terraform/config/lang/eval.go

package lang

import (
	"bytes"
	"fmt"
	"sync"

	"github.com/hashicorp/terraform/config/lang/ast"
)

// EvalConfig is the configuration for evaluating.
type EvalConfig struct {
	// GlobalScope is the global scope of execution for evaluation.
	GlobalScope *ast.BasicScope

	// SemanticChecks is a list of additional semantic checks that will be run
	// on the tree prior to evaluating it. The type checker, identifier checker,
	// etc. will be run before these automatically.
	SemanticChecks []SemanticChecker
}

// SemanticChecker is the type that must be implemented to do a
// semantic check on an AST tree. This will be called with the root node.
type SemanticChecker func(ast.Node) error

// Eval evaluates the given AST tree and returns its output value, the type
// of the output, and any error that occurred.
func Eval(root ast.Node, config *EvalConfig) (interface{}, ast.Type, error) {
	// Copy the scope so we can add our builtins
	scope := registerBuiltins(config.GlobalScope)
	implicitMap := map[ast.Type]map[ast.Type]string{
		ast.TypeInt: {
			ast.TypeString: "__builtin_IntToString",
		},
		ast.TypeString: {
			ast.TypeInt: "__builtin_StringToInt",
		},
	}

	// Build our own semantic checks that we always run
	tv := &TypeCheck{Scope: scope, Implicit: implicitMap}
	ic := &IdentifierCheck{Scope: scope}

	// Build up the semantic checks for execution
	checks := make(
		[]SemanticChecker,
		len(config.SemanticChecks), len(config.SemanticChecks)+2)
	copy(checks, config.SemanticChecks)
	checks = append(checks, ic.Visit)
	checks = append(checks, tv.Visit)

	// Run the semantic checks
	for _, check := range checks {
		if err := check(root); err != nil {
			return nil, ast.TypeInvalid, err
		}
	}

	// Execute
	v := &evalVisitor{Scope: scope}
	return v.Visit(root)
}

// EvalNode is the interface that must be implemented by any ast.Node
// to support evaluation. This will be called in visitor pattern order.
// The result of each call to Eval is automatically pushed onto the
// stack as a LiteralNode. Pop elements off the stack to get child
// values.
type EvalNode interface {
	Eval(ast.Scope, *ast.Stack) (interface{}, ast.Type, error)
}

type evalVisitor struct {
	Scope ast.Scope
	Stack ast.Stack

	err  error
	lock sync.Mutex
}

func (v *evalVisitor) Visit(root ast.Node) (interface{}, ast.Type, error) {
	// Run the actual visitor pattern
	root.Accept(v.visit)

	// Get our result and clear out everything else
	var result *ast.LiteralNode
	if v.Stack.Len() > 0 {
		result = v.Stack.Pop().(*ast.LiteralNode)
	} else {
		result = new(ast.LiteralNode)
	}
	resultErr := v.err

	// Clear everything else so we aren't just dangling
	v.Stack.Reset()
	v.err = nil

	t, err := result.Type(v.Scope)
	if err != nil {
		return nil, ast.TypeInvalid, err
	}

	return result.Value, t, resultErr
}

func (v *evalVisitor) visit(raw ast.Node) ast.Node {
	if v.err != nil {
		return raw
	}

	en, err := evalNode(raw)
	if err != nil {
		v.err = err
		return raw
	}

	out, outType, err := en.Eval(v.Scope, &v.Stack)
	if err != nil {
		v.err = err
		return raw
	}

	v.Stack.Push(&ast.LiteralNode{
		Value: out,
		Typex: outType,
	})
	return raw
}

// evalNode is a private function that returns an EvalNode for built-in
// types as well as any other EvalNode implementations.
func evalNode(raw ast.Node) (EvalNode, error) {
	switch n := raw.(type) {
	case *ast.Call:
		return &evalCall{n}, nil
	case *ast.Concat:
		return &evalConcat{n}, nil
	case *ast.LiteralNode:
		return &evalLiteralNode{n}, nil
	case *ast.VariableAccess:
		return &evalVariableAccess{n}, nil
	default:
		en, ok := n.(EvalNode)
		if !ok {
			return nil, fmt.Errorf("node doesn't support evaluation: %#v", raw)
		}

		return en, nil
	}
}

type evalCall struct{ *ast.Call }

func (v *evalCall) Eval(s ast.Scope, stack *ast.Stack) (interface{}, ast.Type, error) {
	// Look up the function in the map
	function, ok := s.LookupFunc(v.Func)
	if !ok {
		return nil, ast.TypeInvalid, fmt.Errorf(
			"unknown function called: %s", v.Func)
	}

	// The arguments are on the stack in reverse order, so pop them off.
	args := make([]interface{}, len(v.Args))
	for i, _ := range v.Args {
		node := stack.Pop().(*ast.LiteralNode)
		args[len(v.Args)-1-i] = node.Value
	}

	// Call the function
	result, err := function.Callback(args)
	if err != nil {
		return nil, ast.TypeInvalid, fmt.Errorf("%s: %s", v.Func, err)
	}

	return result, function.ReturnType, nil
}

type evalConcat struct{ *ast.Concat }

func (v *evalConcat) Eval(s ast.Scope, stack *ast.Stack) (interface{}, ast.Type, error) {
	// The expressions should all be on the stack in reverse
	// order. So pop them off, reverse their order, and concatenate.
	nodes := make([]*ast.LiteralNode, 0, len(v.Exprs))
	for range v.Exprs {
		nodes = append(nodes, stack.Pop().(*ast.LiteralNode))
	}

	var buf bytes.Buffer
	for i := len(nodes) - 1; i >= 0; i-- {
		buf.WriteString(nodes[i].Value.(string))
	}

	return buf.String(), ast.TypeString, nil
}

type evalLiteralNode struct{ *ast.LiteralNode }

func (v *evalLiteralNode) Eval(ast.Scope, *ast.Stack) (interface{}, ast.Type, error) {
	return v.Value, v.Typex, nil
}

type evalVariableAccess struct{ *ast.VariableAccess }

func (v *evalVariableAccess) Eval(scope ast.Scope, _ *ast.Stack) (interface{}, ast.Type, error) {
	// Look up the variable in the map
	variable, ok := scope.LookupVar(v.Name)
	if !ok {
		return nil, ast.TypeInvalid, fmt.Errorf(
			"unknown variable accessed: %s", v.Name)
	}

	return variable.Value, variable.Type, nil
}