nim-libp2p/libp2p/muxers/mplex/lpchannel.nim

## Nim-LibP2P
## Copyright (c) 2019 Status Research & Development GmbH
## Licensed under either of
##  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE))
##  * MIT license ([LICENSE-MIT](LICENSE-MIT))
## at your option.
## This file may not be copied, modified, or distributed except according to
## those terms.

{.push raises: [Defect].}

import std/[oids, strformat]
import pkg/[chronos, chronicles, metrics, nimcrypto/utils]
import ./coder,
       ../muxer,
       ../../stream/[bufferstream, connection, streamseq],
       ../../peerinfo

export connection

logScope:
  topics = "libp2p mplexchannel"

when defined(libp2p_network_protocols_metrics):
  declareCounter libp2p_protocols_bytes, "total sent or received bytes", ["protocol", "direction"]

## Channel half-closed states
##
## | State    | Closed local      | Closed remote
## |=============================================
## | Read     | Yes (until EOF)   | No
## | Write    | No                | Yes
##
## Channels are considered fully closed when both outgoing and incoming
## directions are closed and when the reader of the channel has read the
## EOF marker

const
  MaxWrites = 1024 ##\
    ## Maximum number of in-flight writes - after this, we disconnect the peer

  LPChannelTrackerName* = "LPChannel"

type
  LPChannel* = ref object of BufferStream
    id*: uint64                   # channel id
    name*: string                 # name of the channel (for debugging)
    conn*: Connection             # wrapped connection used to for writing
    initiator*: bool              # initiated remotely or locally flag
    isOpen*: bool                 # has channel been opened
    closedLocal*: bool            # has channel been closed locally
    msgCode*: MessageType         # cached in/out message code
    closeCode*: MessageType       # cached in/out close code
    resetCode*: MessageType       # cached in/out reset code
    writes*: int                  # In-flight writes

func shortLog*(s: LPChannel): auto =
  try:
    if s.isNil: "LPChannel(nil)"
    elif s.name != $s.oid and s.name.len > 0:
      &"{shortLog(s.conn.peerId)}:{s.oid}:{s.name}"
    else: &"{shortLog(s.conn.peerId)}:{s.oid}"
  except ValueError as exc:
    raise newException(Defect, exc.msg)

chronicles.formatIt(LPChannel): shortLog(it)

proc open*(s: LPChannel) {.async, gcsafe.} =
  trace "Opening channel", s, conn = s.conn
  if s.conn.isClosed:
    return
  try:
    await s.conn.writeMsg(s.id, MessageType.New, s.name)
    s.isOpen = true
  except CatchableError as exc:
    await s.conn.close()
    raise exc

method closed*(s: LPChannel): bool {.raises: [Defect].} =
  s.closedLocal

proc closeUnderlying(s: LPChannel): Future[void] {.async.} =
  ## Channels may be closed for reading and writing in any order - we'll close
  ## the underlying bufferstream when both directions are closed
  if s.closedLocal and s.atEof():
    await procCall BufferStream(s).close()

proc reset*(s: LPChannel) {.async, gcsafe.} =
  if s.isClosed:
    trace "Already closed", s
    return

  s.isClosed = true
  s.closedLocal = true

  trace "Resetting channel", s, len = s.len

  if s.isOpen and not s.conn.isClosed:
    # If the connection is still active, notify the other end
    proc resetMessage() {.async.} =
      try:
        trace "sending reset message", s, conn = s.conn
        await s.conn.writeMsg(s.id, s.resetCode) # write reset
      except CatchableError as exc:
        # No cancellations
        await s.conn.close()
        trace "Can't send reset message", s, conn = s.conn, msg = exc.msg

    asyncSpawn resetMessage()

  await s.closeImpl() # noraises, nocancels

  trace "Channel reset", s

method close*(s: LPChannel) {.async, gcsafe.} =
  ## Close channel for writing - a message will be sent to the other peer
  ## informing them that the channel is closed and that we're waiting for
  ## their acknowledgement.
  if s.closedLocal:
    trace "Already closed", s
    return
  s.closedLocal = true

  trace "Closing channel", s, conn = s.conn, len = s.len

  if s.isOpen and not s.conn.isClosed:
    try:
      await s.conn.writeMsg(s.id, s.closeCode) # write close
    except CancelledError as exc:
      await s.conn.close()
      raise exc
    except CatchableError as exc:
      # It's harmless that close message cannot be sent - the connection is
      # likely down already
      await s.conn.close()
      trace "Cannot send close message", s, id = s.id, msg = exc.msg

