bsc/swarm/pss/writeup.md

PSS tests failures explanation

This document aims to explain the changes in https://github.com/ethersphere/go-ethereum/pull/126 and how those changes affect the pss_test.go TestNetwork tests.

Problem

When running the TestNetwork test, execution sometimes:

• deadlocks
• panics
• failures with wrong result, such as:

$ go test -v ./swarm/pss -cpu 4 -run TestNetwork

--- FAIL: TestNetwork (68.13s)
    --- FAIL: TestNetwork/3/10/4/sim (68.13s)
        pss_test.go:697: 7 of 10 messages received
        pss_test.go:700: 3 messages were not received
FAIL

Moreover execution almost always deadlocks with sim adapter, and sock adapter (when buffer is low), but is mostly stable with exec and tcp adapters.

Findings and Fixes

1. Addressing panics

Panics were caused due to concurrent map read/writes and unsynchronised access to shared memory by multiple goroutines. This is visible when running the test with the -race flag.

go test -race -v ./swarm/pss -cpu 4 -run TestNetwork

  1 ==================
  2 WARNING: DATA RACE
  3 Read at 0x00c424d456a0 by goroutine 1089:
  4   github.com/ethereum/go-ethereum/swarm/pss.(*Pss).forward.func1()
  5       /Users/nonsense/code/src/github.com/ethereum/go-ethereum/swarm/pss/pss.go:654 +0x44f
  6   github.com/ethereum/go-ethereum/swarm/network.(*Kademlia).eachConn.func1()
  7       /Users/nonsense/code/src/github.com/ethereum/go-ethereum/swarm/network/kademlia.go:350 +0xc9
  8   github.com/ethereum/go-ethereum/pot.(*Pot).eachNeighbour.func1()
  9       /Users/nonsense/code/src/github.com/ethereum/go-ethereum/pot/pot.go:599 +0x59
  ...

 28
 29 Previous write at 0x00c424d456a0 by goroutine 829:
 30   github.com/ethereum/go-ethereum/swarm/pss.(*Pss).Run()
 31       /Users/nonsense/code/src/github.com/ethereum/go-ethereum/swarm/pss/pss.go:192 +0x16a
 32   github.com/ethereum/go-ethereum/swarm/pss.(*Pss).Run-fm()
 33       /Users/nonsense/code/src/github.com/ethereum/go-ethereum/swarm/pss/pss.go:185 +0x63
 34   github.com/ethereum/go-ethereum/p2p.(*Peer).startProtocols.func1()
 35       /Users/nonsense/code/src/github.com/ethereum/go-ethereum/p2p/peer.go:347 +0x8b
 ...

Current solution

Adding a mutex around all shared data.

2. Failures with wrong result

The validation phase of the TestNetwork test is done using an RPC subscription:

    ...
	triggerChecks := func(trigger chan enode.ID, id enode.ID, rpcclient *rpc.Client) error {
		msgC := make(chan APIMsg)
		ctx, cancel := context.WithTimeout(context.Background(), time.Second)
		defer cancel()
		sub, err := rpcclient.Subscribe(ctx, "pss", msgC, "receive", hextopic)
		...

By design the RPC uses a subscription buffer with a max length. When this length is reached, the subscription is dropped. The current config value is not suitable for stress tests.

Current solution

Increase the max length of the RPC subscription buffer.

const (
	// Subscriptions are removed when the subscriber cannot keep up.
	//
	// This can be worked around by supplying a channel with sufficiently sized buffer,
	// but this can be inconvenient and hard to explain in the docs. Another issue with
	// buffered channels is that the buffer is static even though it might not be needed
	// most of the time.
	//
	// The approach taken here is to maintain a per-subscription linked list buffer
	// shrinks on demand. If the buffer reaches the size below, the subscription is
	// dropped.
	maxClientSubscriptionBuffer = 20000
)

3. Deadlocks

Deadlocks are triggered when using:

• sim adapter - synchronous, unbuffered channel
• sock adapter - asynchronous, buffered channel (when using a 1K buffer)

No deadlocks were triggered when using:

• tcp adapter - asynchronous, buffered channel
• exec adapter - asynchronous, buffered channel

Ultimately the deadlocks happen due to blocking pp.Send() call at:

	 // attempt to send the message
	err := pp.Send(msg)
	if err != nil {
		log.Debug(fmt.Sprintf("%v: failed forwarding: %v", sendMsg, err))
		return true
	}

p2p request handling is synchronous (as discussed at https://github.com/ethersphere/go-ethereum/issues/130), pss is also synchronous, therefore if two nodes happen to be processing a request, while at the same time waiting for response on pp.Send(msg), deadlock occurs.

pp.Send(msg) is only blocking when the underlying adapter is blocking (read sim or sock) or the buffer of the connection is full.

Current solution

Make no assumption on the undelying connection, and call pp.Send asynchronously in a go-routine.

Alternatively, get rid of the sim and sock adapters, and use tcp adapter for testing.