Invalidate host context provider cache when FQDN feature flag is toggled

internal/pkg/composable/providers/host/host.go

@@ -49,13 +49,29 @@ func (c *contextProvider) Run(comm corecomp.ContextProviderComm) error {
 		return errors.New(err, "failed to set mapping", errors.TypeUnexpected)
 	}
 
+	const fqdnFeatureFlagCallbackID = "host_provider"
+	fqdnFFChangeCh := make(chan struct{})
+	err = features.AddFQDNOnChangeCallback(
+		onFQDNFeatureFlagChange(fqdnFFChangeCh),
+		fqdnFeatureFlagCallbackID,
+	)
+	if err != nil {
+		return fmt.Errorf("unable to add FQDN onChange callback in host provider: %w", err)
+	}
+
+	defer func() {
+		features.RemoveFQDNOnChangeCallback(fqdnFeatureFlagCallbackID)
+		close(fqdnFFChangeCh)
+	}()
+
 	// Update context when any host information changes.
 	for {
 		t := time.NewTimer(c.CheckInterval)
 		select {
 		case <-comm.Done():
 			t.Stop()
 			return comm.Err()
+		case <-fqdnFFChangeCh:
 		case <-t.C:
 		}
 
@@ -76,6 +92,13 @@ func (c *contextProvider) Run(comm corecomp.ContextProviderComm) error {
 	}
 }
 
+func onFQDNFeatureFlagChange(fqdnFFChangeCh chan struct{}) features.BoolValueOnChangeCallback {
+	return func(new, old bool) {
+		// FQDN feature flag was toggled, so notify on channel
+		fqdnFFChangeCh <- struct{}{}
+	}
+}
+
 // ContextProviderBuilder builds the context provider.
 func ContextProviderBuilder(log *logger.Logger, c *config.Config, _ bool) (corecomp.ContextProvider, error) {
 	p := &contextProvider{

internal/pkg/composable/providers/host/host_test.go

@@ -7,6 +7,8 @@ package host
 import (
 	"context"
 
+	"github.com/elastic/elastic-agent/pkg/features"
+
 	"sync"
 	"testing"
 	"time"
@@ -82,6 +84,66 @@ func TestContextProvider(t *testing.T) {
 	assert.Equal(t, next, comm.Current())
 }
 
+func TestFQDNFeatureFlagToggle(t *testing.T) {
+	log, err := logger.New("host_test", false)
+	require.NoError(t, err)
+
+	c, err := config.NewConfigFrom(map[string]interface{}{
+		// Use a long check interval so we can ensure that any
+		// calls to hostProvider.fetcher are not happening due
+		// to the interval timer elapsing. We want such calls
+		// to happen only due to explicit actions in our
+		// test below.
+		"check_interval": 10 * time.Minute,
+	})
+	require.NoError(t, err)
+
+	builder, _ := composable.Providers.GetContextProvider("host")
+	provider, err := builder(log, c, true)
+	require.NoError(t, err)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer func() {
+		cancel()
+	}()
+	comm := ctesting.NewContextComm(ctx)
+
+	// Track the number of times hostProvider.fetcher is called.
+	numCalled := 0
+	hostProvider, ok := provider.(*contextProvider)
+	require.True(t, ok)
+
+	hostProvider.fetcher = func() (map[string]interface{}, error) {
+		numCalled++
+		return nil, nil
+	}
+
+	// Run the provider
+	go func() {
+		err = provider.Run(comm)
+	}()
+
+	// Wait long enough for provider.Run to register
+	// the FQDN feature flag onChange callback.
+	time.Sleep(10 * time.Millisecond)
+
+	// Trigger the FQDN feature flag callback by
+	// toggling the FQDN feature flag
+	err = features.Apply(config.MustNewConfigFrom(map[string]interface{}{
+		"agent.features.fqdn.enabled": true,
+	}))
+	require.NoError(t, err)
+
+	// Wait long enough for the FQDN feature flag onChange
+	// callback to be called.
+	time.Sleep(10 * time.Millisecond)
+
+	// hostProvider.fetcher should be called twice:
+	// - once, right after the provider is run, and
+	// - once again, when the FQDN feature flag callback is triggered
+	require.Equal(t, 2, numCalled)
+}
+
 func returnHostMapping(log *logger.Logger) infoFetcher {
 	i := -1
 	fetcher := getHostInfo(log)

pkg/features/features.go

@@ -19,10 +19,14 @@ var (
 	current = Flags{}
 )
 
