CD daemon: send SIGUSR1 to IMEX daemon on node updates

cmd/compute-domain-daemon/controller.go

@@ -111,9 +111,9 @@ func (c *Controller) Run(ctx context.Context) error {
 	return nil
 }
 
-// GetNodesUpdateChan() returns a channel that only ever yields a full set of nodes,
-// i.e. during startup this blocks until the expected number of nodes is present
-// in CD status.
+// GetNodesUpdateChan() returns a channel that yields updates for the nodes
+// currently present in the CD status. This is only a complete set of nodes
+// (size `numNodes`) if IMEXDaemonsWithDNSNames=false.
 func (c *Controller) GetNodesUpdateChan() chan []*nvapi.ComputeDomainNode {
 	return c.computeDomainManager.GetNodesUpdateChan()
 }

cmd/compute-domain-daemon/dnsnames.go

@@ -57,8 +57,10 @@ func NewDNSNameManager(cliqueID string, maxNodesPerIMEXDomain int, nodesConfigPa
 	}
 }
 
-// UpdateDNSNameMappings updates the /etc/hosts file with any new IP to DNS name mappings.
-func (m *DNSNameManager) UpdateDNSNameMappings(nodes []*nvapi.ComputeDomainNode) error {
+// UpdateDNSNameMappings updates the /etc/hosts file with any new IP to DNS name
+// mappings. The boolean return value indicates whether the hosts file was
+// updated or not (it must be ignored when the returned error is non-nil).
+func (m *DNSNameManager) UpdateDNSNameMappings(nodes []*nvapi.ComputeDomainNode) (bool, error) {
 	m.Lock()
 	defer m.Unlock()
 
@@ -78,7 +80,7 @@ func (m *DNSNameManager) UpdateDNSNameMappings(nodes []*nvapi.ComputeDomainNode)
 		// Construct the DNS name from the node index
 		dnsName, err := m.constructDNSName(node)
 		if err != nil {
-			return fmt.Errorf("failed to allocate DNS name for IP %s: %w", node.IPAddress, err)
+			return false, fmt.Errorf("failed to allocate DNS name for IP %s: %w", node.IPAddress, err)
 		}
 
 		// Assign the IP -> DNS name mapping
@@ -87,14 +89,14 @@ func (m *DNSNameManager) UpdateDNSNameMappings(nodes []*nvapi.ComputeDomainNode)
 
 	// If the existing ipToDNSName mappings are unchanged, exit early
 	if maps.Equal(ipToDNSName, m.ipToDNSName) {
-		return nil
+		return false, nil
 	}
 
 	// Otherwise, update the cached ipToDNSName mapping
 	m.ipToDNSName = ipToDNSName
 
-	// And updated the hosts file with new mappings
-	return m.updateHostsFile()
+	// And update the hosts file with the new mapping
+	return true, m.updateHostsFile()
 }
 
 // LogDNSNameMappings logs the current compute-domain-daemon mappings from memory.

cmd/compute-domain-daemon/main.go

@@ -322,13 +322,37 @@ func IMEXDaemonUpdateLoopWithDNSNames(ctx context.Context, controller *Controlle
 			klog.Infof("shutdown: stop IMEXDaemonUpdateLoopWithDNSNames")
 			return nil
 		case nodes := <-controller.GetNodesUpdateChan():
-			if err := dnsNameManager.UpdateDNSNameMappings(nodes); err != nil {
+			updated, err := dnsNameManager.UpdateDNSNameMappings(nodes)
+			if err != nil {
 				return fmt.Errorf("failed to update DNS name => IP mappings: %w", err)
 			}
-			if err := processManager.EnsureStarted(); err != nil {
+
+			fresh, err := processManager.EnsureStarted()
+			if err != nil {
 				return fmt.Errorf("failed to ensure IMEX daemon is started: %w", err)
 			}
+
 			dnsNameManager.LogDNSNameMappings()
+
+			// Skip sending SIGUSR1 when the process is fresh (has newly been
+			// created) or when thiss was a noop update. TODO: review skipping
+			// this also if the new set of IP addresses only strictly removes
+			// addresses compared to the old set (then we don't need to force
+			// the daemon to re-resolve & re-connect).
+			if !updated || fresh {
+				continue
+			}
+
+			// Actively ask the IMEX daemon to re-read its config and to
+			// re-connect to its peers (involving DNS name re-resolution).
+			klog.Infof("updated DNS/IP mapping, old process: send SIGUSR1")
+			if err := processManager.Signal(syscall.SIGUSR1); err != nil {
+				// Only log (ignore this error for now: if the process went away
+				// unexpectedly, the process manager will handle that. If any
+				// other error resulted in bad signal delivery, we may get away
+				// with it).
+				klog.Errorf("failed to send SIGUSR1 to child process: %s", err)
+			}
 		}
 	}
 }

cmd/compute-domain-daemon/process.go

@@ -56,12 +56,27 @@ func (m *ProcessManager) Restart() error {
 	return m.start()
 }
 
-// EnsureStarted starts the process if it is not already running. If the process is already started, this is a no-op.
-func (m *ProcessManager) EnsureStarted() error {
+// EnsureStarted starts the process if it is not already running. If the process
+// is already started, this is a no-op. The boolean return value indicates
+// `new`, i.e. it is `true` if the process was _newly_ started. It must be
+// ignored when the returned error is non-nil.
+func (m *ProcessManager) EnsureStarted() (bool, error) {
 	if m.handle != nil {
-		return nil
+		return false, nil
 	}
-	return m.start()
+	return true, m.start()
+}
+
+// Signal() attempts to send the provided signal to the managed child process.
+// Any error is emitted to the caller and must be handled there.
+func (m *ProcessManager) Signal(s os.Signal) error {
+	m.Lock()
+	defer m.Unlock()
+
+	if m.handle == nil {
+		return fmt.Errorf("pm: sending signal %s failed: not started", s)
+	}
+	return m.handle.Process.Signal(s)
 }
 
 func (m *ProcessManager) start() error {