[Internal] Add ConvertToAttribute() to convert blocks in a resource/data source schema to attributes (#4284)

## Changes
Blocks in the Plugin Framework cannot be computed. Terraform checks that
the number of blocks in the plan matches the number in the config
(https://github.com/hashicorp/terraform/blob/81441564dd29b4aa5fb9576323ef90b9787d1967/internal/plans/objchange/plan_valid.go#L115).
Today, for compatibility with existing resources in the TF provider
implemented with the SDKv2, we treat nested structures as lists with a
single element. As a result, this precludes any nested structures from
being read-only (since those would be converted into ListNestedBlocks,
which cannot be computed). This blocks databricks_app from being
implemented on the plugin framework, as its app_status and
compute_status fields
are objects and not primitives. Note that TF recommends computing
computed-only blocks to attributes
[here](https://developer.hashicorp.com/terraform/plugin/framework/migrating/attributes-blocks/blocks-computed).
Other useful discussions can be found
[here](https://discuss.hashicorp.com/t/set-list-attribute-migration-from-sdkv2-to-framework/56472)
and
[here](https://discuss.hashicorp.com/t/optional-computed-block-handling-in-plugin-framework/56337).
The migration guide for blocks can be found
[here](https://developer.hashicorp.com/terraform/plugin/framework/migrating/attributes-blocks/blocks).

As an intermediate step, we add `ConvertToAttribute()` in
`CustomizableSchema`. This allows resource maintainers to convert
specific, known read-only blocks to attributes in accordance with [the
migration
guide](https://developer.hashicorp.com/terraform/plugin/framework/migrating/attributes-blocks/blocks-computed),
which then can be marked as read-only.

As blocks can contain other blocks, this method also recursively
converts nested blocks into nested attributes.

This is based on
#4283,
so we should wait for that PR to merge & rebase this before merging
this.

## Tests
Unit tests validate that ListNestedBlocks are converted to
ListNestedAttributes and that SingleNestedBlocks are converted to
SingleNestedAttributes.

Author: mgyucht

internal/providers/pluginfw/tfschema/attribute_converter.go

@@ -0,0 +1,6 @@
+package tfschema
+
+type BlockToAttributeConverter interface {
+	// ConvertBlockToAttribute converts a contained block to its corresponding attribute type.
+	ConvertBlockToAttribute(string) BaseSchemaBuilder
+}

internal/providers/pluginfw/tfschema/block_builder.go

@@ -10,6 +10,12 @@ import (
 // This common interface prevents us from keeping two copies of StructToSchema and CustomizableSchema.
 type BlockBuilder interface {
 	BaseSchemaBuilder
+
+	// ToAttribute converts a block to its corresponding attribute type. Currently, ResourceStructToSchema converts all
+	// nested struct fields and slices to blocks. This method is used to convert those blocks to their corresponding
+	// attribute type. The resulting attribute will not have any of the Computed/Optional/Required/Sensitive flags set.
+	ToAttribute() AttributeBuilder
+
 	BuildDataSourceBlock() dataschema.Block
 	BuildResourceBlock() schema.Block
 }

internal/providers/pluginfw/tfschema/customizable_schema.go

@@ -10,6 +10,7 @@ import (
 )
 
 // CustomizableSchema is a wrapper struct on top of BaseSchemaBuilder that can be used to navigate through nested schema add customizations.
+// The methods of CustomizableSchema that modify the underlying schema should return the same CustomizableSchema object to allow chaining.
 type CustomizableSchema struct {
 	attr BaseSchemaBuilder
 }
@@ -199,6 +200,35 @@ func (s *CustomizableSchema) SetReadOnly(path ...string) *CustomizableSchema {
 	return s
 }
 
