JSON formatting of IAM: Workaround for optional fields

AWS IAM is very strict and doesn't support `Resource: []` for example.
We implement a custom MarshalJSON method to work around that.

pkg/model/iam.go

@@ -338,7 +338,7 @@ func (b *IAMModelBuilder) buildAWSIAMRolePolicy(role iam.Subject) (fi.Resource,
 			Statement: []*iam.Statement{
 				{
 					Effect: "Allow",
-					Principal: &iam.Principal{
+					Principal: iam.Principal{
 						Federated: "arn:aws:iam::" + b.AWSAccountID + ":oidc-provider/" + oidcProvider,
 					},
 					Action: stringorslice.String("sts:AssumeRoleWithWebIdentity"),

pkg/model/iam/iam_builder.go

@@ -77,17 +77,112 @@ type Condition map[string]interface{}
 // http://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_elements.html#Statement
 type Statement struct {
 	Effect    StatementEffect
-	Principal *Principal                  `json:",omitempty"`
-	Action    stringorslice.StringOrSlice `json:",omitempty"`
-	Resource  stringorslice.StringOrSlice `json:",omitempty"`
-	Condition Condition                   `json:",omitempty"`
+	Principal Principal
+	Action    stringorslice.StringOrSlice
+	Resource  stringorslice.StringOrSlice
+	Condition Condition
+}
+
+type jsonWriter struct {
+	w   io.Writer
+	err error
+}
+
+func (j *jsonWriter) Error() error {
+	return j.err
+}
+
+func (j *jsonWriter) WriteLiteral(b []byte) {
+	if j.err != nil {
+		return
+	}
+	_, err := j.w.Write(b)
+	if err != nil {
+		j.err = err
+	}
+}
+
+func (j *jsonWriter) StartObject() {
+	j.WriteLiteral([]byte("{"))
+}
+
+func (j *jsonWriter) EndObject() {
+	j.WriteLiteral([]byte("}"))
+}
+
+func (j *jsonWriter) Comma() {
+	j.WriteLiteral([]byte(","))
+}
+
+func (j *jsonWriter) Field(s string) {
+	if j.err != nil {
+		return
+	}
+	b, err := json.Marshal(s)
+	if err != nil {
+		j.err = err
+		return
+	}
+	j.WriteLiteral(b)
+	j.WriteLiteral([]byte(": "))
+}
+
+func (j *jsonWriter) Marshal(v interface{}) {
+	if j.err != nil {
+		return
+	}
+	b, err := json.Marshal(v)
+	if err != nil {
+		j.err = err
+		return
+	}
+	j.WriteLiteral(b)
+}
+
+// MarshalJSON formats the IAM statement for the AWS IAM restrictions.
+// For example, `Resource: []` is not allowed, but golang would force us to use pointers.
+func (s *Statement) MarshalJSON() ([]byte, error) {
+	var b bytes.Buffer
+
+	jw := &jsonWriter{w: &b}
+	jw.StartObject()
+	jw.Field("Effect")
+	jw.Marshal(s.Effect)
+
+	if !s.Principal.IsEmpty() {
+		jw.Comma()
+		jw.Field("Principal")
+		jw.Marshal(s.Principal)
+	}
+	if !s.Action.IsEmpty() {
+		jw.Comma()
+		jw.Field("Action")
+		jw.Marshal(s.Action)
+	}
+	if !s.Resource.IsEmpty() {
+		jw.Comma()
+		jw.Field("Resource")
+		jw.Marshal(s.Resource)
+	}
+	if len(s.Condition) != 0 {
+		jw.Comma()
+		jw.Field("Condition")
+		jw.Marshal(s.Condition)
+	}
+	jw.EndObject()
+
+	return b.Bytes(), jw.Error()
 }
 
 type Principal struct {
 	Federated string `json:",omitempty"`
 	Service   string `json:",omitempty"`
 }
 
+func (p *Principal) IsEmpty() bool {
+	return *p == Principal{}
+}
+
 // Equal compares two IAM Statements and returns a bool
 // TODO: Extend to support Condition Keys
 func (l *Statement) Equal(r *Statement) bool {

pkg/model/iam/iam_builder_test.go

@@ -48,6 +48,26 @@ func TestRoundTrip(t *testing.T) {
 			},
 			JSON: "{\"Effect\":\"Deny\",\"Action\":[\"ec2:DescribeRegions\",\"ec2:DescribeInstances\"],\"Resource\":[\"a\",\"b\"]}",
 		},
+		{
+			IAM: &Statement{
+				Effect:    StatementEffectDeny,
+				Principal: Principal{Federated: "federated"},
+				Condition: map[string]interface{}{
+					"foo": 1,
+				},
+			},
+			JSON: "{\"Effect\":\"Deny\",\"Principal\":{\"Federated\":\"federated\"},\"Condition\":{\"foo\":1}}",
+		},
+		{
+			IAM: &Statement{
+				Effect:    StatementEffectDeny,
+				Principal: Principal{Service: "service"},
+				Condition: map[string]interface{}{
+					"bar": "baz",
+				},
+			},
+			JSON: "{\"Effect\":\"Deny\",\"Principal\":{\"Service\":\"service\"},\"Condition\":{\"bar\":\"baz\"}}",
+		},
 	}
 	for _, g := range grid {
 		actualJSON, err := json.Marshal(g.IAM)

pkg/util/stringorslice/stringorslice.go

@@ -27,6 +27,10 @@ type StringOrSlice struct {
 	forceEncodeAsArray bool
 }
 
+func (s *StringOrSlice) IsEmpty() bool {
+	return len(s.values) == 0
+}
+
 // Slice will build a value that marshals to a JSON array
 func Slice(v []string) StringOrSlice {
 	return StringOrSlice{values: v, forceEncodeAsArray: true}

tests/integration/update_cluster/public-jwks/data/aws_iam_role_dns-controller.kube-system.sa.minimal.example.com_policy

@@ -7,7 +7,6 @@
         "Federated": "arn:aws:iam::123456789012:oidc-provider/api.minimal.example.com"
       },
       "Action": "sts:AssumeRoleWithWebIdentity",
-      "Resource": [],
       "Condition": {
         "StringEquals": {
           "api.minimal.example.com:sub": "system:serviceaccount:kube-system:dns-controller"