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remove send lock (#334) 

* remove send lock

When mplex receives data it will block until a reader has processed the
data. Thus, when a large message is received, such as a gossipsub
subscription table, all of mplex will be blocked until all reading is
finished.

However, if at the same time a `dial` to establish a gossipsub send
connection is ongoing, that `dial` will be blocked because mplex is no
longer reading data - specifically, it might indeed be the connection
that's processing the previous data that is waiting for a send
connection.

There are other problems with the current code:
* If an exception is raised, it is not necessarily raised for the same
connection as `p.sendConn`, so resetting `p.sendConn` in the exception
handling is wrong
* `p.isConnected` is checked before taking the lock - thus, if it
returns false, a new dial will be started. If a new task enters `send`
before dial is finished, it will also determine `p.isConnected` is
false, then get stuck on the lock - when the previous task finishes and
releases the lock, the new task will _also_ dial and thus reset
`p.sendConn` causing a leak.

* prefer existing connection

simplifies flow
This commit is contained in:
Jacek Sieka 2020-08-17 12:38:27 +02:00 committed by GitHub
parent b12145dff7
commit f46bf0faa4
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
3 changed files with 55 additions and 28 deletions
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2
libp2p/protocols/pubsub/pubsub.nim
View File

@ -55,7 +55,6 @@ type
triggerSelf*: bool # trigger own local handler on publish
verifySignature*: bool # enable signature verification
sign*: bool # enable message signing
cleanupLock: AsyncLock
validators*: Table[string, HashSet[ValidatorHandler]]
observers: ref seq[PubSubObserver] # ref as in smart_ptr
msgIdProvider*: MsgIdProvider # Turn message into message id (not nil)
@ -338,7 +337,6 @@ proc init*(
sign: sign,
peers: initTable[PeerID, PubSubPeer](),
topics: initTable[string, Topic](),
cleanupLock: newAsyncLock(),
msgIdProvider: msgIdProvider)
result.initPubSub()

79
libp2p/protocols/pubsub/pubsubpeer.nim
View File

@ -7,7 +7,7 @@
## This file may not be copied, modified, or distributed except according to
## those terms.
import std/[hashes, options, sequtils, strutils, tables]
import std/[hashes, options, strutils, tables]
import chronos, chronicles, nimcrypto/sha2, metrics
import rpc/[messages, message, protobuf],
timedcache,
@ -39,14 +39,13 @@ type
PubSubPeer* = ref object of RootObj
switch*: Switch # switch instance to dial peers
codec*: string # the protocol that this peer joined from
sendConn: Connection
sendConn: Connection # cached send connection
peerId*: PeerID
handler*: RPCHandler
sentRpcCache: TimedCache[string] # cache for already sent messages
recvdRpcCache: TimedCache[string] # cache for already received messages
observers*: ref seq[PubSubObserver] # ref as in smart_ptr
subscribed*: bool # are we subscribed to this peer
sendLock*: AsyncLock # send connection lock
RPCHandler* = proc(peer: PubSubPeer, msg: seq[RPCMsg]): Future[void] {.gcsafe.}
@ -117,11 +116,53 @@ proc handle*(p: PubSubPeer, conn: Connection) {.async.} =
debug "exiting pubsub peer read loop"
await conn.close()
if p.sendConn == conn:
p.sendConn = nil
except CancelledError as exc:
raise exc
except CatchableError as exc:
trace "Exception occurred in PubSubPeer.handle", exc = exc.msg
proc getSendConn(p: PubSubPeer): Future[Connection] {.async.} =
# get a cached send connection or create a new one
block: # check if there's an existing connection that can be reused
let current = p.sendConn
if not current.isNil:
if not (current.closed() or current.atEof):
# The existing send connection looks like it might work - reuse it
return current
# Send connection is set but broken - get rid of it
p.sendConn = nil
# Careful, p.sendConn might change after here!
await current.close() # TODO this might be unnecessary
# Grab a new send connection
let newConn = await p.switch.dial(p.peerId, p.codec) # ...and here
if newConn == nil:
return p.sendConn # A concurrent attempt perhaps succeeded?
# Because of the awaits above, a concurrent `getSendConn` call might have
# set up a send connection already. We cannot take a lock here because
# it might block the reading of data from mplex which will cause its
# backpressure handling to stop reading from the socket and thus prevent the
# channel negotiation from finishing
if p.sendConn != nil and not(p.sendConn.closed or p.sendConn.atEof):
let current = p.sendConn
# Either the new or the old connection could potentially be closed - it's
# slightly easier to sequence the closing of the new connection because the
# old one might still be in use.
await newConn.close()
return current
p.sendConn = newConn
asyncCheck p.handle(newConn) # start a read loop on the new connection
return newConn
proc send*(
p: PubSubPeer,
msg: RPCMsg,
@ -154,27 +195,17 @@ proc send*(
libp2p_pubsub_skipped_sent_messages.inc(labelValues = [p.id])
return
var conn: Connection
try:
trace "about to send message"
if not p.connected:
try:
await p.sendLock.acquire()
trace "no send connection, dialing peer"
# get a send connection if there is none
p.sendConn = await p.switch.dial(
p.peerId, p.codec)
if not p.connected:
raise newException(CatchableError, "unable to get send pubsub stream")
# install a reader on the send connection
asyncCheck p.handle(p.sendConn)
finally:
if p.sendLock.locked:
p.sendLock.release()
conn = await p.getSendConn()
if conn == nil:
debug "Couldn't get send connection, dropping message"
return
trace "sending encoded msgs to peer"
await p.sendConn.writeLp(encoded).wait(timeout)
await conn.writeLp(encoded).wait(timeout)
p.sentRpcCache.put(digest)
trace "sent pubsub message to remote"
@ -186,9 +217,10 @@ proc send*(
except CatchableError as exc:
trace "unable to send to remote", exc = exc.msg
if not(isNil(p.sendConn)):
await p.sendConn.close()
p.sendConn = nil
# Next time sendConn is used, it will be have its close flag set and thus
# will be recycled
if not isNil(conn):
await conn.close()
raise exc
@ -204,4 +236,3 @@ proc newPubSubPeer*(peerId: PeerID,
result.peerId = peerId
result.sentRpcCache = newTimedCache[string](2.minutes)
result.recvdRpcCache = newTimedCache[string](2.minutes)
result.sendLock = newAsyncLock()

2
tests/pubsub/testfloodsub.nim
View File

@ -15,9 +15,7 @@ import utils,
../../libp2p/[errors,
switch,
stream/connection,
stream/bufferstream,
crypto/crypto,
protocols/pubsub/pubsubpeer,
protocols/pubsub/pubsub,
protocols/pubsub/floodsub,
protocols/pubsub/rpc/messages,