+// ConvertToAttribute converts the last element of the path from a block to an attribute.
+// It panics if the path is empty, if the path does not exist in the schema, or if the path
+// points to an attribute, not a block.
+func (s *CustomizableSchema) ConvertToAttribute(path ...string) *CustomizableSchema {
+	if len(path) == 0 {
+		panic(fmt.Errorf("ConvertToAttribute called on root schema. %s", common.TerraformBugErrorMessage))
+	}
+	field := path[len(path)-1]
+
+	cb := func(attr BaseSchemaBuilder) BaseSchemaBuilder {
+		switch a := attr.(type) {
+		case BlockToAttributeConverter:
+			return a.ConvertBlockToAttribute(field)
+		default:
+			panic(fmt.Errorf("ConvertToAttribute called on invalid attribute type: %s. %s", reflect.TypeOf(attr).String(), common.TerraformBugErrorMessage))
+		}
+	}
+
+	// We have to go only as far as the second-to-last entry, since we need to change the parent schema
+	// by moving the last entry from a block to an attribute.
+	if len(path) == 1 {
+		s.attr = cb(s.attr)
+	} else {
+		navigateSchemaWithCallback(&s.attr, cb, path[0:len(path)-1]...)
+	}
+
+	return s
+}
+
 // navigateSchemaWithCallback navigates through schema attributes and executes callback on the target, panics if path does not exist or invalid.
 func navigateSchemaWithCallback(s *BaseSchemaBuilder, cb func(BaseSchemaBuilder) BaseSchemaBuilder, path ...string) (BaseSchemaBuilder, error) {
 	currentScm := s

