cmd/tailscaled,ipn: show a health warning when state store fails to open (#17883)

With the introduction of node sealing, store.New fails in some cases due
to the TPM device being reset or unavailable. Currently it results in
tailscaled crashing at startup, which is not obvious to the user until
they check the logs.

Instead of crashing tailscaled at startup, start with an in-memory store
with a health warning about state initialization and a link to (future)
docs on what to do. When this health message is set, also block any
login attempts to avoid masking the problem with an ephemeral node
registration.

Updates #15830
Updates #17654

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

ipn/ipnlocal/local.go

@@ -3747,6 +3747,9 @@ func (b *LocalBackend) StartLoginInteractive(ctx context.Context) error {
 // the control plane sends us one. Otherwise, the notification will be delivered to all
 // active [watchSession]s.
 func (b *LocalBackend) StartLoginInteractiveAs(ctx context.Context, user ipnauth.Actor) error {
+	if b.health.IsUnhealthy(ipn.StateStoreHealth) {
+		return errors.New("cannot log in when state store is unhealthy")
+	}
 	b.mu.Lock()
 	defer b.mu.Unlock()
 	if b.cc == nil {
@@ -5677,6 +5680,9 @@ func (b *LocalBackend) NodeKey() key.NodePublic {
 //
 // b.mu must be held
 func (b *LocalBackend) nextStateLocked() ipn.State {
+	if b.health.IsUnhealthy(ipn.StateStoreHealth) {
+		return ipn.NoState
+	}
 	var (
 		cc         = b.cc
 		cn         = b.currentNode()
@@ -6936,6 +6942,9 @@ func (b *LocalBackend) CurrentProfile() ipn.LoginProfileView {
 
 // NewProfile creates and switches to the new profile.
 func (b *LocalBackend) NewProfile() error {
+	if b.health.IsUnhealthy(ipn.StateStoreHealth) {
+		return errors.New("cannot log in when state store is unhealthy")
+	}
 	b.mu.Lock()
 	defer b.mu.Unlock()