terraform: add reason to diff mismatch error

Helps with debugging those pesky "diffs did not match" errors.

terraform/diff.go

@@ -357,18 +357,20 @@ func (d *InstanceDiff) RequiresNew() bool {
 // we say "same", it is not necessarily exactly equal. Instead, it is
 // just checking that the same attributes are changing, a destroy
 // isn't suddenly happening, etc.
-func (d *InstanceDiff) Same(d2 *InstanceDiff) bool {
+func (d *InstanceDiff) Same(d2 *InstanceDiff) (bool, string) {
 	if d == nil && d2 == nil {
-		return true
+		return true, ""
 	} else if d == nil || d2 == nil {
-		return false
+		return false, "both nil"
 	}
 
 	if d.Destroy != d2.Destroy {
-		return false
+		return false, fmt.Sprintf(
+			"diff: Destroy; old: %t, new: %t", d.Destroy, d2.Destroy)
 	}
 	if d.RequiresNew() != d2.RequiresNew() {
-		return false
+		return false, fmt.Sprintf(
+			"diff RequiresNew; old: %t, new: %t", d.RequiresNew(), d2.RequiresNew())
 	}
 
 	// Go through the old diff and make sure the new diff has all the
@@ -420,7 +422,7 @@ func (d *InstanceDiff) Same(d2 *InstanceDiff) bool {
 				}
 				re, err := regexp.Compile("^" + strings.Join(parts2, `\.`) + "$")
 				if err != nil {
-					return false
+					return false, fmt.Sprintf("regexp failed to compile; err: %#v", err)
 				}
 				for k2, _ := range checkNew {
 					if re.MatchString(k2) {
@@ -452,7 +454,7 @@ func (d *InstanceDiff) Same(d2 *InstanceDiff) bool {
 			}
 
 			if !ok {
-				return false
+				return false, fmt.Sprintf("attribute mismatch: %s", k)
 			}
 		}
 
@@ -477,8 +479,13 @@ func (d *InstanceDiff) Same(d2 *InstanceDiff) bool {
 
 	// Check for leftover attributes
 	if len(checkNew) > 0 {
-		return false
+		extras := make([]string, 0, len(checkNew))
+		for attr, _ := range checkNew {
+			extras = append(extras, attr)
+		}
+		return false,
+			fmt.Sprintf("extra attributes: %s", strings.Join(extras, ", "))
 	}
 
-	return true
+	return true, ""
 }

terraform/diff_test.go

@@ -361,29 +361,34 @@ func TestInstanceDiffSame(t *testing.T) {
 	cases := []struct {
 		One, Two *InstanceDiff
 		Same     bool
+		Reason   string
 	}{
 		{
 			&InstanceDiff{},
 			&InstanceDiff{},
 			true,
+			"",
 		},
 
 		{
 			nil,
 			nil,
 			true,
+			"",
 		},
 
 		{
 			&InstanceDiff{Destroy: false},
 			&InstanceDiff{Destroy: true},
 			false,
+			"diff: Destroy; old: false, new: true",
 		},
 
 		{
 			&InstanceDiff{Destroy: true},
 			&InstanceDiff{Destroy: true},
 			true,
+			"",
 		},
 
 		{
@@ -398,6 +403,7 @@ func TestInstanceDiffSame(t *testing.T) {
 				},
 			},
 			true,
+			"",
 		},
 
 		{
@@ -412,6 +418,7 @@ func TestInstanceDiffSame(t *testing.T) {
 				},
 			},
 			false,
+			"attribute mismatch: bar",
 		},
 
 		// Extra attributes
@@ -428,6 +435,7 @@ func TestInstanceDiffSame(t *testing.T) {
 				},
 			},
 			false,
+			"extra attributes: bar",
 		},
 
 		{
@@ -442,6 +450,7 @@ func TestInstanceDiffSame(t *testing.T) {
 				},
 			},
 			false,
+			"diff RequiresNew; old: true, new: false",
 		},
 
 		{
@@ -463,6 +472,7 @@ func TestInstanceDiffSame(t *testing.T) {
 				},
 			},
 			true,
+			"",
 		},
 
 		{
@@ -491,6 +501,7 @@ func TestInstanceDiffSame(t *testing.T) {
 				},
 			},
 			true,
+			"",
 		},
 
 		{
@@ -506,14 +517,19 @@ func TestInstanceDiffSame(t *testing.T) {
 				Attributes: map[string]*ResourceAttrDiff{},
 			},
 			true,
+			"",
 		},
 	}
 
 	for i, tc := range cases {
-		actual := tc.One.Same(tc.Two)
-		if actual != tc.Same {
+		same, reason := tc.One.Same(tc.Two)
+		if same != tc.Same {
 			t.Fatalf("%d:\n\n%#v\n\n%#v", i, tc.One, tc.Two)
 		}
+		if reason != tc.Reason {
+			t.Fatalf(
+				"%d: bad reason\n\nexpected: %#v\n\ngot: %#v", i, tc.Reason, reason)
+		}
 	}
 }
 

terraform/eval_diff.go

@@ -38,8 +38,9 @@ func (n *EvalCompareDiff) Eval(ctx EvalContext) (interface{}, error) {
 		}
 	}()
 
-	if !one.Same(two) {
-		log.Printf("[ERROR] %s: diff's didn't match", n.Info.Id)
+	if same, reason := one.Same(two); !same {
+		log.Printf("[ERROR] %s: diffs didn't match", n.Info.Id)
+		log.Printf("[ERROR] %s: reason: %s", n.Info.Id, reason)
 		log.Printf("[ERROR] %s: diff one: %#v", n.Info.Id, one)
 		log.Printf("[ERROR] %s: diff two: %#v", n.Info.Id, two)
 		return nil, fmt.Errorf(