internal/providers/pluginfw/tfschema/customizable_schema_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"github.com/hashicorp/terraform-plugin-framework/resource/schema"
+	"github.com/hashicorp/terraform-plugin-framework/resource/schema/planmodifier"
 	"github.com/hashicorp/terraform-plugin-framework/resource/schema/stringplanmodifier"
 	"github.com/hashicorp/terraform-plugin-framework/schema/validator"
 	"github.com/hashicorp/terraform-plugin-framework/types"
@@ -175,3 +176,219 @@ func TestCustomizeSchema_SetComputed_PanicOnBlock(t *testing.T) {
 		})
 	})
 }
+
+type mockPlanModifier struct{}
+
+// Description implements planmodifier.List.
+func (m mockPlanModifier) Description(context.Context) string {
+	panic("unimplemented")
+}
+
+// MarkdownDescription implements planmodifier.List.
+func (m mockPlanModifier) MarkdownDescription(context.Context) string {
+	panic("unimplemented")
+}
+
+// PlanModifyList implements planmodifier.List.
+func (m mockPlanModifier) PlanModifyList(context.Context, planmodifier.ListRequest, *planmodifier.ListResponse) {
+	panic("unimplemented")
+}
+
+// PlanModifyList implements planmodifier.List.
+func (m mockPlanModifier) PlanModifyObject(context.Context, planmodifier.ObjectRequest, *planmodifier.ObjectResponse) {
+	panic("unimplemented")
+}
+
+var _ planmodifier.List = mockPlanModifier{}
+var _ planmodifier.Object = mockPlanModifier{}
+
+type mockValidator struct{}
+
+// Description implements validator.List.
+func (m mockValidator) Description(context.Context) string {
+	panic("unimplemented")
+}
+
+// MarkdownDescription implements validator.List.
+func (m mockValidator) MarkdownDescription(context.Context) string {
+	panic("unimplemented")
+}
+
+// ValidateList implements validator.List.
+func (m mockValidator) ValidateList(context.Context, validator.ListRequest, *validator.ListResponse) {
+	panic("unimplemented")
+}
+
+// ValidateList implements validator.Object.
+func (m mockValidator) ValidateObject(context.Context, validator.ObjectRequest, *validator.ObjectResponse) {
+	panic("unimplemented")
+}
+
+var _ validator.List = mockValidator{}
+var _ validator.Object = mockValidator{}
+
+func TestCustomizeSchema_ConvertToAttribute(t *testing.T) {
+	v := mockValidator{}
+	pm := mockPlanModifier{}
+	testCases := []struct {
+		name        string
+		baseSchema  NestedBlockObject
+		path        []string
+		want        NestedBlockObject
+		expectPanic bool
+	}{
+		{
+			name: "ListNestedBlock",
+			baseSchema: NestedBlockObject{
+				Blocks: map[string]BlockBuilder{
+					"list": ListNestedBlockBuilder{
+						NestedObject: NestedBlockObject{
+							Attributes: map[string]AttributeBuilder{
+								"attr": StringAttributeBuilder{},
+							},
+						},
+						DeprecationMessage: "deprecated",
+						Validators:         []validator.List{v},
+						PlanModifiers:      []planmodifier.List{pm},
+					},
+				},
+			},
+			path: []string{"list"},
+			want: NestedBlockObject{
+				Attributes: map[string]AttributeBuilder{
+					"list": ListNestedAttributeBuilder{
+						NestedObject: NestedAttributeObject{
+							Attributes: map[string]AttributeBuilder{
+								"attr": StringAttributeBuilder{},
+							},
+						},
+						DeprecationMessage: "deprecated",
+						Validators:         []validator.List{v},
+						PlanModifiers:      []planmodifier.List{pm},
+					},
+				},
+			},
+		},
+		{
+			name: "ListNestedBlock/CalledOnInnerBlock",
+			baseSchema: NestedBlockObject{
+				Blocks: map[string]BlockBuilder{
+					"list": ListNestedBlockBuilder{
+						NestedObject: NestedBlockObject{
+							Blocks: map[string]BlockBuilder{
+								"nested_block": ListNestedBlockBuilder{
+									NestedObject: NestedBlockObject{
+										Attributes: map[string]AttributeBuilder{
+											"attr": StringAttributeBuilder{},
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+			path: []string{"list", "nested_block"},
+			want: NestedBlockObject{
+				Blocks: map[string]BlockBuilder{
+					"list": ListNestedBlockBuilder{
+						NestedObject: NestedBlockObject{
+							Attributes: map[string]AttributeBuilder{
+								"nested_block": ListNestedAttributeBuilder{
+									NestedObject: NestedAttributeObject{
+										Attributes: map[string]AttributeBuilder{
+											"attr": StringAttributeBuilder{},
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+		},
+		{
+			name: "SingleNestedBlock",
+			baseSchema: NestedBlockObject{
+				Blocks: map[string]BlockBuilder{
+					"single": SingleNestedBlockBuilder{
+						NestedObject: NestedBlockObject{
+							Attributes: map[string]AttributeBuilder{
+								"attr": StringAttributeBuilder{},
+							},
+						},
+						DeprecationMessage: "deprecated",
+						Validators:         []validator.Object{v},
+						PlanModifiers:      []planmodifier.Object{pm},
+					},
+				},
+			},
+			path: []string{"single"},
+			want: NestedBlockObject{
+				Attributes: map[string]AttributeBuilder{
+					"single": SingleNestedAttributeBuilder{
+						Attributes: map[string]AttributeBuilder{
+							"attr": StringAttributeBuilder{},
+						},
+						DeprecationMessage: "deprecated",
+						Validators:         []validator.Object{v},
+						PlanModifiers:      []planmodifier.Object{pm},
+					},
+				},
+			},
+		},
+		{
+			name: "SingleNestedBlock/RecursiveBlocks",
+			baseSchema: NestedBlockObject{
+				Blocks: map[string]BlockBuilder{
+					"single": SingleNestedBlockBuilder{
+						NestedObject: NestedBlockObject{
+							Blocks: map[string]BlockBuilder{
+								"nested_block": ListNestedBlockBuilder{
+									NestedObject: NestedBlockObject{
+										Attributes: map[string]AttributeBuilder{
+											"attr": StringAttributeBuilder{},
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+			path: []string{"single"},
+			want: NestedBlockObject{
+				Attributes: map[string]AttributeBuilder{
+					"single": SingleNestedAttributeBuilder{
+						Attributes: map[string]AttributeBuilder{
+							"nested_block": ListNestedAttributeBuilder{
+								NestedObject: NestedAttributeObject{
+									Attributes: map[string]AttributeBuilder{
+										"attr": StringAttributeBuilder{},
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+		},
+		{
+			name:        "PanicOnEmptyPath",
+			path:        nil,
+			expectPanic: true,
+		},
+	}
+	for _, c := range testCases {
+		t.Run(c.name, func(t *testing.T) {
+			if c.expectPanic {
+				assert.Panics(t, func() {
+					ConstructCustomizableSchema(c.baseSchema).ConvertToAttribute(c.path...)
+				})
+			} else {
+				got := ConstructCustomizableSchema(c.baseSchema).ConvertToAttribute(c.path...)
+				assert.Equal(t, c.want, got.attr.(SingleNestedBlockBuilder).NestedObject)
+			}
+		})
+	}
+}

internal/providers/pluginfw/tfschema/list_nested_attribute.go

@@ -44,7 +44,7 @@ func (a ListNestedAttributeBuilder) BuildResourceAttribute() schema.Attribute {
 	}
 }
 
