cmd/tailscale: combine serve and funnel for debug wip funnel stream model (#9169)

> **Note**
> Behind the `TAILSCALE_USE_WIP_CODE` flag

In preparing for incoming CLI changes, this PR merges the code path for the `serve` and `funnel` subcommands.

See the parent issue for more context.

The following commands will run in foreground mode when using the environment flag.
```
tailscale serve localhost:3000
tailscae funnel localhost:3000
```

Replaces #9134
Updates #8489

Signed-off-by: Tyler Smalley <tyler@tailscale.com>
Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>
Co-authored-by: Marwan Sulaiman <marwan@tailscale.com>

ipn/ipnlocal/serve.go

@@ -356,10 +356,12 @@ func setHandler(sc *ipn.ServeConfig, req ipn.ServeStreamRequest) {
 	wsc.Handlers[req.MountPoint] = &ipn.HTTPHandler{
 		Proxy: req.Source,
 	}
-	if sc.AllowFunnel == nil {
-		sc.AllowFunnel = make(map[ipn.HostPort]bool)
+	if req.Funnel {
+		if sc.AllowFunnel == nil {
+			sc.AllowFunnel = make(map[ipn.HostPort]bool)
+		}
+		sc.AllowFunnel[req.HostPort] = true
 	}
-	sc.AllowFunnel[req.HostPort] = true
 }
 
 func deleteHandler(sc *ipn.ServeConfig, req ipn.ServeStreamRequest, port uint16) {

ipn/localapi/localapi.go

@@ -864,6 +864,10 @@ func (h *Handler) serveServeConfig(w http.ResponseWriter, r *http.Request) {
 // serveStreamServe handles foreground serve and funnel streams. This is
 // currently in development per https://github.com/tailscale/tailscale/issues/8489
 func (h *Handler) serveStreamServe(w http.ResponseWriter, r *http.Request) {
+	if !envknob.UseWIPCode() {
+		http.Error(w, "stream serve not yet available", http.StatusNotImplemented)
+		return
+	}
 	if !h.PermitWrite {
 		// Write permission required because we modify the ServeConfig.
 		http.Error(w, "serve stream denied", http.StatusForbidden)

ipn/serve.go

@@ -93,6 +93,10 @@ type ServeStreamRequest struct {
 	// MountPoint is the path prefix for
 	// the given HostPort.
 	MountPoint string `json:",omitempty"`
+
+	// Funnel indicates whether the request
+	// is a serve request or a funnel one.
+	Funnel bool `json:",omitempty"`
 }
 
 // FunnelRequestLog is the JSON type written out to io.Writers