core: stabilize ResourceAddress.Less results

The implementation of ResourceAddress.Less was flawed because it was only
testing each field in the "less than" direction, and falling through in
cases where an earlier field compared greater than a later one.

Now we test for inequality first as the selector, and only fall through
if the two values for a given field are equal.

terraform/resource_address.go

@@ -362,40 +362,41 @@ func (addr *ResourceAddress) Less(other *ResourceAddress) bool {
 
 	switch {
 
-	case len(addr.Path) < len(other.Path):
+	case len(addr.Path) != len(other.Path):
-		return true
+		return len(addr.Path) < len(other.Path)
 
 	case !reflect.DeepEqual(addr.Path, other.Path):
 		// If the two paths are the same length but don't match, we'll just
 		// cheat and compare the string forms since it's easier than
-		// comparing all of the path segments in turn.
+		// comparing all of the path segments in turn, and lexicographic
+		// comparison is correct for the module path portion.
 		addrStr := addr.String()
 		otherStr := other.String()
 		return addrStr < otherStr
 
-	case addr.Mode == config.DataResourceMode && other.Mode != config.DataResourceMode:
+	case addr.Mode != other.Mode:
-		return true
+		return addr.Mode == config.DataResourceMode
 
-	case addr.Type < other.Type:
+	case addr.Type != other.Type:
-		return true
+		return addr.Type < other.Type
 
-	case addr.Name < other.Name:
+	case addr.Name != other.Name:
-		return true
+		return addr.Name < other.Name
 
-	case addr.Index < other.Index:
+	case addr.Index != other.Index:
 		// Since "Index" is -1 for an un-indexed address, this also conveniently
 		// sorts unindexed addresses before indexed ones, should they both
 		// appear for some reason.
-		return true
+		return addr.Index < other.Index
 
-	case other.InstanceTypeSet && !addr.InstanceTypeSet:
+	case addr.InstanceTypeSet != other.InstanceTypeSet:
-		return true
+		return !addr.InstanceTypeSet
 
-	case addr.InstanceType < other.InstanceType:
+	case addr.InstanceType != other.InstanceType:
 		// InstanceType is actually an enum, so this is just an arbitrary
 		// sort based on the enum numeric values, and thus not particularly
 		// meaningful.
-		return true
+		return addr.InstanceType < other.InstanceType
 
 	default:
 		return false

terraform/resource_address_test.go

@@ -1233,6 +1233,11 @@ func TestResourceAddressLess(t *testing.T) {
 			"module.baz.foo.bar",
 			true,
 		},
+		{
+			"module.baz.foo.bar",
+			"zzz.bar", // would sort after "module" in lexicographical sort
+			false,
+		},
 		{
 			"module.baz.foo.bar",
 			"module.baz.foo.bar",
@@ -1243,6 +1248,11 @@ func TestResourceAddressLess(t *testing.T) {
 			"module.boz.foo.bar",
 			true,
 		},
+		{
+			"module.boz.foo.bar",
+			"module.baz.foo.bar",
+			false,
+		},
 		{
 			"a.b",
 			"b.c",
@@ -1253,11 +1263,21 @@ func TestResourceAddressLess(t *testing.T) {
 			"a.c",
 			true,
 		},
+		{
+			"c.b",
+			"b.c",
+			false,
+		},
 		{
 			"a.b[9]",
 			"a.b[10]",
 			true,
 		},
+		{
+			"b.b[9]",
+			"a.b[10]",
+			false,
+		},
 		{
 			"a.b",
 			"a.b.deposed",