ipn: add IPCVersion override func

I've done this a handful of times in the past and again today.
Time to make it a supported thing for the future.

Used while debugging tailscale/corp#4559 (macsys CLI issues)

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

client/tailscale/tailscale.go

@@ -36,7 +36,6 @@ import (
 	"tailscale.com/paths"
 	"tailscale.com/safesocket"
 	"tailscale.com/tailcfg"
-	"tailscale.com/version"
 )
 
 var (
@@ -106,8 +105,8 @@ func DoLocalRequest(req *http.Request) (*http.Response, error) {
 func doLocalRequestNiceError(req *http.Request) (*http.Response, error) {
 	res, err := DoLocalRequest(req)
 	if err == nil {
-		if server := res.Header.Get("Tailscale-Version"); server != "" && server != version.Long && onVersionMismatch != nil {
+		if server := res.Header.Get("Tailscale-Version"); server != "" && server != ipn.IPCVersion() && onVersionMismatch != nil {
-			onVersionMismatch(version.Long, server)
+			onVersionMismatch(ipn.IPCVersion(), server)
 		}
 		if res.StatusCode == 403 {
 			all, _ := ioutil.ReadAll(res.Body)

cmd/tailscaled/depaware.txt

@@ -274,7 +274,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/util/uniq                                      from tailscale.com/wgengine/magicsock
         tailscale.com/util/winutil                                   from tailscale.com/cmd/tailscaled+
    W 💣 tailscale.com/util/winutil/vss                               from tailscale.com/util/winutil
-        tailscale.com/version                                        from tailscale.com/client/tailscale+
+        tailscale.com/version                                        from tailscale.com/cmd/tailscaled+
         tailscale.com/version/distro                                 from tailscale.com/cmd/tailscaled+
    W    tailscale.com/wf                                             from tailscale.com/cmd/tailscaled
         tailscale.com/wgengine                                       from tailscale.com/cmd/tailscaled+

ipn/message.go

@@ -15,6 +15,7 @@ import (
 	"log"
 	"time"
 
+	"tailscale.com/envknob"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/structs"
@@ -116,7 +117,7 @@ func (bs *BackendServer) send(n Notify) {
 	if bs.sendNotifyMsg == nil {
 		return
 	}
-	n.Version = version.Long
+	n.Version = ipcVersion
 	bs.sendNotifyMsg(n)
 }
 
@@ -153,9 +154,9 @@ func (bs *BackendServer) GotCommandMsg(ctx context.Context, b []byte) error {
 const ErrMsgPermissionDenied = "permission denied"
 
 func (bs *BackendServer) GotCommand(ctx context.Context, cmd *Command) error {
-	if cmd.Version != version.Long && !cmd.AllowVersionSkew {
+	if cmd.Version != ipcVersion && !cmd.AllowVersionSkew {
 		vs := fmt.Sprintf("GotCommand: Version mismatch! frontend=%#v backend=%#v",
-			cmd.Version, version.Long)
+			cmd.Version, ipcVersion)
 		bs.logf("%s", vs)
 		// ignore the command, but send a message back to the
 		// caller so it can realize the version mismatch too.
@@ -228,6 +229,19 @@ func NewBackendClient(logf logger.Logf, sendCommandMsg func(jsonb []byte)) *Back
 	}
 }
 
+// IPCVersion returns version.Long usually, unless TS_DEBUG_FAKE_IPC_VERSION is
+// set, in which it contains that value. This is only used for weird development
+// cases when testing mismatched versions and you want the client to act like it's
+// compatible with the server.
+func IPCVersion() string {
+	if v := envknob.String("TS_DEBUG_FAKE_IPC_VERSION"); v != "" {
+		return v
+	}
+	return version.Long
+}
+
+var ipcVersion = IPCVersion()
+
 func (bc *BackendClient) GotNotifyMsg(b []byte) {
 	if len(b) == 0 {
 		// not interesting
@@ -240,9 +254,9 @@ func (bc *BackendClient) GotNotifyMsg(b []byte) {
 	if err := json.Unmarshal(b, &n); err != nil {
 		log.Fatalf("BackendClient.Notify: cannot decode message (length=%d, %#q): %v", len(b), b, err)
 	}
-	if n.Version != version.Long && !bc.AllowVersionSkew {
+	if n.Version != ipcVersion && !bc.AllowVersionSkew {
 		vs := fmt.Sprintf("GotNotify: Version mismatch! frontend=%#v backend=%#v",
-			version.Long, n.Version)
+			ipcVersion, n.Version)
 		bc.logf("%s", vs)
 		// delete anything in the notification except the version,
 		// to prevent incorrect operation.
@@ -257,7 +271,7 @@ func (bc *BackendClient) GotNotifyMsg(b []byte) {
 }
 
 func (bc *BackendClient) send(cmd Command) {
-	cmd.Version = version.Long
+	cmd.Version = ipcVersion
 	b, err := json.Marshal(cmd)
 	if err != nil {
 		log.Fatalf("Failed json.Marshal(cmd): %v\n", err)