MM-34389: Reliable websockets: First commit (#17297)

* MM-34389: Reliable websockets: First commit

This is the first commit which makes some basic changes
to get it ready for the actual implementation.

Changes include:
- A config field to conditionally enable it.
- Refactoring the WriteMessage along with setting the deadline
to a separate method.

The basic idea is that the client sends the connection_id
and sequence_number either during the handshake (via query params),
or during challenge_auth (via added parameters in the map).

If the conn_id is empty, then we create a new one and set it.
Otherwise, we get the queues from a connection manager (TBD)
and attach them to WebConn.

```release-note
NONE
```

https://mattermost.atlassian.net/browse/MM-34389

* Incorporate review comments

* Trigger CI

* removing telemetry

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/websocket.go

@@ -5,10 +5,17 @@ package api4
 
 import (
 	"net/http"
+	"strconv"
 
 	"github.com/gorilla/websocket"
 
 	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+)
+
+const (
+	connectionIDParam   = "connection_id"
+	sequenceNumberParam = "sequence_number"
 )
 
 func (api *API) InitWebSocket() {
@@ -31,6 +38,51 @@ func connectWebSocket(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	wc := c.App.NewWebConn(ws, *c.App.Session(), c.App.T, "")
 
+	if *c.App.Config().ServiceSettings.EnableReliableWebSockets {
+		connID := r.URL.Query().Get(connectionIDParam)
+		if connID == "" {
+			// If not present, we assume client is not capable yet, or it's a fresh connection.
+			// We just create a new ID.
+			connID = model.NewId()
+		} else {
+			if !model.IsValidId(connID) {
+				mlog.Error("Invalid connection ID", mlog.String("id", connID))
+				wc.WebSocket.Close()
+				return
+			}
+			// If present, we check if it's present in the connection manager.
+			// TODO: the connection manager internally should forward the request
+			// to the cluster if it does not have it.
+			//
+			// If the connection is not present, then we assume either timeout,
+			// or server restart. In that case, we set a new one.
+			//
+			// Now we get the sequence number
+			seqVal := r.URL.Query().Get(sequenceNumberParam)
+			if seqVal == "" {
+				// Sequence_number must be sent with connection id.
+				// A client must be either non-compliant or fully compliant.
+				mlog.Error("Sequence number not present in websocket request")
+				wc.WebSocket.Close()
+				return
+			}
+			seq, err := strconv.Atoi(seqVal)
+			if err != nil || seq < 0 {
+				mlog.Error("Invalid sequence number set in query param",
+					mlog.String("query", seqVal),
+					mlog.Err(err))
+				wc.WebSocket.Close()
+				return
+			}
+			wc.Sequence = int64(seq)
+			// Now if there have been past entries to be back-filled, we do it.
+			// First we find the right sequence number point.
+			// We start consuming from dead queue first, and then move to active queue
+		}
+		// In case of fresh connection id, sequence number is already zero.
+		wc.SetConnectionID(connID)
+	}
+
 	if c.App.Session().UserId != "" {
 		c.App.HubRegister(wc)
 	}