tailcfg,ipn/ipnlocal: add C2NUpdateRequest to c2n /update handler

An optional (for now) request body for POST /update messages to carry
the suggested version. This removes the need from clients to figure out
what the latest available version is.

Updates #6907

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -324,7 +324,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/types/views                                    from tailscale.com/ipn/ipnlocal+
         tailscale.com/util/clientmetric                              from tailscale.com/control/controlclient+
         tailscale.com/util/cloudenv                                  from tailscale.com/net/dns/resolver+
-  LW    tailscale.com/util/cmpver                                    from tailscale.com/net/dns+
+        tailscale.com/util/cmpver                                    from tailscale.com/net/dns+
         tailscale.com/util/cmpx                                      from tailscale.com/derp/derphttp+
      💣 tailscale.com/util/deephash                                  from tailscale.com/ipn/ipnlocal+
    L 💣 tailscale.com/util/dirwalk                                   from tailscale.com/metrics+

ipn/ipnlocal/c2n.go

@@ -21,6 +21,7 @@ import (
 	"tailscale.com/net/sockstats"
 	"tailscale.com/tailcfg"
 	"tailscale.com/util/clientmetric"
+	"tailscale.com/util/cmpver"
 	"tailscale.com/util/goroutines"
 	"tailscale.com/version"
 )
@@ -121,6 +122,16 @@ func (b *LocalBackend) handleC2NUpdate(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	var req tailcfg.C2NUpdateRequest
+	if r.ContentLength > 0 {
+		if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+			b.logf("c2n: bad /update request body: %v", err)
+			http.Error(w, "failed to parse request body", http.StatusBadRequest)
+			return
+		}
+		// TODO(awly): store the requested version somewhere to surface in UI.
+	}
+
 	// If NewUpdater does not return an error, we can update the installation.
 	// Exception: When version.IsMacSysExt returns true, we don't support that
 	// yet. TODO(cpalmer, #6995): Implement it.
@@ -141,6 +152,7 @@ func (b *LocalBackend) handleC2NUpdate(w http.ResponseWriter, r *http.Request) {
 	if r.Method == "GET" {
 		return
 	}
+
 	if !res.Enabled {
 		res.Err = "not enabled"
 		return
@@ -171,7 +183,18 @@ func (b *LocalBackend) handleC2NUpdate(w http.ResponseWriter, r *http.Request) {
 		res.Err = "cmd/tailscale version mismatch"
 		return
 	}
-	cmd := exec.Command(cmdTS, "update", "--yes")
+	var cmd *exec.Cmd
+	if req.LatestVer != "" {
+		if cmpver.Compare(req.LatestVer, version.Short()) > 0 {
+			b.logf("c2n: update to version %q requested", req.LatestVer)
+		} else {
+			res.Err = fmt.Sprintf("requested update version %q is older or the same as current version %q", req.LatestVer, version.Short())
+			return
+		}
+		cmd = exec.Command(cmdTS, "update", "--yes", "--version", req.LatestVer)
+	} else {
+		cmd = exec.Command(cmdTS, "update", "--yes")
+	}
 	if err := cmd.Start(); err != nil {
 		res.Err = fmt.Sprintf("failed to start cmd/tailscale update: %v", err)
 		return

tailcfg/c2ntypes.go

@@ -34,6 +34,15 @@ type C2NSSHUsernamesResponse struct {
 	Usernames []string
 }
 
+// C2NUpdateRequest is the request (from control to node) to the /update
+// handler. It tells the node what the latest version is. The node may choose
+// to ignore this update, notify the user, or apply it automatically (for POST
+// requests only).
+type C2NUpdateRequest struct {
+	// LatestVer is the latest released version.
+	LatestVer string
+}
+
 // C2NUpdateResponse is the response (from node to control) from the /update
 // handler. It tells control the status of its request for the node to update
 // its Tailscale installation.