fix: skip null entries when mapping enums to OneOf validators

DESIGN.md

@@ -260,6 +260,8 @@ If the field is only present in a schema other than the `read` operation `parame
 | [pattern](https://json-schema.org/draft/2020-12/json-schema-validation.html#name-pattern)             | [`validators`](https://developer.hashicorp.com/terraform/plugin/code-generation/specification#validators)                            |
 | [uniqueItems](https://json-schema.org/draft/2020-12/json-schema-validation.html#name-uniqueItems)     | [`validators`](https://developer.hashicorp.com/terraform/plugin/code-generation/specification#validators)                            |
 
+A `null` entry in an `enum`, which OpenAPI 3.0 requires for a `nullable` enum to accept `null`, is not mapped to an allowed value in the `OneOf` validator: Terraform already represents `null` as an unset value. An `enum` that only lists `null` produces no validator.
+
 ### Attribute Names
 After all attributes have been [mapped](#oas-types-to-provider-attributes) and any overrides/aliases have been applied, the attribute names mapped from the OAS will be converted (if needed) to valid [Terraform Identifiers](https://developer.hashicorp.com/terraform/language/syntax/configuration#identifiers). This [logic](https://github.com/hashicorp/terraform-plugin-codegen-openapi/blob/main/internal/mapper/util/framework_identifier.go#L25) performs the following, in order:
 1. Removes all characters that are NOT alphanumeric or an underscore

internal/cmd/testdata/edgecase/generator_config.yml

@@ -33,6 +33,14 @@ resources:
       path: /composition_siblings
       method: GET
 
+  nullable_enum:
+    create:
+      path: /nullable_enum
+      method: POST
+    read:
+      path: /nullable_enum
+      method: GET
+
   nested_path:
     create:
       path: /parents/{parent_id}/children

internal/cmd/testdata/edgecase/openapi_spec.yml

@@ -199,6 +199,30 @@ paths:
                       deprecated: true
                   allOf:
                   - $ref: "#/components/schemas/composition_refinable_object"
+  /nullable_enum:
+    get:
+      summary: Test for nullable enums that list null as an allowed value
+      responses:
+        "200":
+          description: OK
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/nullable_enum_object"
+    post:
+      summary: Test for nullable enums that list null as an allowed value
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/nullable_enum_object"
+      responses:
+        "200":
+          description: OK
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/nullable_enum_object"
   /parents/{parent_id}/children:
     # Declared on the path item rather than the operation, so that resolving the create
     # operation's path parameters has to merge the path item in.
@@ -309,6 +333,36 @@ components:
           - active
           - disabled
           - expired
+    nullable_enum_object:
+      type: object
+      properties:
+        string_prop:
+          description: The null entry must not become an empty string in the OneOf validator
+          type: [string, "null"]
+          enum:
+          - a
+          - b
+          - null
+        integer_prop:
+          description: The null entry must not become 0 in the OneOf validator
+          type: [integer, "null"]
+          enum:
+          - 1
+          - 2
+          - null
+        number_prop:
+          description: The null entry must not become 0 in the OneOf validator
+          type: [number, "null"]
+          format: double
+          enum:
+          - 1.5
+          - 2.5
+          - null
+        only_null_prop:
+          description: An enum that only lists null produces no validator
+          type: [string, "null"]
+          enum:
+          - null
     composition_object:
       description: This description loses to the composing schema's
       type: object

