derp/derphttp: don't link websockets other than on GOOS=js

Or unless the new "ts_debug_websockets" build tag is set.

Updates #1278

Change-Id: Ic4c4f81c1924250efd025b055585faec37a5491d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derphttp/derphttp_client.go

@@ -313,6 +313,9 @@ func (c *Client) preferIPv6() bool {
 var dialWebsocketFunc func(ctx context.Context, urlStr string) (net.Conn, error)
 
 func useWebsockets() bool {
+	if !canWebsockets {
+		return false
+	}
 	if runtime.GOOS == "js" {
 		return true
 	}
@@ -383,7 +386,7 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 	var node *tailcfg.DERPNode // nil when using c.url to dial
 	var idealNodeInRegion bool
 	switch {
-	case useWebsockets():
+	case canWebsockets && useWebsockets():
 		var urlStr string
 		if c.url != nil {
 			urlStr = c.url.String()

derp/derphttp/derphttp_test.go

@@ -17,7 +17,9 @@ import (
 
 	"tailscale.com/derp"
 	"tailscale.com/net/netmon"
+	"tailscale.com/tstest/deptest"
 	"tailscale.com/types/key"
+	"tailscale.com/util/set"
 )
 
 func TestSendRecv(t *testing.T) {
@@ -485,3 +487,23 @@ func TestProbe(t *testing.T) {
 		}
 	}
 }
+
+func TestDeps(t *testing.T) {
+	deptest.DepChecker{
+		GOOS:   "darwin",
+		GOARCH: "arm64",
+		BadDeps: map[string]string{
+			"github.com/coder/websocket": "shouldn't link websockets except on js/wasm",
+		},
+	}.Check(t)
+
+	deptest.DepChecker{
+		GOOS:   "darwin",
+		GOARCH: "arm64",
+		Tags:   "ts_debug_websockets",
+		WantDeps: set.Of(
+			"github.com/coder/websocket",
+		),
+	}.Check(t)
+
+}

derp/derphttp/websocket.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build linux || js
+//go:build js || ((linux || darwin) && ts_debug_websockets)
 
 package derphttp
 
@@ -14,6 +14,8 @@ import (
 	"tailscale.com/net/wsconn"
 )
 
+const canWebsockets = true
+
 func init() {
 	dialWebsocketFunc = dialWebsocket
 }

derp/derphttp/websocket_stub.go

@@ -0,0 +1,8 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !(js || ((linux || darwin) && ts_debug_websockets))
+
+package derphttp
+
+const canWebsockets = false