time, but this is harder to express in a vector read. Guess makes sense when coming from a world of single buffer reads, as it’s easy to slice the destination down to size. But a vectored read is not so clear. I don’t know exactly how this can work (hence this issue), but I am thinking some how returning a negative number for guess() mean disabling it.

@normanmaurer @njhill thoughts?

[normanmaurer]

@carl-mastrangelo couldn’t you do something like https://github.com/netty/netty/pull/9509 ?

[carl-mastrangelo]

@normanmaurer it can be closed.