cc/config/bp2build.go - platform/build/soong - Git at Google

 // Copyright 2021 Google Inc. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 package config

 import (
 	"fmt"
 	"reflect"
 	"regexp"
 	"sort"
 	"strings"

 	"android/soong/android"
 	"android/soong/starlark_fmt"

 	"github.com/google/blueprint"
 )

 type bazelVarExporter interface {
 	asBazel(android.Config, exportedStringVariables, exportedStringListVariables, exportedConfigDependingVariables) []bazelConstant
 }

 // Helpers for exporting cc configuration information to Bazel.
 var (
 	// Maps containing toolchain variables that are independent of the
 	// environment variables of the build.
 	exportedStringListVars     = exportedStringListVariables{}
 	exportedStringVars         = exportedStringVariables{}
 	exportedStringListDictVars = exportedStringListDictVariables{}
 	// Note: these can only contain references to other variables and must be printed last
 	exportedVariableReferenceDictVars = exportedVariableReferenceDictVariables{}

 	/// Maps containing variables that are dependent on the build config.
 	exportedConfigDependingVars = exportedConfigDependingVariables{}
 )

 type exportedConfigDependingVariables map[string]interface{}

 func (m exportedConfigDependingVariables) Set(k string, v interface{}) {
 	m[k] = v
 }

 // Ensure that string s has no invalid characters to be generated into the bzl file.
 func validateCharacters(s string) string {
 	for _, c := range []string{`\n`, `"`, `\`} {
 		if strings.Contains(s, c) {
 			panic(fmt.Errorf("%s contains illegal character %s", s, c))
 		}
 	}
 	return s
 }

 type bazelConstant struct {
 	variableName       string
 	internalDefinition string
 	sortLast           bool
 }

 type exportedStringVariables map[string]string

 func (m exportedStringVariables) Set(k string, v string) {
 	m[k] = v
 }

 func (m exportedStringVariables) asBazel(config android.Config,
 	stringVars exportedStringVariables, stringListVars exportedStringListVariables, cfgDepVars exportedConfigDependingVariables) []bazelConstant {
 	ret := make([]bazelConstant, 0, len(m))
 	for k, variableValue := range m {
 		expandedVar, err := expandVar(config, variableValue, stringVars, stringListVars, cfgDepVars)
 		if err != nil {
 			panic(fmt.Errorf("error expanding config variable %s: %s", k, err))
 		}
 		if len(expandedVar) > 1 {
 			panic(fmt.Errorf("%s expands to more than one string value: %s", variableValue, expandedVar))
 		}
 		ret = append(ret, bazelConstant{
 			variableName:       k,
 			internalDefinition: fmt.Sprintf(`"%s"`, validateCharacters(expandedVar[0])),
 		})
 	}
 	return ret
 }

 // Convenience function to declare a static variable and export it to Bazel's cc_toolchain.
 func exportStringStaticVariable(name string, value string) {
 	pctx.StaticVariable(name, value)
 	exportedStringVars.Set(name, value)
 }

 type exportedStringListVariables map[string][]string

 func (m exportedStringListVariables) Set(k string, v []string) {
 	m[k] = v
 }

 func (m exportedStringListVariables) asBazel(config android.Config,
 	stringScope exportedStringVariables, stringListScope exportedStringListVariables,
 	exportedVars exportedConfigDependingVariables) []bazelConstant {
 	ret := make([]bazelConstant, 0, len(m))
 	// For each exported variable, recursively expand elements in the variableValue
 	// list to ensure that interpolated variables are expanded according to their values
 	// in the variable scope.
 	for k, variableValue := range m {
 		var expandedVars []string
 		for _, v := range variableValue {
 			expandedVar, err := expandVar(config, v, stringScope, stringListScope, exportedVars)
 			if err != nil {
 				panic(fmt.Errorf("Error expanding config variable %s=%s: %s", k, v, err))
 			}
 			expandedVars = append(expandedVars, expandedVar...)
 		}
 		// Assign the list as a bzl-private variable; this variable will be exported
 		// out through a constants struct later.
 		ret = append(ret, bazelConstant{
 			variableName:       k,
 			internalDefinition: starlark_fmt.PrintStringList(expandedVars, 0),
 		})
 	}
 	return ret
 }

 // Convenience function to declare a static "source path" variable and export it to Bazel's cc_toolchain.
 func exportVariableConfigMethod(name string, method interface{}) blueprint.Variable {
 	exportedConfigDependingVars.Set(name, method)
 	return pctx.VariableConfigMethod(name, method)
 }

