errors: Drop Cause in favor of Go 1.13 error handling

The only use cases in the code were in errors.IsFatal, backend/b2,
which needs a workaround, and backend.ParseLayout. The last of these
requires all backends to implement error unwrapping in IsNotExist.
All backends except gs already did that.

cmd/restic/integration_test.go

@@ -1197,7 +1197,7 @@ func TestRestoreFilter(t *testing.T) {
 			if ok, _ := filter.Match(pat, filepath.Base(testFile.name)); !ok {
 				rtest.OK(t, err)
 			} else {
-				rtest.Assert(t, os.IsNotExist(errors.Cause(err)),
+				rtest.Assert(t, os.IsNotExist(err),
 					"expected %v to not exist in restore step %v, but it exists, err %v", testFile.name, i+1, err)
 			}
 		}
@@ -1283,15 +1283,15 @@ func TestRestoreLatest(t *testing.T) {
 
 	testRunRestoreLatest(t, env.gopts, filepath.Join(env.base, "restore1"), []string{filepath.Dir(p1)}, nil)
 	rtest.OK(t, testFileSize(p1rAbs, int64(102)))
-	if _, err := os.Stat(p2rAbs); os.IsNotExist(errors.Cause(err)) {
-		rtest.Assert(t, os.IsNotExist(errors.Cause(err)),
+	if _, err := os.Stat(p2rAbs); os.IsNotExist(err) {
+		rtest.Assert(t, os.IsNotExist(err),
 			"expected %v to not exist in restore, but it exists, err %v", p2rAbs, err)
 	}
 
 	testRunRestoreLatest(t, env.gopts, filepath.Join(env.base, "restore2"), []string{filepath.Dir(p2)}, nil)
 	rtest.OK(t, testFileSize(p2rAbs, int64(103)))
-	if _, err := os.Stat(p1rAbs); os.IsNotExist(errors.Cause(err)) {
-		rtest.Assert(t, os.IsNotExist(errors.Cause(err)),
+	if _, err := os.Stat(p1rAbs); os.IsNotExist(err) {
+		rtest.Assert(t, os.IsNotExist(err),
 			"expected %v to not exist in restore, but it exists, err %v", p1rAbs, err)
 	}
 }
@@ -1861,7 +1861,7 @@ func TestHardLink(t *testing.T) {
 
 	datafile := filepath.Join("testdata", "test.hl.tar.gz")
 	fd, err := os.Open(datafile)
-	if os.IsNotExist(errors.Cause(err)) {
+	if os.IsNotExist(err) {
 		t.Skipf("unable to find data file %q, skipping", datafile)
 		return
 	}

internal/backend/b2/b2.go

@@ -184,7 +184,14 @@ func (be *b2Backend) HasAtomicReplace() bool {
 
 // IsNotExist returns true if the error is caused by a non-existing file.
 func (be *b2Backend) IsNotExist(err error) bool {
-	return b2.IsNotExist(errors.Cause(err))
+	// blazer/b2 does not export its error types and values,
+	// so we can't use errors.{As,Is}.
+	for ; err != nil; err = errors.Unwrap(err) {
+		if b2.IsNotExist(err) {
+			return true
+		}
+	}
+	return false
 }
 
 // Load runs fn with a reader that yields the contents of the file at h at the
@@ -386,7 +393,7 @@ func (be *b2Backend) Delete(ctx context.Context) error {
 		}
 	}
 	err := be.Remove(ctx, restic.Handle{Type: restic.ConfigFile})
-	if err != nil && b2.IsNotExist(errors.Cause(err)) {
+	if err != nil && be.IsNotExist(err) {
 		err = nil
 	}
 

internal/backend/gs/gs.go

@@ -174,13 +174,8 @@ func (be *Backend) IsNotExist(err error) bool {
 		return true
 	}
 
-	if er, ok := err.(*googleapi.Error); ok {
-		if er.Code == 404 {
-			return true
-		}
-	}
-
-	return false
+	var gerr *googleapi.Error
+	return errors.As(err, &gerr) && gerr.Code == 404
 }
 
 // Join combines path components with slashes.

internal/backend/layout.go

@@ -71,7 +71,7 @@ var backendFilename = regexp.MustCompile(fmt.Sprintf("^[a-fA-F0-9]{%d}$", backen
 
 func hasBackendFile(ctx context.Context, fs Filesystem, dir string) (bool, error) {
 	entries, err := fs.ReadDir(ctx, dir)
-	if err != nil && fs.IsNotExist(errors.Cause(err)) {
+	if err != nil && fs.IsNotExist(err) {
 		return false, nil
 	}
 

internal/errors/errors.go

@@ -1,9 +1,8 @@
 package errors
 
 import (
-	"net/url"
+	stderrors "errors"
 
-	"github.com/cenkalti/backoff/v4"
 	"github.com/pkg/errors"
 )
 
@@ -29,29 +28,10 @@ var WithMessage = errors.WithMessage
 
 var WithStack = errors.WithStack
 
-// Cause returns the cause of an error. It will also unwrap certain errors,
-// e.g. *url.Error returned by the net/http client.
-func Cause(err error) error {
-	type Causer interface {
-		Cause() error
-	}
-
-	for {
-		switch e := err.(type) {
-		case Causer: // github.com/pkg/errors
-			err = e.Cause()
-		case *backoff.PermanentError:
-			err = e.Err
-		case *url.Error:
-			err = e.Err
-		default:
-			return err
-		}
-	}
-}
-
 // Go 1.13-style error handling.
 
-func As(err error, tgt interface{}) bool { return errors.As(err, tgt) }
+func As(err error, tgt interface{}) bool { return stderrors.As(err, tgt) }
 
-func Is(x, y error) bool { return errors.Is(x, y) }
+func Is(x, y error) bool { return stderrors.Is(x, y) }
+
+func Unwrap(err error) error { return stderrors.Unwrap(err) }

internal/errors/fatal.go

@@ -1,6 +1,9 @@
 package errors
 
-import "fmt"
+import (
+	"errors"
+	"fmt"
+)
 
 // fatalError is an error that should be printed to the user, then the program
 // should exit with an error code.
@@ -10,31 +13,19 @@ func (e fatalError) Error() string {
 	return string(e)
 }
 
-func (e fatalError) Fatal() bool {
-	return true
-}
-
-// Fataler is an error which should be printed to the user directly.
-// Afterwards, the program should exit with an error.
-type Fataler interface {
-	Fatal() bool
-}
-
 // IsFatal returns true if err is a fatal message that should be printed to the
 // user. Then, the program should exit.
 func IsFatal(err error) bool {
-	// unwrap "Wrap" method
-	err = Cause(err)
-	e, ok := err.(Fataler)
-	return ok && e.Fatal()
+	var fatal fatalError
+	return errors.As(err, &fatal)
 }
 
-// Fatal returns a wrapped error which implements the Fataler interface.
+// Fatal returns an error that is marked fatal.
 func Fatal(s string) error {
 	return Wrap(fatalError(s), "Fatal")
 }
 
-// Fatalf returns an error which implements the Fataler interface.
+// Fatalf returns an error that is marked fatal.
 func Fatalf(s string, data ...interface{}) error {
 	return Wrap(fatalError(fmt.Sprintf(s, data...)), "Fatal")
 }

internal/restic/backend.go

@@ -64,6 +64,9 @@ type Backend interface {
 
 	// IsNotExist returns true if the error was caused by a non-existing file
 	// in the backend.
+	//
+	// The argument may be a wrapped error. The implementation is responsible
+	// for unwrapping it.
 	IsNotExist(err error) bool
 
 	// Delete removes all data in the backend.