config: can detect variables in config strings

config/config.go

@@ -1,5 +1,9 @@
 package config
 
+import (
+	"strings"
+)
+
 // Config is the configuration that comes from loading a collection
 // of Terraform templates.
 type Config struct {
@@ -7,13 +11,71 @@ type Config struct {
 	Resources []Resource
 }
 
+// A resource represents a single Terraform resource in the configuration.
+// A Terraform resource is something that represents some component that
+// can be created and managed, and has some properties associated with it.
 type Resource struct {
-	Name   string
-	Type   string
-	Config map[string]interface{}
+	Name      string
+	Type      string
+	Config    map[string]interface{}
+	Variables map[string]InterpolatedVariable
 }
 
 type Variable struct {
 	Default     string
 	Description string
 }
+
+// An InterpolatedVariable is a variable that is embedded within a string
+// in the configuration, such as "hello ${world}" (world in this case is
+// an interpolated variable).
+//
+// These variables can come from a variety of sources, represented by
+// implementations of this interface.
+type InterpolatedVariable interface {
+	FullKey() string
+}
+
+// A ResourceVariable is a variable that is referencing the field
+// of a resource, such as "${aws_instance.foo.ami}"
+type ResourceVariable struct {
+	Type  string
+	Name  string
+	Field string
+
+	key string
+}
+
+// A UserVariable is a variable that is referencing a user variable
+// that is inputted from outside the configuration. This looks like
+// "${var.foo}"
+type UserVariable struct {
+	name string
+	key  string
+}
+
+func NewResourceVariable(key string) (*ResourceVariable, error) {
+	parts := strings.SplitN(key, ".", 3)
+	return &ResourceVariable{
+		Type:  parts[0],
+		Name:  parts[1],
+		Field: parts[2],
+		key:   key,
+	}, nil
+}
+
+func (v *ResourceVariable) FullKey() string {
+	return v.key
+}
+
+func NewUserVariable(key string) (*UserVariable, error) {
+	name := key[len("var."):]
+	return &UserVariable{
+		key:  key,
+		name: name,
+	}, nil
+}
+
+func (v *UserVariable) FullKey() string {
+	return v.key
+}

config/loader_libucl.go

@@ -5,6 +5,7 @@ import (
 	"path/filepath"
 
 	"github.com/mitchellh/go-libucl"
+	"github.com/mitchellh/reflectwalk"
 )
 
 // Put the parse flags we use for libucl in a constant so we can get
@@ -176,10 +177,20 @@ func loadResourcesLibucl(o *libucl.Object) ([]Resource, error) {
 					err)
 			}
 
+			walker := new(variableDetectWalker)
+			if err := reflectwalk.Walk(config, walker); err != nil {
+				return nil, fmt.Errorf(
+					"Error reading config for %s[%s]: %s",
+					t.Key(),
+					r.Key(),
+					err)
+			}
+
 			result = append(result, Resource{
-				Name:   r.Key(),
-				Type:   t.Key(),
-				Config: config,
+				Name:      r.Key(),
+				Type:      t.Key(),
+				Config:    config,
+				Variables: walker.Variables,
 			})
 		}
 	}

config/loader_test.go

@@ -69,6 +69,23 @@ func resourcesStr(rs []Resource) string {
 		for k, _ := range r.Config {
 			result += fmt.Sprintf("  %s\n", k)
 		}
+
+		if len(r.Variables) > 0 {
+			result += fmt.Sprintf("  vars\n")
+			for _, rawV := range r.Variables {
+				kind := "unknown"
+				str := rawV.FullKey()
+
+				switch rawV.(type) {
+				case *ResourceVariable:
+					kind = "resource"
+				case *UserVariable:
+					kind = "user"
+				}
+
+				result += fmt.Sprintf("    %s: %s\n", kind, str)
+			}
+		}
 	}
 
 	return strings.TrimSpace(result)
@@ -101,6 +118,9 @@ aws_security_group[firewall]
 aws_instance[web]
   ami
   security_groups
+  vars
+    user: var.foo
+    resource: aws_security_group.firewall.foo
 `
 
 const basicVariablesStr = `

config/test-fixtures/basic.tf

@@ -7,7 +7,7 @@ resource "aws_security_group" "firewall" {
 }
 
 resource aws_instance "web" {
-    ami = "ami-123456"
+    ami = "${var.foo}"
     security_groups = [
         "foo",
         "${aws_security_group.firewall.foo}"

config/variable.go

@@ -0,0 +1,70 @@
+package config
+
+import (
+	"reflect"
+	"regexp"
+	"strings"
+)
+
+// varRegexp is a regexp that matches variables such as ${foo.bar}
+var varRegexp *regexp.Regexp
+
+func init() {
+	varRegexp = regexp.MustCompile(`(?i)(\$+)\{([-.a-z0-9_]+)\}`)
+}
+
+// variableDetectWalker implements interfaces for the reflectwalk package
+// (github.com/mitchellh/reflectwalk) that can be used to automatically
+// pull out the variables that need replacing.
+type variableDetectWalker struct {
+	Variables map[string]InterpolatedVariable
+}
+
+func (w *variableDetectWalker) Primitive(v reflect.Value) error {
+	// We only care about strings
+	if v.Kind() != reflect.String {
+		return nil
+	}
+
+	// XXX: This can be a lot more efficient if we used a real
+	// parser. A regexp is a hammer though that will get this working.
+
+	matches := varRegexp.FindAllStringSubmatch(v.String(), -1)
+	if len(matches) == 0 {
+		return nil
+	}
+
+	for _, match := range matches {
+		dollars := len(match[1])
+
+		// If there are even amounts of dollar signs, then it is escaped
+		if dollars%2 == 0 {
+			continue
+		}
+
+		// Otherwise, record it
+		key := match[2]
+		if w.Variables == nil {
+			w.Variables = make(map[string]InterpolatedVariable)
+		}
+		if _, ok := w.Variables[key]; ok {
+			continue
+		}
+
+		var err error
+		var iv InterpolatedVariable
+		if strings.HasPrefix(key, "var.") {
+			iv, err = NewUserVariable(key)
+		} else {
+			iv, err = NewResourceVariable(key)
+		}
+
+		if err != nil {
+			return err
+		}
+
+		w.Variables[key] = iv
+	}
+
+	return nil
+}