+type BoolValueOnChangeCallback func(new, old bool)
+
 type Flags struct {
 	mu     sync.RWMutex
 	source *structpb.Struct
-	fqdn   bool
+
+	fqdn          bool
+	fqdnCallbacks map[string]BoolValueOnChangeCallback
 }
 
 type cfg struct {
@@ -51,12 +55,42 @@ func (f *Flags) AsProto() *proto.Features {
 	}
 }
 
+// AddFQDNOnChangeCallback takes a callback function that will be called with the new and old values
+// of `flags.fqdnEnabled` whenever it changes. It also takes a string ID - this is useful
+// in calling `RemoveFQDNOnChangeCallback` to de-register the callback.
+func AddFQDNOnChangeCallback(cb BoolValueOnChangeCallback, id string) error {
+	current.mu.Lock()
+	defer current.mu.Unlock()
+
+	// Initialize callbacks map if necessary.
+	if current.fqdnCallbacks == nil {
+		current.fqdnCallbacks = map[string]BoolValueOnChangeCallback{}
+	}
+
+	current.fqdnCallbacks[id] = cb
+	return nil
+}
+
+// RemoveFQDNOnChangeCallback removes the callback function associated with the given ID (originally
+// returned by `AddFQDNOnChangeCallback` so that function will be no longer be called when
+// `flags.fqdnEnabled` changes.
+func RemoveFQDNOnChangeCallback(id string) {
+	current.mu.Lock()
+	defer current.mu.Unlock()
+
+	delete(current.fqdnCallbacks, id)
+}
+
 // setFQDN sets the value of the FQDN flag in Flags.
 func (f *Flags) setFQDN(newValue bool) {
 	f.mu.Lock()
 	defer f.mu.Unlock()
 
+	oldValue := f.fqdn
 	f.fqdn = newValue
+	for _, cb := range f.fqdnCallbacks {
+		cb(newValue, oldValue)
+	}
 }
 
 // setSource sets the source from he given cfg.

pkg/features/features_test.go

@@ -7,6 +7,8 @@ package features
 import (
 	"testing"
 
+	"github.com/stretchr/testify/require"
+
 	"github.com/elastic/elastic-agent/internal/pkg/config"
 )
 
@@ -79,3 +81,38 @@ agent:
 		})
 	}
 }
+
+func TestFQDNCallbacks(t *testing.T) {
+	cb1Called, cb2Called := false, false
+
+	err := AddFQDNOnChangeCallback(func(new, old bool) {
+		cb1Called = true
+	}, "cb1")
+	require.NoError(t, err)
+
+	err = AddFQDNOnChangeCallback(func(new, old bool) {
+		cb2Called = true
+	}, "cb2")
+	require.NoError(t, err)
+
+	defer func() {
+		// Cleanup in case we don't get to the end of
+		// this test successfully.
+		if _, exists := current.fqdnCallbacks["cb1"]; exists {
+			RemoveFQDNOnChangeCallback("cb1")
+		}
+		if _, exists := current.fqdnCallbacks["cb2"]; exists {
+			RemoveFQDNOnChangeCallback("cb2")
+		}
+	}()
+
+	require.Len(t, current.fqdnCallbacks, 2)
+	current.setFQDN(false)
+	require.True(t, cb1Called)
+	require.True(t, cb2Called)
+
+	RemoveFQDNOnChangeCallback("cb1")
+	require.Len(t, current.fqdnCallbacks, 1)
+	RemoveFQDNOnChangeCallback("cb2")
+	require.Len(t, current.fqdnCallbacks, 0)
+}