Add unwrap functionality to topdown.Error

OPA currently doesn't support wrapping errors returned by builtin
functions. This change modifies topdown.Error to include a wrapped
error so that additional context about the builtin error is available.

Fixes open-policy-agent#5890

Signed-off-by: Ajith Subramanian <ajith_subramanian@trimble.com>

rego/rego.go

@@ -2647,12 +2647,14 @@ func finishFunction(name string, bctx topdown.BuiltinContext, result *ast.Term,
 				Code:     topdown.BuiltinErr,
 				Message:  fmt.Sprintf("%v: %v", name, e.Error()),
 				Location: bctx.Location,
+				Err:      e,
 			}}
 		}
 		return &topdown.Error{
 			Code:     topdown.BuiltinErr,
 			Message:  fmt.Sprintf("%v: %v", name, err.Error()),
 			Location: bctx.Location,
+			Err:      err,
 		}
 	}
 	if result == nil {

topdown/builtins.go

@@ -186,12 +186,14 @@ func handleBuiltinErr(name string, loc *ast.Location, err error) error {
 			Code:     TypeErr,
 			Message:  fmt.Sprintf("%v: %v", name, err.Error()),
 			Location: loc,
+			Err:      err,
 		}
 	default:
 		return &Error{
 			Code:     BuiltinErr,
 			Message:  fmt.Sprintf("%v: %v", name, err.Error()),
 			Location: loc,
+			Err:      err,
 		}
 	}
 }

topdown/errors.go

@@ -29,6 +29,7 @@ type Error struct {
 	Code     string        `json:"code"`
 	Message  string        `json:"message"`
 	Location *ast.Location `json:"location,omitempty"`
+	Err      error         `json:"-"`
 }
 
 const (
@@ -90,6 +91,10 @@ func (e *Error) Error() string {
 	return msg
 }
 
+func (e *Error) Unwrap() error {
+	return e.Err
+}
+
 func functionConflictErr(loc *ast.Location) error {
 	return &Error{
 		Code:     ConflictErr,

topdown/errors_test.go

@@ -14,13 +14,17 @@ func TestErrorWrapping(t *testing.T) {
 		return errors.As(err, &topdown.Halt{})
 	}
 
+	builtinErr := errors.New("builtin error")
+	loc := location.Location{
+		File: "b.rego",
+		Col:  10,
+		Row:  12,
+	}
+
 	e0 := topdown.Error{Code: topdown.BuiltinErr,
-		Message: "builtin error",
-		Location: &location.Location{
-			File: "b.rego",
-			Col:  10,
-			Row:  12,
-		},
+		Message:  "builtin error",
+		Location: &loc,
+		Err:      builtinErr,
 	}
 
 	tests := []struct {
@@ -63,6 +67,13 @@ func TestErrorWrapping(t *testing.T) {
 			err:   fmt.Errorf("meh: %w", &topdown.Error{Code: topdown.CancelErr}),
 			check: topdown.IsCancel,
 		},
+		{
+			note: "wrapped builtin error",
+			err:  &e0,
+			check: func(err error) bool {
+				return errors.Is(err, builtinErr)
+			},
+		},
 		{
 			note: "matching errors, code",
 			err:  &e0,
@@ -78,7 +89,14 @@ func TestErrorWrapping(t *testing.T) {
 			},
 		},
 		{
-			note: "matching errors, code and message and location",
+			note: "matching errors, code, message and location",
+			err:  &e0,
+			check: func(err error) bool {
+				return errors.Is(err, &topdown.Error{Code: topdown.BuiltinErr, Message: "builtin error", Location: &loc})
+			},
+		},
+		{
+			note: "matching errors, code, message, location and builtin error",
 			err:  &e0,
 			check: func(err error) bool {
 				return errors.Is(err, &e0)