 // Convenience function to declare a static "source path" variable and export it to Bazel's cc_toolchain.
 func exportSourcePathVariable(name string, value string) {
 	pctx.SourcePathVariable(name, value)
 	exportedStringVars.Set(name, value)
 }

 // Convenience function to declare a static variable and export it to Bazel's cc_toolchain.
 func exportStringListStaticVariable(name string, value []string) {
 	pctx.StaticVariable(name, strings.Join(value, " "))
 	exportedStringListVars.Set(name, value)
 }

 func ExportStringList(name string, value []string) {
 	exportedStringListVars.Set(name, value)
 }

 type exportedStringListDictVariables map[string]map[string][]string

 func (m exportedStringListDictVariables) Set(k string, v map[string][]string) {
 	m[k] = v
 }

 // Since dictionaries are not supported in Ninja, we do not expand variables for dictionaries
 func (m exportedStringListDictVariables) asBazel(_ android.Config, _ exportedStringVariables,
 	_ exportedStringListVariables, _ exportedConfigDependingVariables) []bazelConstant {
 	ret := make([]bazelConstant, 0, len(m))
 	for k, dict := range m {
 		ret = append(ret, bazelConstant{
 			variableName:       k,
 			internalDefinition: starlark_fmt.PrintStringListDict(dict, 0),
 		})
 	}
 	return ret
 }

 type exportedVariableReferenceDictVariables map[string]map[string]string

 func (m exportedVariableReferenceDictVariables) Set(k string, v map[string]string) {
 	m[k] = v
 }

 func (m exportedVariableReferenceDictVariables) asBazel(_ android.Config, _ exportedStringVariables,
 	_ exportedStringListVariables, _ exportedConfigDependingVariables) []bazelConstant {
 	ret := make([]bazelConstant, 0, len(m))
 	for n, dict := range m {
 		for k, v := range dict {
 			matches, err := variableReference(v)
 			if err != nil {
 				panic(err)
 			} else if !matches.matches {
 				panic(fmt.Errorf("Expected a variable reference, got %q", v))
 			} else if len(matches.fullVariableReference) != len(v) {
 				panic(fmt.Errorf("Expected only a variable reference, got %q", v))
 			}
 			dict[k] = "_" + matches.variable
 		}
 		ret = append(ret, bazelConstant{
 			variableName:       n,
 			internalDefinition: starlark_fmt.PrintDict(dict, 0),
 			sortLast:           true,
 		})
 	}
 	return ret
 }

 // BazelCcToolchainVars generates bzl file content containing variables for
 // Bazel's cc_toolchain configuration.
 func BazelCcToolchainVars(config android.Config) string {
 	return bazelToolchainVars(
 		config,
 		exportedStringListDictVars,
 		exportedStringListVars,
 		exportedStringVars,
 		exportedVariableReferenceDictVars)
 }

 func bazelToolchainVars(config android.Config, vars ...bazelVarExporter) string {
 	ret := "# GENERATED FOR BAZEL FROM SOONG. DO NOT EDIT.\n\n"

 	results := []bazelConstant{}
 	for _, v := range vars {
 		results = append(results, v.asBazel(config, exportedStringVars, exportedStringListVars, exportedConfigDependingVars)...)
 	}

 	sort.Slice(results, func(i, j int) bool {
 		if results[i].sortLast != results[j].sortLast {
 			return !results[i].sortLast
 		}
 		return results[i].variableName < results[j].variableName
 	})

 	definitions := make([]string, 0, len(results))
 	constants := make([]string, 0, len(results))
 	for _, b := range results {
 		definitions = append(definitions,
 			fmt.Sprintf("_%s = %s", b.variableName, b.internalDefinition))
 		constants = append(constants,
 			fmt.Sprintf("%[1]s%[2]s = _%[2]s,", starlark_fmt.Indention(1), b.variableName))
 	}