  await s.closeUnderlying() # maybe already eofed

  trace "Closed channel", s, len = s.len

method initStream*(s: LPChannel) =
  if s.objName.len == 0:
    s.objName = LPChannelTrackerName

  s.timeoutHandler = proc(): Future[void] {.gcsafe.} =
    trace "Idle timeout expired, resetting LPChannel", s
    s.reset()

  procCall BufferStream(s).initStream()

method readOnce*(s: LPChannel,
                 pbytes: pointer,
                 nbytes: int):
                 Future[int] {.async.} =
  ## Mplex relies on reading being done regularly from every channel, or all
  ## channels are blocked - in particular, this means that reading from one
  ## channel must not be done from within a callback / read handler of another
  ## or the reads will lock each other.
  try:
    let bytes = await procCall BufferStream(s).readOnce(pbytes, nbytes)
    when defined(libp2p_network_protocols_metrics):
      if s.tag.len > 0:
        libp2p_protocols_bytes.inc(bytes.int64, labelValues=[s.tag, "in"])

    trace "readOnce", s, bytes
    if bytes == 0:
      await s.closeUnderlying()
    return bytes
  except CatchableError as exc:
    # readOnce in BufferStream generally raises on EOF or cancellation - for
    # the former, resetting is harmless, for the latter it's necessary because
    # data has been lost in s.readBuf and there's no way to gracefully recover /
    # use the channel any more
    await s.reset()
    raise exc

proc prepareWrite(s: LPChannel, msg: seq[byte]): Future[void] {.async.} =
  # prepareWrite is the slow path of writing a message - see conditions in
  # write
  if s.closedLocal or s.conn.closed:
    raise newLPStreamClosedError()

  if msg.len == 0:
    return

  if s.writes >= MaxWrites:
    debug "Closing connection, too many in-flight writes on channel",
      s, conn = s.conn, writes = s.writes
    await s.reset()
    await s.conn.close()
    return

  if not s.isOpen:
    await s.open()

  await s.conn.writeMsg(s.id, s.msgCode, msg)

proc completeWrite(
    s: LPChannel, fut: Future[void], msgLen: int): Future[void] {.async.} =
  try:
    s.writes += 1

    await fut
    when defined(libp2p_network_protocols_metrics):
      if s.tag.len > 0:
        libp2p_protocols_bytes.inc(msgLen.int64, labelValues=[s.tag, "out"])

    s.activity = true
  except CatchableError as exc:
    trace "exception in lpchannel write handler", s, msg = exc.msg
    await s.reset()
    await s.conn.close()
    raise exc
  finally:
    s.writes -= 1

method write*(s: LPChannel, msg: seq[byte]): Future[void] =
  ## Write to mplex channel - there may be up to MaxWrite concurrent writes
  ## pending after which the peer is disconnected

  let
    closed = s.closedLocal or s.conn.closed

  let fut =
    if (not closed) and msg.len > 0 and s.writes < MaxWrites and s.isOpen:
      # Fast path: Avoid a copy of msg being kept in the closure created by
      # `{.async.}` as this drives up memory usage - the conditions are laid out
      # in prepareWrite
      s.conn.writeMsg(s.id, s.msgCode, msg)
    else:
      prepareWrite(s, msg)

  s.completeWrite(fut, msg.len)

proc init*(
  L: type LPChannel,
  id: uint64,
  conn: Connection,
  initiator: bool,
  name: string = "",
  timeout: Duration = DefaultChanTimeout): LPChannel =

  let chann = L(
    id: id,
    name: name,
    conn: conn,
    initiator: initiator,
    timeout: timeout,
    isOpen: if initiator: false else: true,
    msgCode: if initiator: MessageType.MsgOut else: MessageType.MsgIn,
    closeCode: if initiator: MessageType.CloseOut else: MessageType.CloseIn,
    resetCode: if initiator: MessageType.ResetOut else: MessageType.ResetIn,
    dir: if initiator: Direction.Out else: Direction.In)

  chann.initStream()

  when chronicles.enabledLogLevel == LogLevel.TRACE:
    chann.name = if chann.name.len > 0: chann.name else: $chann.oid

  trace "Created new lpchannel", s = chann, id, initiator

  return chann