Segmentation fault when closing the client with server disconnected

[cailloumajor]

Description

When the client is trying to reconnect to the server, closing it with Close or CloseWithContext method causes a segmentation fault.

I investigated and found that the connection (conn field of the Client struct) becomes nil in Dial method during the process of trying to reconnect:

opcua/client.go

Line 534 in 166c1c7

c.conn, err = d.Dial(ctx, c.endpointURL)

Later, closing the client causes c.conn.Close method to be called on a nil pointer:

opcua/client.go

Line 588 in 166c1c7

c.conn.Close()

This ends trying to dereference a nil pointer in the method:

opcua/uacp/conn.go

Lines 215 to 218 in 166c1c7

	func (c *Conn) close() error {
	debug.Printf("uacp %d: close", c.id)
	return c.TCPConn.Close()
	}

Reproduction

main.go:

package main

import (
	"context"
	"flag"
	"log"
	"os"
	"os/signal"

	"github.com/gopcua/opcua"
)

func main() {
	endpoint := flag.String("endpoint", "", "OPC-UA server endpoint URL")
	flag.Parse()
	client := opcua.NewClient(*endpoint)
	if err := client.Connect(context.Background()); err != nil {
		log.Fatalf("error connecting: %v", err)
	}
	sc := make(chan os.Signal, 1)
	signal.Notify(sc, os.Interrupt)
	<-sc
	if err := client.CloseWithContext(context.Background()); err != nil {
		log.Printf("error closing: %v\n", err)
	}
}

1. With an OPC-UA server ready to accept connections (anonymous auth and no message security), run the code in above main.go:

$ OPC_DEBUG=debug go run ./main.go -endpoint opc.tcp://172.17.0.3:4840
debug: uacp: connecting to opc.tcp://172.17.0.3:4840
debug: uacp 1: start HEL/ACK handshake
debug: uacp 1: sent HELF with 57 bytes
debug: uacp 1: recv ACKF with 28 bytes
debug: uacp 1: server has no chunk limit. Using 512
debug: uacp 1: server has no message size limit. Using 2097152
debug: uacp 1: recv &uacp.Acknowledge{Version:0x0, ReceiveBufSize:0xffff, SendBufSize:0xffff, MaxMessageSize:0x200000, MaxChunkCount:0x200}
debug: uasc 1/1: send *ua.OpenSecureChannelRequest with 132 bytes
debug: uacp 1: recv OPNF with 135 bytes
debug: uasc 1/1: recv OPNF with 135 bytes
debug: uasc 1/1: recv *ua.OpenSecureChannelResponse
debug: uasc 1/1: sending *ua.OpenSecureChannelResponse to handler
debug: uasc 1: received security token. channelID=1 tokenID=1 createdAt=2022-03-22T08:32:11Z lifetime=10m0s
debug: uasc 1: security token is refreshed at 2022-03-22T08:39:41Z (7m30s). channelID=1 tokenID=1
debug: uasc 1/2: send *ua.CreateSessionRequest with 264 bytes
debug: uacp 1: recv MSGF with 5487 bytes
debug: uasc 1/2: recv MSGF with 5487 bytes
debug: uasc 1: security token expires at 2022-03-22T08:44:41Z. channelID=1 tokenID=1
debug: uasc 1/2: recv *ua.CreateSessionResponse
debug: uasc 1/2: sending *ua.CreateSessionResponse to handler
debug: uasc 1/3: send *ua.ActivateSessionRequest with 146 bytes
debug: uacp 1: recv MSGF with 96 bytes
debug: uasc 1/3: recv MSGF with 96 bytes
debug: uasc 1/3: recv *ua.ActivateSessionResponse
debug: uasc 1/3: sending *ua.ActivateSessionResponse to handler
debug: client: monitor: start
debug: uasc 1/4: send *ua.ReadRequest with 111 bytes
debug: sub: pause
debug: uacp 1: recv MSGF with 142 bytes
debug: uasc 1/4: recv MSGF with 142 bytes
debug: uasc 1/4: recv *ua.ReadResponse
debug: uasc 1/4: sending *ua.ReadResponse to handler

3. Now stop the OPC-UA server, causing the client to try to reconnect

debug: uasc 1: readChunk EOF
debug: client: monitor: disconnected
debug: client: monitor: auto-reconnecting
debug: client: monitor: action: createSecureChannel
debug: uacp 1: close
debug: sub: pause: pause
debug: uasc 1: Close()
debug: client: monitor: trying to recreate secure channel
debug: uacp: connecting to opc.tcp://172.17.0.3:4840
debug: client: monitor: trying to recreate secure channel
debug: uacp: connecting to opc.tcp://172.17.0.3:4840

4. Hit Ctrl+C

^Cdebug: client: monitor: done
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x5820f6]

goroutine 1 [running]:
github.com/gopcua/opcua/uacp.(*Conn).Close(0xc0001fe0e0)
        /go/src/github.com/gopcua/opcua/uacp/conn.go:211 +0x36
github.com/gopcua/opcua.(*Client).CloseWithContext(0xc0001fe0e0, {0x6e1cb0, 0xc00001c0d8})
        /go/src/github.com/gopcua/opcua/client.go:588 +0x12a
main.main()
        /workspaces/gopcua-sigsev-repro/main.go:23 +0x16f
exit status 2

[cailloumajor]

Thank you.
And sorry for the dup...