internal/cmd/testdata/edgecase/provider_code_spec.json

@@ -579,6 +579,77 @@
 				]
 			}
 		},
+		{
+			"name": "nullable_enum",
+			"schema": {
+				"attributes": [
+					{
+						"name": "integer_prop",
+						"int64": {
+							"computed_optional_required": "computed_optional",
+							"description": "The null entry must not become 0 in the OneOf validator",
+							"validators": [
+								{
+									"custom": {
+										"imports": [
+											{
+												"path": "github.com/hashicorp/terraform-plugin-framework-validators/int64validator"
+											}
+										],
+										"schema_definition": "int64validator.OneOf(\n1,\n2,\n)"
+									}
+								}
+							]
+						}
+					},
+					{
+						"name": "number_prop",
+						"float64": {
+							"computed_optional_required": "computed_optional",
+							"description": "The null entry must not become 0 in the OneOf validator",
+							"validators": [
+								{
+									"custom": {
+										"imports": [
+											{
+												"path": "github.com/hashicorp/terraform-plugin-framework-validators/float64validator"
+											}
+										],
+										"schema_definition": "float64validator.OneOf(\n1.5,\n2.5,\n)"
+									}
+								}
+							]
+						}
+					},
+					{
+						"name": "only_null_prop",
+						"string": {
+							"computed_optional_required": "computed_optional",
+							"description": "An enum that only lists null produces no validator"
+						}
+					},
+					{
+						"name": "string_prop",
+						"string": {
+							"computed_optional_required": "computed_optional",
+							"description": "The null entry must not become an empty string in the OneOf validator",
+							"validators": [
+								{
+									"custom": {
+										"imports": [
+											{
+												"path": "github.com/hashicorp/terraform-plugin-framework-validators/stringvalidator"
+											}
+										],
+										"schema_definition": "stringvalidator.OneOf(\n\"a\",\n\"b\",\n)"
+									}
+								}
+							]
+						}
+					}
+				]
+			}
+		},
 		{
 			"name": "set_test",
 			"schema": {

internal/mapper/oas/integer.go

@@ -86,6 +86,12 @@ func (s *OASSchema) GetIntegerValidators() []schema.Int64Validator {
 		var enum []int64
 
 		for _, valueNode := range s.Schema.Enum {
+			// A nullable enum lists null (OpenAPI 3.0 requires it), but Terraform represents null as an unset value,
+			// not an allowed one. Decoding null would otherwise yield the zero value.
+			if valueNode.ShortTag() == "!!null" {
+				continue
+			}
+
 			var value int64
 			if err := valueNode.Decode(&value); err != nil {
 				// could consider error/panic here to notify developers

internal/mapper/oas/integer_test.go

@@ -595,6 +595,41 @@ func TestGetIntegerValidators(t *testing.T) {
 				},
 			},
 		},
+		"enum-with-null": {
+			schema: oas.OASSchema{
+				Schema: &base.Schema{
+					Type: []string{"integer"},
+					Enum: []*yaml.Node{
+						{Kind: yaml.ScalarNode, Value: "1"},
+						{Kind: yaml.ScalarNode, Value: "2"},
+						{Kind: yaml.ScalarNode, Tag: "!!null", Value: "null"},
+					},
+				},
+			},
+			expected: []schema.Int64Validator{
+				{
+					Custom: &schema.CustomValidator{
+						Imports: []code.Import{
+							{
+								Path: "github.com/hashicorp/terraform-plugin-framework-validators/int64validator",
+							},
+						},
+						SchemaDefinition: "int64validator.OneOf(\n1,\n2,\n)",
+					},
+				},
+			},
+		},
+		"enum-only-null": {
+			schema: oas.OASSchema{
+				Schema: &base.Schema{
+					Type: []string{"integer"},
+					Enum: []*yaml.Node{
+						{Kind: yaml.ScalarNode, Tag: "!!null", Value: "null"},
+					},
+				},
+			},
+			expected: nil,
+		},
 		"maximum": {
 			schema: oas.OASSchema{
 				Schema: &base.Schema{

internal/mapper/oas/number.go

@@ -128,6 +128,12 @@ func (s *OASSchema) GetFloatValidators() []schema.Float64Validator {
 		var enum []float64
 
 		for _, valueNode := range s.Schema.Enum {
+			// A nullable enum lists null (OpenAPI 3.0 requires it), but Terraform represents null as an unset value,
+			// not an allowed one. Decoding null would otherwise yield the zero value.
+			if valueNode.ShortTag() == "!!null" {
+				continue
+			}
+
 			var value float64
 			if err := valueNode.Decode(&value); err != nil {
 				// could consider error/panic here to notify developers

