config: validate that multi-variables are only used in slices

config/config.go

@@ -388,6 +388,17 @@ func (c *Config) Validate() error {
 		}
 	}
 
+	// Check that all variables are in the proper context
+	for source, rc := range c.rawConfigs() {
+		walker := &interpolationWalker{
+			ContextF: c.validateVarContextFn(source, &errs),
+		}
+		if err := reflectwalk.Walk(rc.Raw, walker); err != nil {
+			errs = append(errs, fmt.Errorf(
+				"%s: error reading config: %s", source, err))
+		}
+	}
+
 	if len(errs) > 0 {
 		return &multierror.Error{Errors: errs}
 	}
@@ -400,33 +411,57 @@ func (c *Config) Validate() error {
 // are valid in the Validate step.
 func (c *Config) InterpolatedVariables() map[string][]InterpolatedVariable {
 	result := make(map[string][]InterpolatedVariable)
-	for _, pc := range c.ProviderConfigs {
-		source := fmt.Sprintf("provider config '%s'", pc.Name)
-		for _, v := range pc.RawConfig.Variables {
+	for source, rc := range c.rawConfigs() {
+		for _, v := range rc.Variables {
 			result[source] = append(result[source], v)
 		}
 	}
+	return result
+}
+
+// rawConfigs returns all of the RawConfigs that are available keyed by
+// a human-friendly source.
+func (c *Config) rawConfigs() map[string]*RawConfig {
+	result := make(map[string]*RawConfig)
+	for _, pc := range c.ProviderConfigs {
+		source := fmt.Sprintf("provider config '%s'", pc.Name)
+		result[source] = pc.RawConfig
+	}
 
 	for _, rc := range c.Resources {
 		source := fmt.Sprintf("resource '%s'", rc.Id())
-		for _, v := range rc.RawCount.Variables {
-			result[source] = append(result[source], v)
-		}
-		for _, v := range rc.RawConfig.Variables {
-			result[source] = append(result[source], v)
-		}
+		result[source+" count"] = rc.RawCount
+		result[source+" config"] = rc.RawConfig
 	}
 
 	for _, o := range c.Outputs {
 		source := fmt.Sprintf("output '%s'", o.Name)
-		for _, v := range o.RawConfig.Variables {
-			result[source] = append(result[source], v)
-		}
+		result[source] = o.RawConfig
 	}
 
 	return result
 }
 
+func (c *Config) validateVarContextFn(
+	source string, errs *[]error) interpolationWalkerContextFunc {
+	return func(loc reflectwalk.Location, i Interpolation) {
+		vi, ok := i.(*VariableInterpolation)
+		if !ok {
+			return
+		}
+
+		rv, ok := vi.Variable.(*ResourceVariable)
+		if !ok {
+			return
+		}
+
+		if rv.Multi && loc != reflectwalk.SliceElem {
+			*errs = append(*errs, fmt.Errorf(
+				"%s: multi-variable must be in a slice", source))
+		}
+	}
+}
+
 func (m *Module) mergerName() string {
 	return m.Id()
 }

config/config_test.go

@@ -221,6 +221,13 @@ func TestConfigValidate_varDefaultInterpolate(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_varMultiNonSlice(t *testing.T) {
+	c := testConfig(t, "validate-var-multi-non-slice")
+	if err := c.Validate(); err == nil {
+		t.Fatal("should not be valid")
+	}
+}
+
 func TestConfigValidate_varModule(t *testing.T) {
 	c := testConfig(t, "validate-var-module")
 	if err := c.Validate(); err != nil {

config/interpolate_walk.go

@@ -22,8 +22,17 @@ var interpRegexp *regexp.Regexp = regexp.MustCompile(
 // (github.com/mitchellh/reflectwalk) that can be used to automatically
 // execute a callback for an interpolation.
 type interpolationWalker struct {
-	F       interpolationWalkerFunc
-	Replace bool
+	// F is the function to call for every interpolation. It can be nil.
+	//
+	// If Replace is true, then the return value of F will be used to
+	// replace the interpolation.
+	F           interpolationWalkerFunc
+	Replace     bool
+
+	// ContextF is an advanced version of F that also receives the
+	// location of where it is in the structure. This lets you do
+	// context-aware validation.
+	ContextF interpolationWalkerContextFunc
 
 	key         []string
 	lastValue   reflect.Value
@@ -43,6 +52,14 @@ type interpolationWalker struct {
 // value can be anything as it will have no effect.
 type interpolationWalkerFunc func(Interpolation) (string, error)
 
+// interpolationWalkerContextFunc is called by interpolationWalk if
+// ContextF is set. This receives both the interpolation and the location
+// where the interpolation is.
+//
+// This callback can be used to validate the location of the interpolation
+// within the configuration.
+type interpolationWalkerContextFunc func(reflectwalk.Location, Interpolation)
+
 func (w *interpolationWalker) Enter(loc reflectwalk.Location) error {
 	w.loc = loc
 	return nil
@@ -129,6 +146,14 @@ func (w *interpolationWalker) Primitive(v reflect.Value) error {
 			return err
 		}
 
+		if w.ContextF != nil {
+			w.ContextF(w.loc, i)
+		}
+
+		if w.F == nil {
+			continue
+		}
+
 		replaceVal, err := w.F(i)
 		if err != nil {
 			return fmt.Errorf(

config/test-fixtures/validate-var-multi-non-slice/main.tf

@@ -0,0 +1,7 @@
+resource "aws_instance" "foo" {
+    count = 3
+}
+
+resource "aws_instance" "bar" {
+    foo = "${aws_instance.foo.*.id}"
+}