Add BufferChannel for fsm incoming/outgoing channels.

[taitov]

Added the ability to select a type of fsm incoming/outgoing channels:

1. InfiniteChannel (old behavior, default)
2. BufferChannel

---

BufferChannel allows to accumulate and combine paths in a separate thread, in the case when the reading side (for example, BgpServer.Serve) is busy.

A map with NLRI key (bufferChannel.pathIdxs) is also used to combine multiple changes of one path into a single update.

BufferChannel prevents infinite memory consumption because its maximum size is comparable to the size of the adj-in (or adj-out for the fsm outgoing channel).

[taitov]

cc #2862

[bayrinat]

@fujita could you please take a look at it?

[fujita]

I don't think it's a good idea for GoBGP's most core code to have two different implementations.
What's the maximum buffer size of Go's buffered channel?
If it's large, then we could use buffered channel instead of infinite channel like #2862 The buffered size can be specified by a command line option or such.

[fujita]

btw, what kind of use case do you have that puts such a heavy load on GoBGP?

[taitov]

I don't think it's a good idea for GoBGP's most core code to have two different implementations. What's the maximum buffer size of Go's buffered channel? If it's large, then we could use buffered channel instead of infinite channel like #2862 The buffered size can be specified by a command line option or such.

A regular channel with a limited size will not work, as it can still be reached, which may lead to disconnection with peer.

BufferChannel can combine multiple updates into one, discarding outdated path's attributes. The most effectiveness of this channel is shown when there is a storm of attribute updates for one path from peer.

[taitov]

Сonfiguration example:

peer-groups:
  - config:
      peer-group-name: group1
      peer-as: 12345
      local-as: 54321
      incoming-channel-timeout: 1
      incoming-channel:
        channel-type: buffer
        size: 64
      outgoing-channel:
        channel-type: buffer
        size: 64

neighbors:
  - config:
      neighbor-address: 10.0.0.1
      peer-group: group1

or

neighbors:
  - config:
      neighbor-address: 10.0.0.1
      incoming-channel:
        channel-type: buffer
        size: 64
      outgoing-channel:
        channel-type: buffer
        size: 64
  - config:
      neighbor-address: 10.0.0.2
      incoming-channel:
        channel-type: buffer
        size: 256

[taitov]

btw, what kind of use case do you have that puts such a heavy load on GoBGP?

Paths are being cyclically updated from peers due to attribute updates. And main thread BgpServer.Serve() a busy (ex, processes the mgmtOperation or propagateUpdate paths when peer went disconnect). In this scenario, InfiniteChannel begins to fill up quickly.