internal/mapper/oas/number_test.go

@@ -1108,6 +1108,41 @@ func TestGetFloatValidators(t *testing.T) {
 				},
 			},
 		},
+		"enum-with-null": {
+			schema: oas.OASSchema{
+				Schema: &base.Schema{
+					Type: []string{"number"},
+					Enum: []*yaml.Node{
+						{Kind: yaml.ScalarNode, Value: "1.2"},
+						{Kind: yaml.ScalarNode, Value: "2.3"},
+						{Kind: yaml.ScalarNode, Tag: "!!null", Value: "null"},
+					},
+				},
+			},
+			expected: []schema.Float64Validator{
+				{
+					Custom: &schema.CustomValidator{
+						Imports: []code.Import{
+							{
+								Path: "github.com/hashicorp/terraform-plugin-framework-validators/float64validator",
+							},
+						},
+						SchemaDefinition: "float64validator.OneOf(\n1.2,\n2.3,\n)",
+					},
+				},
+			},
+		},
+		"enum-only-null": {
+			schema: oas.OASSchema{
+				Schema: &base.Schema{
+					Type: []string{"number"},
+					Enum: []*yaml.Node{
+						{Kind: yaml.ScalarNode, Tag: "!!null", Value: "null"},
+					},
+				},
+			},
+			expected: nil,
+		},
 	}
 
 	for name, testCase := range testCases {

internal/mapper/oas/string.go

@@ -89,6 +89,12 @@ func (s *OASSchema) GetStringValidators() []schema.StringValidator {
 		var enum []string
 
 		for _, valueNode := range s.Schema.Enum {
+			// A nullable enum lists null (OpenAPI 3.0 requires it), but Terraform represents null as an unset value,
+			// not an allowed one. Decoding null would otherwise yield the zero value.
+			if valueNode.ShortTag() == "!!null" {
+				continue
+			}
+
 			var value string
 			if err := valueNode.Decode(&value); err != nil {
 				// could consider error/panic here to notify developers

internal/mapper/oas/string_test.go

@@ -601,6 +601,64 @@ func TestGetStringValidators(t *testing.T) {
 				},
 			},
 		},
+		"enum-with-null": {
+			schema: oas.OASSchema{
+				Schema: &base.Schema{
+					Type: []string{"string"},
+					Enum: []*yaml.Node{
+						{Kind: yaml.ScalarNode, Value: "one"},
+						{Kind: yaml.ScalarNode, Value: "two"},
+						{Kind: yaml.ScalarNode, Tag: "!!null", Value: "null"},
+					},
+				},
+			},
+			expected: []schema.StringValidator{
+				{
+					Custom: &schema.CustomValidator{
+						Imports: []code.Import{
+							{
+								Path: "github.com/hashicorp/terraform-plugin-framework-validators/stringvalidator",
+							},
+						},
+						SchemaDefinition: "stringvalidator.OneOf(\n\"one\",\n\"two\",\n)",
+					},
+				},
+			},
+		},
+		"enum-only-null": {
+			schema: oas.OASSchema{
+				Schema: &base.Schema{
+					Type: []string{"string"},
+					Enum: []*yaml.Node{
+						{Kind: yaml.ScalarNode, Tag: "!!null", Value: "null"},
+					},
+				},
+			},
+			expected: nil,
+		},
+		"enum-with-quoted-null-string": {
+			schema: oas.OASSchema{
+				Schema: &base.Schema{
+					Type: []string{"string"},
+					Enum: []*yaml.Node{
+						{Kind: yaml.ScalarNode, Value: "one"},
+						{Kind: yaml.ScalarNode, Style: yaml.DoubleQuotedStyle, Value: "null"},
+					},
+				},
+			},
+			expected: []schema.StringValidator{
+				{
+					Custom: &schema.CustomValidator{
+						Imports: []code.Import{
+							{
+								Path: "github.com/hashicorp/terraform-plugin-framework-validators/stringvalidator",
+							},
+						},
+						SchemaDefinition: "stringvalidator.OneOf(\n\"one\",\n\"null\",\n)",
+					},
+				},
+			},
+		},
 		"maxLength": {
 			schema: oas.OASSchema{
 				Schema: &base.Schema{