[core] nil check and error handling for client status in heartbeat responses (#17316)

Add a nil check to constructNodeServerInfoResponse to manage an apparent race between deregister and client heartbeats. Fixes #17310

.changelog/17316.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+core: Fix panic around client deregistration and pending heartbeats
+```

client/client.go

@@ -2024,7 +2024,8 @@ func (c *Client) updateNodeStatus() error {
 	}
 
 	// Check heartbeat response for information about the server-side scheduling
-	// state of this node
+	// state of this node. If there are errors on the server side, this will come
+	// back as an empty string.
 	c.UpdateConfig(func(c *config.Config) {
 		if resp.SchedulingEligibility != "" {
 			c.Node.SchedulingEligibility = resp.SchedulingEligibility

nomad/node_endpoint.go

@@ -286,7 +286,7 @@ func equalDevices(n1, n2 *structs.Node) bool {
 	return reflect.DeepEqual(n1.NodeResources.Devices, n2.NodeResources.Devices)
 }
 
-// updateNodeUpdateResponse assumes the n.srv.peerLock is held for reading.
+// constructNodeServerInfoResponse assumes the n.srv.peerLock is held for reading.
 func (n *Node) constructNodeServerInfoResponse(nodeID string, snap *state.StateSnapshot, reply *structs.NodeUpdateResponse) error {
 	reply.LeaderRPCAddr = string(n.srv.raft.Leader())
 
@@ -300,16 +300,30 @@ func (n *Node) constructNodeServerInfoResponse(nodeID string, snap *state.StateS
 			})
 	}
 
+	ws := memdb.NewWatchSet()
+
 	// Add ClientStatus information to heartbeat response.
-	node, _ := snap.NodeByID(nil, nodeID)
-	reply.SchedulingEligibility = node.SchedulingEligibility
+	if node, err := snap.NodeByID(ws, nodeID); err == nil && node != nil {
+		reply.SchedulingEligibility = node.SchedulingEligibility
+	} else if node == nil {
+
+		// If the node is not found, leave reply.SchedulingEligibility as
+		// the empty string. The response handler in the client treats this
+		// as a no-op. As there is no call to action for an operator, log it
+		// at debug level.
+		n.logger.Debug("constructNodeServerInfoResponse: node not found",
+			"node_id", nodeID)
+	} else {
+
+		// This case is likely only reached via a code error in state store
+		return err
+	}
 
 	// TODO(sean@): Use an indexed node count instead
 	//
 	// Snapshot is used only to iterate over all nodes to create a node
 	// count to send back to Nomad Clients in their heartbeat so Clients
 	// can estimate the size of the cluster.
-	ws := memdb.NewWatchSet()
 	iter, err := snap.Nodes(ws)
 	if err == nil {
 		for {

nomad/node_endpoint_test.go

@@ -4434,3 +4434,23 @@ func TestNode_List_PaginationFiltering(t *testing.T) {
 		})
 	}
 }
+
+func TestNode_constructNodeServerInfoResponse_MissingNode(t *testing.T) {
+	ci.Parallel(t)
+
+	s, cleanup := TestServer(t, nil)
+	defer cleanup()
+	testutil.WaitForLeader(t, s.RPC)
+
+	// Create a node that isn't a member of the state to force a not found
+	node := mock.Node()
+	var reply structs.NodeUpdateResponse
+
+	nE := NewNodeEndpoint(s, nil)
+	snap, err := s.State().Snapshot()
+	must.NoError(t, err)
+
+	// call constructNodeServerInfoResponse. Before GH #17316 this would panic
+	require.NoError(t, nE.constructNodeServerInfoResponse(node.ID, snap, &reply))
+	must.NotNil(t, &reply)
+}