 	// Build the exported constants struct.
 	ret += strings.Join(definitions, "\n\n")
 	ret += "\n\n"
 	ret += "constants = struct(\n"
 	ret += strings.Join(constants, "\n")
 	ret += "\n)"

 	return ret
 }

 type match struct {
 	matches               bool
 	fullVariableReference string
 	variable              string
 }

 func variableReference(input string) (match, error) {
 	// e.g. "${ExternalCflags}"
 	r := regexp.MustCompile(`\${(?:config\.)?([a-zA-Z0-9_]+)}`)

 	matches := r.FindStringSubmatch(input)
 	if len(matches) == 0 {
 		return match{}, nil
 	}
 	if len(matches) != 2 {
 		return match{}, fmt.Errorf("Expected to only match 1 subexpression in %s, got %d", input, len(matches)-1)
 	}
 	return match{
 		matches:               true,
 		fullVariableReference: matches[0],
 		// Index 1 of FindStringSubmatch contains the subexpression match
 		// (variable name) of the capture group.
 		variable: matches[1],
 	}, nil
 }

 // expandVar recursively expand interpolated variables in the exportedVars scope.
 //
 // We're using a string slice to track the seen variables to avoid
 // stackoverflow errors with infinite recursion. it's simpler to use a
 // string slice than to handle a pass-by-referenced map, which would make it
 // quite complex to track depth-first interpolations. It's also unlikely the
 // interpolation stacks are deep (n > 1).
 func expandVar(config android.Config, toExpand string, stringScope exportedStringVariables,
 	stringListScope exportedStringListVariables, exportedVars exportedConfigDependingVariables) ([]string, error) {

 	// Internal recursive function.
 	var expandVarInternal func(string, map[string]bool) (string, error)
 	expandVarInternal = func(toExpand string, seenVars map[string]bool) (string, error) {
 		var ret string
 		remainingString := toExpand
 		for len(remainingString) > 0 {
 			matches, err := variableReference(remainingString)
 			if err != nil {
 				panic(err)
 			}
 			if !matches.matches {
 				return ret + remainingString, nil
 			}
 			matchIndex := strings.Index(remainingString, matches.fullVariableReference)
 			ret += remainingString[:matchIndex]
 			remainingString = remainingString[matchIndex+len(matches.fullVariableReference):]

 			variable := matches.variable
 			// toExpand contains a variable.
 			if _, ok := seenVars[variable]; ok {
 				return ret, fmt.Errorf(
 					"Unbounded recursive interpolation of variable: %s", variable)
 			}
 			// A map is passed-by-reference. Create a new map for
 			// this scope to prevent variables seen in one depth-first expansion
 			// to be also treated as "seen" in other depth-first traversals.
 			newSeenVars := map[string]bool{}
 			for k := range seenVars {
 				newSeenVars[k] = true
 			}
 			newSeenVars[variable] = true
 			if unexpandedVars, ok := stringListScope[variable]; ok {
 				expandedVars := []string{}
 				for _, unexpandedVar := range unexpandedVars {
 					expandedVar, err := expandVarInternal(unexpandedVar, newSeenVars)
 					if err != nil {
 						return ret, err
 					}
 					expandedVars = append(expandedVars, expandedVar)
 				}
 				ret += strings.Join(expandedVars, " ")
 			} else if unexpandedVar, ok := stringScope[variable]; ok {
 				expandedVar, err := expandVarInternal(unexpandedVar, newSeenVars)
 				if err != nil {
 					return ret, err
 				}
 				ret += expandedVar
 			} else if unevaluatedVar, ok := exportedVars[variable]; ok {
 				evalFunc := reflect.ValueOf(unevaluatedVar)
 				validateVariableMethod(variable, evalFunc)
 				evaluatedResult := evalFunc.Call([]reflect.Value{reflect.ValueOf(config)})
 				evaluatedValue := evaluatedResult[0].Interface().(string)
 				expandedVar, err := expandVarInternal(evaluatedValue, newSeenVars)
 				if err != nil {
 					return ret, err
 				}
 				ret += expandedVar
 			} else {
 				return "", fmt.Errorf("Unbound config variable %s", variable)
 			}
 		}
 		return ret, nil
 	}
 	var ret []string
 	for _, v := range strings.Split(toExpand, " ") {
 		val, err := expandVarInternal(v, map[string]bool{})
 		if err != nil {
 			return ret, err
 		}
 		ret = append(ret, val)
 	}

 	return ret, nil
 }

 func validateVariableMethod(name string, methodValue reflect.Value) {
 	methodType := methodValue.Type()
 	if methodType.Kind() != reflect.Func {
 		panic(fmt.Errorf("method given for variable %s is not a function",
 			name))
 	}
 	if n := methodType.NumIn(); n != 1 {
 		panic(fmt.Errorf("method for variable %s has %d inputs (should be 1)",
 			name, n))
 	}
 	if n := methodType.NumOut(); n != 1 {
 		panic(fmt.Errorf("method for variable %s has %d outputs (should be 1)",
 			name, n))
 	}
 	if kind := methodType.Out(0).Kind(); kind != reflect.String {
 		panic(fmt.Errorf("method for variable %s does not return a string",
 			name))
 	}
 }
	// Copyright 2021 Google Inc. All rights reserved.
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	package config

	import (
	"fmt"
	"reflect"
	"regexp"
	"sort"
	"strings"

