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

@@ -12,15 +12,18 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"regexp"
 	"runtime"
 	"strconv"
+	"sync"
 	"time"
 
 	"tailscale.com/clientupdate"
 	"tailscale.com/envknob"
 	"tailscale.com/net/sockstats"
 	"tailscale.com/tailcfg"
 	"tailscale.com/util/clientmetric"
+	"tailscale.com/util/cmpver"
 	"tailscale.com/util/goroutines"
 	"tailscale.com/version"
 )
@@ -121,6 +124,15 @@ 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
+		}
+	}
+
 	// 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.
@@ -139,8 +151,16 @@ func (b *LocalBackend) handleC2NUpdate(w http.ResponseWriter, r *http.Request) {
 	}()
 
 	if r.Method == "GET" {
+		if req.LatestVersion != "" {
+			if err := validateUpdateVersion(req.LatestVersion, false); err != nil {
+				res.Err = fmt.Sprintf("LatestVersion is invalid: %v", err)
+				return
+			}
+			// TODO(awly): store req.LatestVersion somewhere to surface in UI.
+		}
 		return
 	}
+
 	if !res.Enabled {
 		res.Err = "not enabled"
 		return
@@ -171,7 +191,17 @@ 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.LatestVersion != "" {
+		if err := validateUpdateVersion(req.LatestVersion, true); err != nil {
+			res.Err = err.Error()
+			return
+		}
+		b.logf("c2n: update to version %q requested", req.LatestVersion)
+		cmd = exec.Command(cmdTS, "update", "--yes", "--version", req.LatestVersion)
+	} 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
@@ -189,6 +219,18 @@ func (b *LocalBackend) handleC2NUpdate(w http.ResponseWriter, r *http.Request) {
 	return
 }
 
+var versionRe = sync.OnceValue(func() *regexp.Regexp { return regexp.MustCompile(`^\d+\.\d+\.\d+$`) })
+
+func validateUpdateVersion(ver string, expectNewer bool) error {
+	if !versionRe().MatchString(ver) {
+		return fmt.Errorf("requested update version %q is malformed", ver)
+	}
+	if expectNewer && cmpver.Compare(ver, version.Short()) <= 0 {
+		return fmt.Errorf("requested update version %q is older or the same as current version %q", ver, version.Short())
+	}
+	return nil
+}
+
 // findCmdTailscale looks for the cmd/tailscale that corresponds to the
 // currently running cmd/tailscaled. It's up to the caller to verify that the
 // two match, but this function does its best to find the right one. Notably, it

tailcfg/c2ntypes.go

@@ -34,6 +34,18 @@ type C2NSSHUsernamesResponse struct {
 	Usernames []string
 }
 
+// C2NUpdateRequest is the request (from control to node) to the c2n /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) based on configuration.
+type C2NUpdateRequest struct {
+	// LatestVersion is the latest released version in the "x.y.z" format.
+	//
+	// The node should not blindly trust this version and perform validation
+	// against downgrades and malformed version strings.
+	LatestVersion 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.