Update interface of Interpolate

Signed-off-by: Daniel Nephin <dnephin@docker.com>

cli/compose/interpolation/interpolation.go

@@ -1,75 +1,88 @@
 package interpolation
 
 import (
+	"os"
+
 	"github.com/docker/cli/cli/compose/template"
 	"github.com/pkg/errors"
 )
 
+// Options supported by Interpolate
+type Options struct {
+	// SectionName of the configuration section
+	SectionName string
+	// LookupValue from a key
+	LookupValue LookupValue
+}
+
+// LookupValue is a function which maps from variable names to values.
+// Returns the value as a string and a bool indicating whether
+// the value is present, to distinguish between an empty string
+// and the absence of a value.
+type LookupValue func(key string) (string, bool)
+
 // Interpolate replaces variables in a string with the values from a mapping
-func Interpolate(config map[string]interface{}, section string, mapping template.Mapping) (map[string]interface{}, error) {
+func Interpolate(config map[string]interface{}, opts Options) (map[string]interface{}, error) {
 	out := map[string]interface{}{}
 
-	for name, item := range config {
+	if opts.LookupValue == nil {
+		opts.LookupValue = os.LookupEnv
+	}
+
+	for key, item := range config {
 		if item == nil {
-			out[name] = nil
+			out[key] = nil
 			continue
 		}
 		mapItem, ok := item.(map[string]interface{})
 		if !ok {
-			return nil, errors.Errorf("Invalid type for %s : %T instead of %T", name, item, out)
+			return nil, errors.Errorf("Invalid type for %s : %T instead of %T", key, item, out)
 		}
-		interpolatedItem, err := interpolateSectionItem(name, mapItem, section, mapping)
+		interpolatedItem, err := interpolateSectionItem(key, mapItem, opts)
 		if err != nil {
 			return nil, err
 		}
-		out[name] = interpolatedItem
+		out[key] = interpolatedItem
 	}
 
 	return out, nil
 }
 
 func interpolateSectionItem(
-	name string,
+	sectionkey string,
 	item map[string]interface{},
-	section string,
-	mapping template.Mapping,
+	opts Options,
 ) (map[string]interface{}, error) {
-
 	out := map[string]interface{}{}
 
 	for key, value := range item {
-		interpolatedValue, err := recursiveInterpolate(value, mapping)
+		interpolatedValue, err := recursiveInterpolate(value, opts)
 		switch err := err.(type) {
 		case nil:
 		case *template.InvalidTemplateError:
 			return nil, errors.Errorf(
 				"Invalid interpolation format for %#v option in %s %#v: %#v. You may need to escape any $ with another $.",
-				key, section, name, err.Template,
+				key, opts.SectionName, sectionkey, err.Template,
 			)
 		default:
-			return nil, errors.Wrapf(err, "error while interpolating %s in %s %s", key, section, name)
+			return nil, errors.Wrapf(err, "error while interpolating %s in %s %s", key, opts.SectionName, sectionkey)
 		}
 		out[key] = interpolatedValue
 	}
 
 	return out, nil
-
 }
 
-func recursiveInterpolate(
-	value interface{},
-	mapping template.Mapping,
-) (interface{}, error) {
-
+func recursiveInterpolate(value interface{}, opts Options) (interface{}, error) {
 	switch value := value.(type) {
 
 	case string:
-		return template.Substitute(value, mapping)
+		return template.Substitute(value, template.Mapping(opts.LookupValue))
 
 	case map[string]interface{}:
 		out := map[string]interface{}{}
 		for key, elem := range value {
-			interpolatedElem, err := recursiveInterpolate(elem, mapping)
+			interpolatedElem, err := recursiveInterpolate(elem, opts)
 			if err != nil {
 				return nil, err
 			}
@@ -80,7 +93,7 @@ func recursiveInterpolate(
 	case []interface{}:
 		out := make([]interface{}, len(value))
 		for i, elem := range value {
-			interpolatedElem, err := recursiveInterpolate(elem, mapping)
+			interpolatedElem, err := recursiveInterpolate(elem, opts)
 			if err != nil {
 				return nil, err
 			}
@@ -92,5 +105,4 @@ func recursiveInterpolate(
 		return value, nil
 
 	}
-
 }

cli/compose/interpolation/interpolation_test.go

@@ -3,6 +3,7 @@ package interpolation
 import (
 	"testing"
 
+	"github.com/gotestyourself/gotestyourself/env"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -41,7 +42,10 @@ func TestInterpolate(t *testing.T) {
 			},
 		},
 	}
-	result, err := Interpolate(services, "service", defaultMapping)
+	result, err := Interpolate(services, Options{
+		SectionName: "service",
+		LookupValue: defaultMapping,
+	})
 	assert.NoError(t, err)
 	assert.Equal(t, expected, result)
 }
@@ -52,6 +56,27 @@ func TestInvalidInterpolation(t *testing.T) {
 			"image": "${",
 		},
 	}
-	_, err := Interpolate(services, "service", defaultMapping)
+	_, err := Interpolate(services, Options{
+		SectionName: "service",
+		LookupValue: defaultMapping,
+	})
 	assert.EqualError(t, err, `Invalid interpolation format for "image" option in service "servicea": "${". You may need to escape any $ with another $.`)
 }
+
+func TestInterpolateWithDefaults(t *testing.T) {
+	defer env.Patch(t, "FOO", "BARZ")()
+
+	config := map[string]interface{}{
+		"networks": map[string]interface{}{
+			"foo": "thing_${FOO}",
+		},
+	}
+	expected := map[string]interface{}{
+		"networks": map[string]interface{}{
+			"foo": "thing_BARZ",
+		},
+	}
+	result, err := Interpolate(config, Options{})
+	assert.NoError(t, err)
+	assert.Equal(t, expected, result)
+}