	"android/soong/android"
	"android/soong/starlark_fmt"

	"github.com/google/blueprint"
	)

	type bazelVarExporter interface {
	asBazel(android.Config, exportedStringVariables, exportedStringListVariables, exportedConfigDependingVariables) []bazelConstant
	}

	// Helpers for exporting cc configuration information to Bazel.
	var (
	// Maps containing toolchain variables that are independent of the
	// environment variables of the build.
	exportedStringListVars = exportedStringListVariables{}
	exportedStringVars = exportedStringVariables{}
	exportedStringListDictVars = exportedStringListDictVariables{}
	// Note: these can only contain references to other variables and must be printed last
	exportedVariableReferenceDictVars = exportedVariableReferenceDictVariables{}

	/// Maps containing variables that are dependent on the build config.
	exportedConfigDependingVars = exportedConfigDependingVariables{}
	)

	type exportedConfigDependingVariables map[string]interface{}

	func (m exportedConfigDependingVariables) Set(k string, v interface{}) {
	m[k] = v
	}

	// Ensure that string s has no invalid characters to be generated into the bzl file.
	func validateCharacters(s string) string {
	for _, c := range []string{`\n`, `"`, `\`} {
	if strings.Contains(s, c) {
	panic(fmt.Errorf("%s contains illegal character %s", s, c))
	}
	}
	return s
	}

	type bazelConstant struct {
	variableName string
	internalDefinition string
	sortLast bool
	}

	type exportedStringVariables map[string]string

	func (m exportedStringVariables) Set(k string, v string) {
	m[k] = v
	}

	func (m exportedStringVariables) asBazel(config android.Config,
	stringVars exportedStringVariables, stringListVars exportedStringListVariables, cfgDepVars exportedConfigDependingVariables) []bazelConstant {
	ret := make([]bazelConstant, 0, len(m))
	for k, variableValue := range m {
	expandedVar, err := expandVar(config, variableValue, stringVars, stringListVars, cfgDepVars)
	if err != nil {
	panic(fmt.Errorf("error expanding config variable %s: %s", k, err))
	}
	if len(expandedVar) > 1 {
	panic(fmt.Errorf("%s expands to more than one string value: %s", variableValue, expandedVar))
	}
	ret = append(ret, bazelConstant{
	variableName: k,
	internalDefinition: fmt.Sprintf(`"%s"`, validateCharacters(expandedVar[0])),
	})
	}
	return ret
	}

	// Convenience function to declare a static variable and export it to Bazel's cc_toolchain.
	func exportStringStaticVariable(name string, value string) {
	pctx.StaticVariable(name, value)
	exportedStringVars.Set(name, value)
	}

	type exportedStringListVariables map[string][]string

	func (m exportedStringListVariables) Set(k string, v []string) {
	m[k] = v
	}

	func (m exportedStringListVariables) asBazel(config android.Config,
	stringScope exportedStringVariables, stringListScope exportedStringListVariables,
	exportedVars exportedConfigDependingVariables) []bazelConstant {
	ret := make([]bazelConstant, 0, len(m))
	// For each exported variable, recursively expand elements in the variableValue
	// list to ensure that interpolated variables are expanded according to their values
	// in the variable scope.
	for k, variableValue := range m {
	var expandedVars []string
	for _, v := range variableValue {
	expandedVar, err := expandVar(config, v, stringScope, stringListScope, exportedVars)
	if err != nil {
	panic(fmt.Errorf("Error expanding config variable %s=%s: %s", k, v, err))
	}
	expandedVars = append(expandedVars, expandedVar...)
	}
	// Assign the list as a bzl-private variable; this variable will be exported
	// out through a constants struct later.
	ret = append(ret, bazelConstant{
	variableName: k,
	internalDefinition: starlark_fmt.PrintStringList(expandedVars, 0),
	})
	}
	return ret
	}