-func (a ListNestedAttributeBuilder) SetOptional() BaseSchemaBuilder {
+func (a ListNestedAttributeBuilder) SetOptional() AttributeBuilder {
 	if a.Optional && !a.Required {
 		panic("attribute is already optional")
 	}
@@ -53,7 +53,7 @@ func (a ListNestedAttributeBuilder) SetOptional() BaseSchemaBuilder {
 	return a
 }
 
-func (a ListNestedAttributeBuilder) SetRequired() BaseSchemaBuilder {
+func (a ListNestedAttributeBuilder) SetRequired() AttributeBuilder {
 	if !a.Optional && a.Required {
 		panic("attribute is already required")
 	}
@@ -62,23 +62,23 @@ func (a ListNestedAttributeBuilder) SetRequired() BaseSchemaBuilder {
 	return a
 }
 
-func (a ListNestedAttributeBuilder) SetSensitive() BaseSchemaBuilder {
+func (a ListNestedAttributeBuilder) SetSensitive() AttributeBuilder {
 	if a.Sensitive {
 		panic("attribute is already sensitive")
 	}
 	a.Sensitive = true
 	return a
 }
 
-func (a ListNestedAttributeBuilder) SetComputed() BaseSchemaBuilder {
+func (a ListNestedAttributeBuilder) SetComputed() AttributeBuilder {
 	if a.Computed {
 		panic("attribute is already computed")
 	}
 	a.Computed = true
 	return a
 }
 
-func (a ListNestedAttributeBuilder) SetReadOnly() BaseSchemaBuilder {
+func (a ListNestedAttributeBuilder) SetReadOnly() AttributeBuilder {
 	if a.Computed && !a.Optional && !a.Required {
 		panic("attribute is already read only")
 	}

internal/providers/pluginfw/tfschema/list_nested_block.go

@@ -1,6 +1,8 @@
 package tfschema
 
 import (
+	"fmt"
+
 	dataschema "github.com/hashicorp/terraform-plugin-framework/datasource/schema"
 	"github.com/hashicorp/terraform-plugin-framework/resource/schema"
 	"github.com/hashicorp/terraform-plugin-framework/resource/schema/planmodifier"
@@ -16,6 +18,15 @@ type ListNestedBlockBuilder struct {
 	PlanModifiers      []planmodifier.List
 }
 
+func (a ListNestedBlockBuilder) ToAttribute() AttributeBuilder {
+	return ListNestedAttributeBuilder{
+		NestedObject:       a.NestedObject.ToNestedAttributeObject(),
+		DeprecationMessage: a.DeprecationMessage,
+		Validators:         a.Validators,
+		PlanModifiers:      a.PlanModifiers,
+	}
+}
+
 func (a ListNestedBlockBuilder) BuildDataSourceBlock() dataschema.Block {
 	return dataschema.ListNestedBlock{
 		NestedObject:       a.NestedObject.BuildDataSourceAttribute(),
@@ -47,3 +58,19 @@ func (a ListNestedBlockBuilder) AddPlanModifier(v planmodifier.List) BaseSchemaB
 	a.PlanModifiers = append(a.PlanModifiers, v)
 	return a
 }
+
+func (a ListNestedBlockBuilder) ConvertBlockToAttribute(field string) BaseSchemaBuilder {
+	elem, ok := a.NestedObject.Blocks[field]
+	if !ok {
+		panic(fmt.Errorf("field %s does not exist in nested block", field))
+	}
+	if a.NestedObject.Attributes == nil {
+		a.NestedObject.Attributes = make(map[string]AttributeBuilder)
+	}
+	a.NestedObject.Attributes[field] = elem.ToAttribute()
+	delete(a.NestedObject.Blocks, field)
+	if len(a.NestedObject.Blocks) == 0 {
+		a.NestedObject.Blocks = nil
+	}
+	return a
+}