Panic in WS client in subscription mechanism

[carpawell]

Have 100ms block machines (4) and our SN nodes (https://github.com/nspcc-dev/neofs-node) are subscribed to some events. Once in a time we have panics and have it more than once, i would say even consistently.

Current Behavior

Create a client, subscribe, use it, panic.

Expected Behavior

Create a client, subscribe, use it, no panic.

Possible Solution

Not sure, i guess some slices (receivers and subscriptions) are not in sync.

Steps to Reproduce

Run SN, go 100ms block time, see panics.

Context

июл 16 14:06:04 node04 neofs-node[642022]: panic: runtime error: invalid memory address or nil pointer dereference
июл 16 14:06:04 node04 neofs-node[642022]: [signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0xa2e02e]
июл 16 14:06:04 node04 neofs-node[642022]: goroutine 114 [running]:
июл 16 14:06:04 node04 neofs-node[642022]: github.com/nspcc-dev/neo-go/pkg/rpcclient.(*WSClient).notifySubscribers(0xc001402540, {0x1a?, {0x1063ca0?, 0xc0000fda40?}})
июл 16 14:06:04 node04 neofs-node[642022]:         github.com/nspcc-dev/[email protected]/pkg/rpcclient/wsclient.go:696 +0x26e
июл 16 14:06:04 node04 neofs-node[642022]: github.com/nspcc-dev/neo-go/pkg/rpcclient.(*WSClient).wsReader(0xc001402540)
июл 16 14:06:04 node04 neofs-node[642022]:         github.com/nspcc-dev/[email protected]/pkg/rpcclient/wsclient.go:592 +0x625
июл 16 14:06:04 node04 neofs-node[642022]: created by github.com/nspcc-dev/neo-go/pkg/rpcclient.NewWS in goroutine 1
июл 16 14:06:04 node04 neofs-node[642022]:         github.com/nspcc-dev/[email protected]/pkg/rpcclient/wsclient.go:493 +0x51b

Regression

Not sure. Have never seen it but also have never tried the client on this short block time.

Your Environment

Neo-go was taken from this branch: #3508. Not a release but looks like some recent master.

[roman-khimov]

Duplicates #3093.