Formatting that message helps make it understandable:

{
  :function_clause,
  [
    {
      WebSockAdapter.CowboyAdapter,
      :terminate,
      [
        {:crash, :error, :function_clause},
        %{
          port: 80,
          scheme: "http",
          version: :"HTTP/1.1",
          path: "/live/websocket",
          host: "app.domain.com",
          peer: {{0, 0, 0, 0, 0, 65535, 44063, 6462}, 37060},
          method: "GET",
          qs: "_csrf_token=FQJ-CHkAYWVEDy5BejkzQhItNCouEx43ERGNMF55sKcqWQQtzDehxJSP&_track_static%5B0%5D=https%3A%2F%2Fapp.domain.com%2Fassets%2Fapp-4d990432c83d65c7274ba6c2f26005e3.css%3Fvsn%3Dd&_track_static%5B1%5D=https%3A%2F%2Fapp.domain.com%2Fassets%2Fapp-a33e271c2b56bde39457fdd88e722922.js%3Fvsn%3Dd&_mounts=0&_live_referer=undefined&vsn=2.0.0"
        },
        {
          Phoenix.LiveView.Socket,
          %{},
          {
            %{channels: %{}, channels_inverse: %{}},
            %Phoenix.Socket{
              assigns: %{},
              channel: nil,
              channel_pid: nil,
              endpoint: AppWeb.Endpoint,
              handler: Phoenix.LiveView.Socket,
              id: "users_sessions:n66qO3JS6Pqw7v2Bcx6iwnQYTasR4SwhP8tDgaNs9cg=",
              joined: false,
              join_ref: nil,
              private: %{
                connect_info: %{
                  session: %{
                    "_csrf_token" => "PP9F4FTP7DM0-hb6hiQBVYMg",
                    "live_socket_id" => "users_sessions:n66qO3JS6Pqw7v2Bcx6iwnQYTasR4SwhP8tDgaNs9cg=",
                    "user_token" => <<159, 174, 170, 59, 114, 82, 232, 250, 176, 238, 253, 129, 115, 30, 162, 194, 116, 24, 77, 171, 17, 225, ...>>
                  }
                }
              },
              pubsub_server: [
                adapter: Phoenix.PubSub.Redis,
                name: App.PubSub,
                node_name: "node-0qw2do37A_U=",
                url: "redis://master.redacted.cache.amazonaws.com:6379",
                ssl: true
              ],
              ref: nil,
              serializer: Phoenix.Socket.V2.JSONSerializer,
              topic: nil,
              transport: :websocket,
              transport_pid: nil
            }
          }
        }
      ],
      [file: ~c"lib/websock_adapter/cowboy_adapter.ex", line: 52]
    },
    {:cowboy_websocket, :handler_call, 6, [file: ~c"/app/deps/cowboy/src/cowboy_websocket.erl", line: 564]},
    {:cowboy_http, :loop, 1, [file: ~c"/app/deps/cowboy/src/cowboy_http.erl", line: 253]},
    {:proc_lib, :init_p_do_apply, 3, [file: ~c"proc_lib.erl", line: 241]}
  ]
}

This tells us some things:

• the ultimate cause of the exit was a function_clause error in lib/websock_adapter/cowboy_adapter.ex:52

• This error happened because WebSockAdapter.CowboyAdapter.terminate/3 only expects 2-tuples in the final argument, but the caller passed a 3-tuple shaped like {Phoenix.LiveView.Socket, %{}, {%{channels: ...}, %Phoenix.Socket{}}}

  The first element of that tuple is a handler like the module expects.

  The third element is a {state, socket} pair like would be expected with Phoenix.Socket

  The second element is ???, and it’s going to be tricky to find where it came from

• however, this isn’t the root cause - the process was trying to call terminate/3 because something else crashed with a function_clause_error. Any noises in the logs before the message you posted?

My suspicion is a cascading error like this:

• something messed up the socket GenServer’s state from {handler, state} to {handler, what_the_heck, state}
• that caused the next function call dispatched against that server to fail to match (the first function_clause_error)
• that made the socket try to shut down, which then ALSO failed to match in terminate/3 (the second function_clause_error)