	// Convenience function to declare a static "source path" variable and export it to Bazel's cc_toolchain.
	func exportVariableConfigMethod(name string, method interface{}) blueprint.Variable {
	exportedConfigDependingVars.Set(name, method)
	return pctx.VariableConfigMethod(name, method)
	}

	// Convenience function to declare a static "source path" variable and export it to Bazel's cc_toolchain.
	func exportSourcePathVariable(name string, value string) {
	pctx.SourcePathVariable(name, value)
	exportedStringVars.Set(name, value)
	}

	// Convenience function to declare a static variable and export it to Bazel's cc_toolchain.
	func exportStringListStaticVariable(name string, value []string) {
	pctx.StaticVariable(name, strings.Join(value, " "))
	exportedStringListVars.Set(name, value)
	}

	func ExportStringList(name string, value []string) {
	exportedStringListVars.Set(name, value)
	}

	type exportedStringListDictVariables map[string]map[string][]string

	func (m exportedStringListDictVariables) Set(k string, v map[string][]string) {
	m[k] = v
	}

	// Since dictionaries are not supported in Ninja, we do not expand variables for dictionaries
	func (m exportedStringListDictVariables) asBazel(_ android.Config, _ exportedStringVariables,
	_ exportedStringListVariables, _ exportedConfigDependingVariables) []bazelConstant {
	ret := make([]bazelConstant, 0, len(m))
	for k, dict := range m {
	ret = append(ret, bazelConstant{
	variableName: k,
	internalDefinition: starlark_fmt.PrintStringListDict(dict, 0),
	})
	}
	return ret
	}

	type exportedVariableReferenceDictVariables map[string]map[string]string

	func (m exportedVariableReferenceDictVariables) Set(k string, v map[string]string) {
	m[k] = v
	}

	func (m exportedVariableReferenceDictVariables) asBazel(_ android.Config, _ exportedStringVariables,
	_ exportedStringListVariables, _ exportedConfigDependingVariables) []bazelConstant {
	ret := make([]bazelConstant, 0, len(m))
	for n, dict := range m {
	for k, v := range dict {
	matches, err := variableReference(v)
	if err != nil {
	panic(err)
	} else if !matches.matches {
	panic(fmt.Errorf("Expected a variable reference, got %q", v))
	} else if len(matches.fullVariableReference) != len(v) {
	panic(fmt.Errorf("Expected only a variable reference, got %q", v))
	}
	dict[k] = "_" + matches.variable
	}
	ret = append(ret, bazelConstant{
	variableName: n,
	internalDefinition: starlark_fmt.PrintDict(dict, 0),
	sortLast: true,
	})
	}
	return ret
	}

	// BazelCcToolchainVars generates bzl file content containing variables for
	// Bazel's cc_toolchain configuration.
	func BazelCcToolchainVars(config android.Config) string {
	return bazelToolchainVars(
	config,
	exportedStringListDictVars,
	exportedStringListVars,
	exportedStringVars,
	exportedVariableReferenceDictVars)
	}

	func bazelToolchainVars(config android.Config, vars ...bazelVarExporter) string {
	ret := "# GENERATED FOR BAZEL FROM SOONG. DO NOT EDIT.\n\n"

	results := []bazelConstant{}
	for _, v := range vars {
	results = append(results, v.asBazel(config, exportedStringVars, exportedStringListVars, exportedConfigDependingVars)...)
	}

	sort.Slice(results, func(i, j int) bool {
	if results[i].sortLast != results[j].sortLast {
	return !results[i].sortLast
	}
	return results[i].variableName < results[j].variableName
	})

	definitions := make([]string, 0, len(results))
	constants := make([]string, 0, len(results))
	for _, b := range results {
	definitions = append(definitions,
	fmt.Sprintf("_%s = %s", b.variableName, b.internalDefinition))
	constants = append(constants,
	fmt.Sprintf("%[1]s%[2]s = _%[2]s,", starlark_fmt.Indention(1), b.variableName))
	}

	// Build the exported constants struct.
	ret += strings.Join(definitions, "\n\n")
	ret += "\n\n"
	ret += "constants = struct(\n"
	ret += strings.Join(constants, "\n")
	ret += "\n)"

	return ret
	}

	type match struct {
	matches bool
	fullVariableReference string
	variable string
	}

	func variableReference(input string) (match, error) {
	// e.g. "${ExternalCflags}"
	r := regexp.MustCompile(`\${(?:config\.)?([a-zA-Z0-9_]+)}`)

	matches := r.FindStringSubmatch(input)
	if len(matches) == 0 {
	return match{}, nil
	}
	if len(matches) != 2 {
	return match{}, fmt.Errorf("Expected to only match 1 subexpression in %s, got %d", input, len(matches)-1)
	}
	return match{
	matches: true,
	fullVariableReference: matches[0],
	// Index 1 of FindStringSubmatch contains the subexpression match
	// (variable name) of the capture group.
	variable: matches[1],
	}, nil
	}

