terraform: context verifies real and shadow state/diff match

terraform/context.go

@@ -613,13 +613,13 @@ func (c *Context) walk(
 	realCtx := c
 
 	// If we have a shadow graph, walk that as well
+	var shadowCtx *Context
 	var shadowCh chan error
 	var shadowCloser io.Closer
 	if shadow != nil {
 		// Build the shadow context. In the process, override the real context
 		// with the one that is wrapped so that the shadow context can verify
 		// the results of the real.
-		var shadowCtx *Context
 		realCtx, shadowCtx, shadowCloser = newShadowContext(c)
 
 		// Build the graph walker for the shadow.
@@ -657,6 +657,11 @@ func (c *Context) walk(
 			c.shadowErr = multierror.Append(c.shadowErr, err)
 		}
 
+		// Verify the contexts (compare)
+		if err := shadowContextVerify(realCtx, shadowCtx); err != nil {
+			c.shadowErr = multierror.Append(c.shadowErr, err)
+		}
+
 		if c.shadowErr == nil {
 			log.Printf("[INFO] Shadow graph success!")
 		} else {

terraform/shadow_context.go

@@ -1,8 +1,11 @@
 package terraform
 
 import (
+	"fmt"
 	"io"
+	"reflect"
 
+	"github.com/hashicorp/go-multierror"
 	"github.com/mitchellh/copystructure"
 )
 
@@ -63,6 +66,32 @@ func newShadowContext(c *Context) (*Context, *Context, io.Closer) {
 	}
 }
 
+// shadowContextVerify takes the real and shadow context and verifies they
+// have equal diffs and states.
+func shadowContextVerify(real, shadow *Context) error {
+	var result error
+
+	// Compare the states
+	if !real.state.Equal(shadow.state) {
+		result = multierror.Append(result, fmt.Errorf(
+			"Real and shadow states do not match! "+
+				"Real state:\n\n%s\n\n"+
+				"Shadow state:\n\n%s\n\n",
+			real.state, shadow.state))
+	}
+
+	// Compare the diffs
+	if !reflect.DeepEqual(real.diff, shadow.diff) {
+		result = multierror.Append(result, fmt.Errorf(
+			"Real and shadow diffs do not match! "+
+				"Real diff:\n\n%s\n\n"+
+				"Shadow diff:\n\n%s\n\n",
+			real.diff, shadow.diff))
+	}
+
+	return result
+}
+
 // shadowContextCloser is the io.Closer returned by newShadowContext that
 // closes all the shadows and returns the results.
 type shadowContextCloser struct {

terraform/shadow_resource_provider.go

@@ -167,6 +167,9 @@ type shadowResourceProviderShadow struct {
 }
 
 type shadowResourceProviderShared struct {
+	// NOTE: Anytime a value is added here, be sure to add it to
+	// the Close() method so that it is closed.
+
 	CloseErr  shadow.Value
 	Input     shadow.Value
 	Validate  shadow.Value