cmd/k8s-operator,ipn/store/kubestore: patch secrets instead of updating

We would call Update on the secret, but that was racey and would occasionaly
fail. Instead use patch whenever we can.

Fixes errors like
```
boot: 2023/08/29 01:03:53 failed to set serve config: sending serve config: updating config: writing ServeConfig to StateStore: Operation cannot be fulfilled on secrets "ts-webdav-kfrzv-0": the object has been modified; please apply your changes to the latest version and try again

{"level":"error","ts":"2023-08-29T01:03:48Z","msg":"Reconciler error","controller":"ingress","controllerGroup":"networking.k8s.io","controllerKind":"Ingress","Ingress":{"name":"webdav","namespace":"default"},"namespace":"default","name":"webdav","reconcileID":"96f5cfed-7782-4834-9b75-b0950fd563ed","error":"failed to provision: failed to create or get API key secret: Operation cannot be fulfilled on secrets \"ts-webdav-kfrzv-0\": the object has been modified; please apply your changes to the latest version and try again","stacktrace":"sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).reconcileHandler\n\tsigs.k8s.io/controller-runtime@v0.15.0/pkg/internal/controller/controller.go:324\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).processNextWorkItem\n\tsigs.k8s.io/controller-runtime@v0.15.0/pkg/internal/controller/controller.go:265\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Start.func2.2\n\tsigs.k8s.io/controller-runtime@v0.15.0/pkg/internal/controller/controller.go:226"}
```

Updates #502
Updates #7895

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/k8s-operator/sts.go

@@ -175,15 +175,15 @@ func (a *tailscaleSTSReconciler) createOrGetSecret(ctx context.Context, logger *
 			Labels:    stsC.ChildResourceLabels,
 		},
 	}
-	alreadyExists := false
+	var orig *corev1.Secret // unmodified copy of secret
 	if err := a.Get(ctx, client.ObjectKeyFromObject(secret), secret); err == nil {
 		logger.Debugf("secret %s/%s already exists", secret.GetNamespace(), secret.GetName())
-		alreadyExists = true
+		orig = secret.DeepCopy()
 	} else if !apierrors.IsNotFound(err) {
 		return "", err
 	}
 
-	if !alreadyExists {
+	if orig == nil {
 		// Secret doesn't exist yet, create one. Initially it contains
 		// only the Tailscale authkey, but once Tailscale starts it'll
 		// also store the daemon state.
@@ -218,8 +218,8 @@ func (a *tailscaleSTSReconciler) createOrGetSecret(ctx context.Context, logger *
 		}
 		mak.Set(&secret.StringData, "serve-config", string(j))
 	}
-	if alreadyExists {
-		if err := a.Update(ctx, secret); err != nil {
+	if orig != nil {
+		if err := a.Patch(ctx, secret, client.MergeFrom(orig)); err != nil {
 			return "", err
 		}
 	} else {

ipn/store/kubestore/store_kube.go

@@ -19,6 +19,7 @@ import (
 // Store is an ipn.StateStore that uses a Kubernetes Secret for persistence.
 type Store struct {
 	client     *kube.Client
+	canPatch   bool
 	secretName string
 }
 
@@ -28,8 +29,13 @@ func New(_ logger.Logf, secretName string) (*Store, error) {
 	if err != nil {
 		return nil, err
 	}
+	canPatch, err := c.CheckSecretPermissions(context.Background(), secretName)
+	if err != nil {
+		return nil, err
+	}
 	return &Store{
 		client:     c,
+		canPatch:   canPatch,
 		secretName: secretName,
 	}, nil
 }
@@ -93,6 +99,19 @@ func (s *Store) WriteState(id ipn.StateKey, bs []byte) error {
 		}
 		return err
 	}
+	if s.canPatch {
+		m := []kube.JSONPatch{
+			{
+				Op:    "add",
+				Path:  "/data/" + sanitizeKey(id),
+				Value: bs,
+			},
+		}
+		if err := s.client.JSONPatchSecret(ctx, s.secretName, m); err != nil {
+			return err
+		}
+		return nil
+	}
 	secret.Data[sanitizeKey(id)] = bs
 	if err := s.client.UpdateSecret(ctx, secret); err != nil {
 		return err

kube/client.go

@@ -233,15 +233,16 @@ func (c *Client) UpdateSecret(ctx context.Context, s *Secret) error {
 //
 // https://tools.ietf.org/html/rfc6902
 type JSONPatch struct {
-	Op   string `json:"op"`
-	Path string `json:"path"`
+	Op    string `json:"op"`
+	Path  string `json:"path"`
+	Value any    `json:"value,omitempty"`
 }
 
 // JSONPatchSecret updates a secret in the Kubernetes API using a JSON patch.
 // It currently (2023-03-02) only supports the "remove" operation.
 func (c *Client) JSONPatchSecret(ctx context.Context, name string, patch []JSONPatch) error {
 	for _, p := range patch {
-		if p.Op != "remove" {
+		if p.Op != "remove" && p.Op != "add" {
 			panic(fmt.Errorf("unsupported JSON patch operation: %q", p.Op))
 		}
 	}