	// expandVar recursively expand interpolated variables in the exportedVars scope.
	//
	// We're using a string slice to track the seen variables to avoid
	// stackoverflow errors with infinite recursion. it's simpler to use a
	// string slice than to handle a pass-by-referenced map, which would make it
	// quite complex to track depth-first interpolations. It's also unlikely the
	// interpolation stacks are deep (n > 1).
	func expandVar(config android.Config, toExpand string, stringScope exportedStringVariables,
	stringListScope exportedStringListVariables, exportedVars exportedConfigDependingVariables) ([]string, error) {

	// Internal recursive function.
	var expandVarInternal func(string, map[string]bool) (string, error)
	expandVarInternal = func(toExpand string, seenVars map[string]bool) (string, error) {
	var ret string
	remainingString := toExpand
	for len(remainingString) > 0 {
	matches, err := variableReference(remainingString)
	if err != nil {
	panic(err)
	}
	if !matches.matches {
	return ret + remainingString, nil
	}
	matchIndex := strings.Index(remainingString, matches.fullVariableReference)
	ret += remainingString[:matchIndex]
	remainingString = remainingString[matchIndex+len(matches.fullVariableReference):]

	variable := matches.variable
	// toExpand contains a variable.
	if _, ok := seenVars[variable]; ok {
	return ret, fmt.Errorf(
	"Unbounded recursive interpolation of variable: %s", variable)
	}
	// A map is passed-by-reference. Create a new map for
	// this scope to prevent variables seen in one depth-first expansion
	// to be also treated as "seen" in other depth-first traversals.
	newSeenVars := map[string]bool{}
	for k := range seenVars {
	newSeenVars[k] = true
	}
	newSeenVars[variable] = true
	if unexpandedVars, ok := stringListScope[variable]; ok {
	expandedVars := []string{}
	for _, unexpandedVar := range unexpandedVars {
	expandedVar, err := expandVarInternal(unexpandedVar, newSeenVars)
	if err != nil {
	return ret, err
	}
	expandedVars = append(expandedVars, expandedVar)
	}
	ret += strings.Join(expandedVars, " ")
	} else if unexpandedVar, ok := stringScope[variable]; ok {
	expandedVar, err := expandVarInternal(unexpandedVar, newSeenVars)
	if err != nil {
	return ret, err
	}
	ret += expandedVar
	} else if unevaluatedVar, ok := exportedVars[variable]; ok {
	evalFunc := reflect.ValueOf(unevaluatedVar)
	validateVariableMethod(variable, evalFunc)
	evaluatedResult := evalFunc.Call([]reflect.Value{reflect.ValueOf(config)})
	evaluatedValue := evaluatedResult[0].Interface().(string)
	expandedVar, err := expandVarInternal(evaluatedValue, newSeenVars)
	if err != nil {
	return ret, err
	}
	ret += expandedVar
	} else {
	return "", fmt.Errorf("Unbound config variable %s", variable)
	}
	}
	return ret, nil
	}
	var ret []string
	for _, v := range strings.Split(toExpand, " ") {
	val, err := expandVarInternal(v, map[string]bool{})
	if err != nil {
	return ret, err
	}
	ret = append(ret, val)
	}

	return ret, nil
	}

	func validateVariableMethod(name string, methodValue reflect.Value) {
	methodType := methodValue.Type()
	if methodType.Kind() != reflect.Func {
	panic(fmt.Errorf("method given for variable %s is not a function",
	name))
	}
	if n := methodType.NumIn(); n != 1 {
	panic(fmt.Errorf("method for variable %s has %d inputs (should be 1)",
	name, n))
	}
	if n := methodType.NumOut(); n != 1 {
	panic(fmt.Errorf("method for variable %s has %d outputs (should be 1)",
	name, n))
	}
	if kind := methodType.Out(0).Kind(); kind != reflect.String {
	panic(fmt.Errorf("method for variable %s does not return a string",
	name))
	}
	}