fixes to extra-record file watcher (#2298)

* Fix excess error message during writes

Fixes #2290

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

* retry filewatcher on removed files

This should handled if files are deleted and added again, and for rename
scenarios.

Fixes #2289

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

* test more write and remove in filewatcher

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

---------

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/dns/extrarecords.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"sync"
 
+	"github.com/cenkalti/backoff/v4"
 	"github.com/fsnotify/fsnotify"
 	"github.com/rs/zerolog/log"
 	"tailscale.com/tailcfg"
@@ -83,12 +84,39 @@ func (e *ExtraRecordsMan) Run() {
 				log.Error().Caller().Msgf("file watcher event channel closing")
 				return
 			}
-
-			log.Trace().Caller().Str("path", event.Name).Str("op", event.Op.String()).Msg("extra records received filewatch event")
-			if event.Name != e.path {
-				continue
+			switch event.Op {
+			case fsnotify.Create, fsnotify.Write, fsnotify.Chmod:
+				log.Trace().Caller().Str("path", event.Name).Str("op", event.Op.String()).Msg("extra records received filewatch event")
+				if event.Name != e.path {
+					continue
+				}
+				e.updateRecords()
+
+				// If a file is removed or renamed, fsnotify will loose track of it
+				// and not watch it. We will therefore attempt to re-add it with a backoff.
+			case fsnotify.Remove, fsnotify.Rename:
+				err := backoff.Retry(func() error {
+					if _, err := os.Stat(e.path); err != nil {
+						return err
+					}
+
+					return nil
+				}, backoff.NewExponentialBackOff())
+
+				if err != nil {
+					log.Error().Caller().Err(err).Msgf("extra records filewatcher retrying to find file after delete")
+					continue
+				}
+
+				err = e.watcher.Add(e.path)
+				if err != nil {
+					log.Error().Caller().Err(err).Msgf("extra records filewatcher re-adding file after delete failed, giving up.")
+					return
+				} else {
+					log.Trace().Caller().Str("path", e.path).Msg("extra records file re-added after delete")
+					e.updateRecords()
+				}
 			}
-			e.updateRecords()
 
 		case err, ok := <-e.watcher.Errors:
 			if !ok {
@@ -116,6 +144,11 @@ func (e *ExtraRecordsMan) updateRecords() {
 		return
 	}
 
+	// If there are no records, ignore the update.
+	if records == nil {
+		return
+	}
+
 	e.mu.Lock()
 	defer e.mu.Unlock()
 
@@ -143,6 +176,12 @@ func readExtraRecordsFromPath(path string) ([]tailcfg.DNSRecord, [32]byte, error
 		return nil, [32]byte{}, fmt.Errorf("reading path: %s, err: %w", path, err)
 	}
 
+	// If the read was triggered too fast, and the file is not complete, ignore the update
+	// if the file is empty. A consecutive update will be triggered when the file is complete.
+	if len(b) == 0 {
+		return nil, [32]byte{}, nil
+	}
+
 	var records []tailcfg.DNSRecord
 	err = json.Unmarshal(b, &records)
 	if err != nil {

integration/dns_test.go

@@ -146,19 +146,34 @@ func TestResolveMagicDNSExtraRecordsPath(t *testing.T) {
 		assertCommandOutputContains(t, client, []string{"dig", "test.myvpn.example.com"}, "6.6.6.6")
 	}
 
+	hs, err := scenario.Headscale()
+	assertNoErr(t, err)
+
+	// Write the file directly into place from the docker API.
+	b0, _ := json.Marshal([]tailcfg.DNSRecord{
+		{
+			Name:  "docker.myvpn.example.com",
+			Type:  "A",
+			Value: "2.2.2.2",
+		},
+	})
+
+	err = hs.WriteFile(erPath, b0)
+	assertNoErr(t, err)
+
+	for _, client := range allClients {
+		assertCommandOutputContains(t, client, []string{"dig", "docker.myvpn.example.com"}, "2.2.2.2")
+	}
+
+	// Write a new file and move it to the path to ensure the reload
+	// works when a file is moved atomically into place.
 	extraRecords = append(extraRecords, tailcfg.DNSRecord{
 		Name:  "otherrecord.myvpn.example.com",
 		Type:  "A",
 		Value: "7.7.7.7",
 	})
 	b2, _ := json.Marshal(extraRecords)
 
-	hs, err := scenario.Headscale()
-	assertNoErr(t, err)
-
-	// Write it to a separate file to ensure Docker's API doesnt
-	// do anything unexpected and rather move it into place to trigger
-	// a reload.
 	err = hs.WriteFile(erPath+"2", b2)
 	assertNoErr(t, err)
 	_, err = hs.Execute([]string{"mv", erPath + "2", erPath})
@@ -168,6 +183,61 @@ func TestResolveMagicDNSExtraRecordsPath(t *testing.T) {
 		assertCommandOutputContains(t, client, []string{"dig", "test.myvpn.example.com"}, "6.6.6.6")
 		assertCommandOutputContains(t, client, []string{"dig", "otherrecord.myvpn.example.com"}, "7.7.7.7")
 	}
+
+	// Write a new file and copy it to the path to ensure the reload
+	// works when a file is copied into place.
+	b3, _ := json.Marshal([]tailcfg.DNSRecord{
+		{
+			Name:  "copy.myvpn.example.com",
+			Type:  "A",
+			Value: "8.8.8.8",
+		},
+	})
+
+	err = hs.WriteFile(erPath+"3", b3)
+	assertNoErr(t, err)
+	_, err = hs.Execute([]string{"cp", erPath + "3", erPath})
+	assertNoErr(t, err)
+
+	for _, client := range allClients {
+		assertCommandOutputContains(t, client, []string{"dig", "copy.myvpn.example.com"}, "8.8.8.8")
+	}
+
+	// Write in place to ensure pipe like behaviour works
+	b4, _ := json.Marshal([]tailcfg.DNSRecord{
+		{
+			Name:  "docker.myvpn.example.com",
+			Type:  "A",
+			Value: "9.9.9.9",
+		},
+	})
+	command := []string{"echo", fmt.Sprintf("'%s'", string(b4)), ">", erPath}
+	_, err = hs.Execute([]string{"bash", "-c", strings.Join(command, " ")})
+	assertNoErr(t, err)
+
+	for _, client := range allClients {
+		assertCommandOutputContains(t, client, []string{"dig", "docker.myvpn.example.com"}, "9.9.9.9")
+	}
+
+	// Delete the file and create a new one to ensure it is picked up again.
+	_, err = hs.Execute([]string{"rm", erPath})
+	assertNoErr(t, err)
+
+	time.Sleep(2 * time.Second)
+
+	// The same paths should still be available as it is not cleared on delete.
+	for _, client := range allClients {
+		assertCommandOutputContains(t, client, []string{"dig", "docker.myvpn.example.com"}, "9.9.9.9")
+	}
+
+	// Write a new file, the backoff mechanism should make the filewatcher pick it up
+	// again.
+	err = hs.WriteFile(erPath, b3)
+	assertNoErr(t, err)
+
+	for _, client := range allClients {
+		assertCommandOutputContains(t, client, []string{"dig", "copy.myvpn.example.com"}, "8.8.8.8")
+	}
 }
 
 // TestValidateResolvConf validates that the resolv.conf file