Add metadata functions structs (#68)

* Fix schema version copy paste mistake

* Add structs for metadata functions command

* Test metadata functions validate

* Apply suggestions from code review

Co-authored-by: Radek Simko <radek.simko@gmail.com>

* Fix plural

---------

Co-authored-by: Radek Simko <radek.simko@gmail.com>

metadata.go

@@ -0,0 +1,104 @@
+package tfjson
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+
+	"github.com/hashicorp/go-version"
+	"github.com/zclconf/go-cty/cty"
+)
+
+// MetadataFunctionsFormatVersionConstraints defines the versions of the JSON
+// metadata functions format that are supported by this package.
+var MetadataFunctionsFormatVersionConstraints = "~> 1.0"
+
+// MetadataFunctions is the top-level object returned when exporting function
+// signatures
+type MetadataFunctions struct {
+	// The version of the format. This should always match the
+	// MetadataFunctionsFormatVersionConstraints in this package, else
+	// unmarshaling will fail.
+	FormatVersion string `json:"format_version"`
+
+	// The signatures of the functions available in a Terraform version.
+	Signatures map[string]*FunctionSignature `json:"function_signatures,omitempty"`
+}
+
+// Validate checks to ensure that MetadataFunctions is present, and the
+// version matches the version supported by this library.
+func (f *MetadataFunctions) Validate() error {
+	if f == nil {
+		return errors.New("metadata functions data is nil")
+	}
+
+	if f.FormatVersion == "" {
+		return errors.New("unexpected metadata functions data, format version is missing")
+	}
+
+	constraint, err := version.NewConstraint(MetadataFunctionsFormatVersionConstraints)
+	if err != nil {
+		return fmt.Errorf("invalid version constraint: %w", err)
+	}
+
+	version, err := version.NewVersion(f.FormatVersion)
+	if err != nil {
+		return fmt.Errorf("invalid format version %q: %w", f.FormatVersion, err)
+	}
+
+	if !constraint.Check(version) {
+		return fmt.Errorf("unsupported metadata functions format version: %q does not satisfy %q",
+			version, constraint)
+	}
+
+	return nil
+}
+
+func (f *MetadataFunctions) UnmarshalJSON(b []byte) error {
+	type rawFunctions MetadataFunctions
+	var functions rawFunctions
+
+	err := json.Unmarshal(b, &functions)
+	if err != nil {
+		return err
+	}
+
+	*f = *(*MetadataFunctions)(&functions)
+
+	return f.Validate()
+}
+
+// FunctionSignature represents a function signature.
+type FunctionSignature struct {
+	// Description is an optional human-readable description
+	// of the function
+	Description string `json:"description,omitempty"`
+
+	// ReturnType is the ctyjson representation of the function's
+	// return types based on supplying all parameters using
+	// dynamic types. Functions can have dynamic return types.
+	ReturnType cty.Type `json:"return_type"`
+
+	// Parameters describes the function's fixed positional parameters.
+	Parameters []*FunctionParameter `json:"parameters,omitempty"`
+
+	// VariadicParameter describes the function's variadic
+	// parameter if it is supported.
+	VariadicParameter *FunctionParameter `json:"variadic_parameter,omitempty"`
+}
+
+// FunctionParameter represents a parameter to a function.
+type FunctionParameter struct {
+	// Name is an optional name for the argument.
+	Name string `json:"name,omitempty"`
+
+	// Description is an optional human-readable description
+	// of the argument
+	Description string `json:"description,omitempty"`
+
+	// IsNullable is true if null is acceptable value for the argument
+	IsNullable bool `json:"is_nullable,omitempty"`
+
+	// A type that any argument for this parameter must conform to.
+	Type cty.Type `json:"type"`
+}

metadata_test.go

@@ -0,0 +1,24 @@
+package tfjson
+
+import (
+	"encoding/json"
+	"os"
+	"testing"
+)
+
+func TestMetadataFunctionsValidate(t *testing.T) {
+	f, err := os.Open("testdata/basic/functions.json")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	var functions *MetadataFunctions
+	if err := json.NewDecoder(f).Decode(&functions); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := functions.Validate(); err != nil {
+		t.Fatal(err)
+	}
+}

schemas.go

@@ -39,7 +39,7 @@ func (p *ProviderSchemas) Validate() error {
 		return errors.New("unexpected provider schema data, format version is missing")
 	}
 
-	constraint, err := version.NewConstraint(PlanFormatVersionConstraints)
+	constraint, err := version.NewConstraint(ProviderSchemasFormatVersionConstraints)
 	if err != nil {
 		return fmt.Errorf("invalid version constraint: %w", err)
 	}