control/controlclient: fix panic regression from earlier load balancer hint header

In the recent 20e9f3369 we made HealthChangeRequest machine requests
include a NodeKey, as it was the oddball machine request that didn't
include one. Unfortunately, that code was sometimes being called (at
least in some of our integration tests) without a node key due to its
registration with health.RegisterWatcher(direct.ReportHealthChange).

Fortunately tests in corp caught this before we cut a release. It's
possible this only affects this particular integration test's
environment, but still worth fixing.

Updates tailscale/corp#1297

Change-Id: I84046779955105763dc1be5121c69fec3c138672
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/direct.go

@@ -1709,7 +1709,10 @@ func (c *Direct) ReportHealthChange(sys health.Subsystem, sysErr error) {
 		// Don't report errors to control if the server doesn't support noise.
 		return
 	}
-	nodeKey := c.GetPersist().PublicNodeKey()
+	nodeKey, ok := c.GetPersist().PublicNodeKeyOK()
+	if !ok {
+		return
+	}
 	req := &tailcfg.HealthChangeRequest{
 		Subsys:  string(sys),
 		NodeKey: nodeKey,

types/persist/persist.go

@@ -51,11 +51,32 @@ func (p *Persist) PublicNodeKey() key.NodePublic {
 	return p.PrivateNodeKey.Public()
 }
 
+// PublicNodeKeyOK returns the public key for the node key.
+//
+// Unlike PublicNodeKey, it returns ok=false if there is no node private key
+// instead of panicking.
+func (p *Persist) PublicNodeKeyOK() (pub key.NodePublic, ok bool) {
+	if p.PrivateNodeKey.IsZero() {
+		return
+	}
+	return p.PrivateNodeKey.Public(), true
+}
+
 // PublicNodeKey returns the public key for the node key.
+//
+// It panics if there is no node private key. See PublicNodeKeyOK.
 func (p PersistView) PublicNodeKey() key.NodePublic {
 	return p.ж.PublicNodeKey()
 }
 
+// PublicNodeKeyOK returns the public key for the node key.
+//
+// Unlike PublicNodeKey, it returns ok=false if there is no node private key
+// instead of panicking.
+func (p PersistView) PublicNodeKeyOK() (_ key.NodePublic, ok bool) {
+	return p.ж.PublicNodeKeyOK()
+}
+
 func (p PersistView) Equals(p2 PersistView) bool {
 	return p.ж.Equals(p2.ж)
 }