Minimal reproduction of connecting to Slack's RTM websocket API to receive messages

SlackRtmTlsBugRepo.ps1

[cmdletbinding()]
param()

function Connect-SlackRtm {
    [cmdletbinding()]
    param(
        [string]$Token
    )

    $loginData = @{}

    $url = "https://slack.com/api/rtm.start?token=$($Token)&pretty=1"
    try {
        $r = Invoke-RestMethod -Uri $url -Method Get -Verbose:$false
        if ($r.ok) {
            Write-Verbose 'Successfully authenticated to Slack Real Time API'

            $loginData['WebSocketUrl'] = $r.url
            $loginData['Domain']       = $r.team.domain
            $loginData['UserName']     = $r.self.name
        } else {
            throw $r
        }
    } catch {
        Write-Error 'Error connecting to Slack Real Time API'
        throw $_
    }

    $loginData
}

function Start-SlackWsReceive {
    param(
        [hashtable]$LoginData
    )

    # Slack enforces TLS12 on the websocket API
    [Net.ServicePointManager]::SecurityProtocol = [Net.ServicePointManager]::SecurityProtocol -bor [Net.SecurityProtocolType]::Tls12

    [Net.WebSockets.ClientWebSocket]$webSocket = [Net.WebSockets.ClientWebSocket]::new()

    # Connect to websocket
    Write-Verbose "[SlackBackend:ReceiveJob] Connecting to websocket at [$($LoginData.WebSocketUrl)]"
    $buffer = [Net.WebSockets.WebSocket]::CreateClientBuffer(1024,1024)
    $cts    = [Threading.CancellationTokenSource]::new()
    $task   = $webSocket.ConnectAsync($LoginData.WebSocketUrl, $cts.Token)
    do {
        [Threading.Thread]::Sleep(10)}
    until ($task.IsCompleted)

    $ct         = [Threading.CancellationToken]::new($false)
    $taskResult = $null

    # Maintain websocket connection and put received messages on the output stream
    function Receive-Msg {
        $jsonResult = ""
        do {
            $taskResult = $webSocket.ReceiveAsync($buffer, $ct)
            while (-not $taskResult.IsCompleted) {
                [Threading.Thread]::Sleep(10)
            }
            $jsonResult += [Text.Encoding]::UTF8.GetString($buffer, 0, $taskResult.Result.Count)
        } until (
            $taskResult.Result.EndOfMessage
        )

        if (-not [string]::IsNullOrEmpty($jsonResult)) {
            try {
                $msgs = ConvertFrom-Json -InputObject $jsonResult
            }
            catch {
                throw $Error[0]
            }

            foreach ($msg in $msgs) {
                ConvertTo-Json -InputObject $msg -Compress
            }
        }
    }

    while ($webSocket.State -eq [Net.WebSockets.WebSocketState]::Open) {
        Receive-Msg
    }
}

$loginData = Connect-SlackRtm -Token '<YOUR-SLACK-TOKEN>'
Start-SlackWsReceive -LoginData $loginData