all: Move away from pkg/errors, easy cases

github.com/pkg/errors is no longer getting updates, because Go 1.13
went with the more flexible errors.{As,Is} function. Use those instead:
errors from pkg/errors already support the Unwrap interface used by 1.13
error handling. Also:

* check for io.EOF with a straight ==. That value should not be wrapped,
  and the chunker (whose error is checked in the cases changed) does not
  wrap it.
* Give custom Error methods pointer receivers, so there's no ambiguity
  when type-switching since the value type will no longer implement error.
* Make restic.ErrAlreadyLocked private, and rename it to
  alreadyLockedError to match the stdlib convention that error type
  names end in Error.
* Same with rest.ErrIsNotExist => rest.notExistError.
* Make s3.Backend.IsAccessDenied a private function.

internal/backend/layout.go

@@ -152,7 +152,7 @@ func ParseLayout(ctx context.Context, repo Filesystem, layout, defaultLayout, pa
 		l, err = DetectLayout(ctx, repo, path)
 
 		// use the default layout if auto detection failed
-		if errors.Cause(err) == ErrLayoutDetectionFailed && defaultLayout != "" {
+		if errors.Is(err, ErrLayoutDetectionFailed) && defaultLayout != "" {
 			debug.Log("error: %v, use default layout %v", err, defaultLayout)
 			return ParseLayout(ctx, repo, defaultLayout, "", path)
 		}

internal/backend/mem/mem_backend.go

@@ -71,7 +71,7 @@ func (be *MemoryBackend) Test(ctx context.Context, h restic.Handle) (bool, error
 
 // IsNotExist returns true if the file does not exist.
 func (be *MemoryBackend) IsNotExist(err error) bool {
-	return errors.Cause(err) == errNotFound
+	return errors.Is(err, errNotFound)
 }
 
 // Save adds new Data to the backend.

internal/backend/rclone/backend_test.go

@@ -29,7 +29,8 @@ func newTestSuite(t testing.TB) *test.Suite {
 			t.Logf("Create()")
 			cfg := config.(rclone.Config)
 			be, err := rclone.Create(context.TODO(), cfg)
-			if e, ok := errors.Cause(err).(*exec.Error); ok && e.Err == exec.ErrNotFound {
+			var e *exec.Error
+			if errors.As(err, &e) && e.Err == exec.ErrNotFound {
 				t.Skipf("program %q not found", e.Name)
 				return nil, nil
 			}

internal/backend/rclone/internal_test.go

@@ -18,7 +18,8 @@ func TestRcloneExit(t *testing.T) {
 	cfg := NewConfig()
 	cfg.Remote = dir
 	be, err := Open(cfg, nil)
-	if e, ok := errors.Cause(err).(*exec.Error); ok && e.Err == exec.ErrNotFound {
+	var e *exec.Error
+	if errors.As(err, &e) && e.Err == exec.ErrNotFound {
 		t.Skipf("program %q not found", e.Name)
 		return
 	}

internal/backend/rest/rest.go

@@ -169,21 +169,20 @@ func (b *Backend) Save(ctx context.Context, h restic.Handle, rd restic.RewindRea
 	return errors.Wrap(cerr, "Close")
 }
 
-// ErrIsNotExist is returned whenever the requested file does not exist on the
+// notExistError is returned whenever the requested file does not exist on the
 // server.
-type ErrIsNotExist struct {
+type notExistError struct {
 	restic.Handle
 }
 
-func (e ErrIsNotExist) Error() string {
+func (e *notExistError) Error() string {
 	return fmt.Sprintf("%v does not exist", e.Handle)
 }
 
 // IsNotExist returns true if the error was caused by a non-existing file.
 func (b *Backend) IsNotExist(err error) bool {
-	err = errors.Cause(err)
-	_, ok := err.(ErrIsNotExist)
-	return ok
+	var e *notExistError
+	return errors.As(err, &e)
 }
 
 // Load runs fn with a reader that yields the contents of the file at h at the
@@ -296,7 +295,7 @@ func (b *Backend) openReader(ctx context.Context, h restic.Handle, length int, o
 
 	if resp.StatusCode == http.StatusNotFound {
 		_ = resp.Body.Close()
-		return nil, ErrIsNotExist{h}
+		return nil, &notExistError{h}
 	}
 
 	if resp.StatusCode != 200 && resp.StatusCode != 206 {
@@ -341,7 +340,7 @@ func (b *Backend) Stat(ctx context.Context, h restic.Handle) (restic.FileInfo, e
 
 	if resp.StatusCode == http.StatusNotFound {
 		_ = resp.Body.Close()
-		return restic.FileInfo{}, ErrIsNotExist{h}
+		return restic.FileInfo{}, &notExistError{h}
 	}
 
 	if resp.StatusCode != 200 {
@@ -392,7 +391,7 @@ func (b *Backend) Remove(ctx context.Context, h restic.Handle) error {
 
 	if resp.StatusCode == http.StatusNotFound {
 		_ = resp.Body.Close()
-		return ErrIsNotExist{h}
+		return &notExistError{h}
 	}
 
 	if resp.StatusCode != 200 {

internal/backend/s3/s3.go

@@ -137,7 +137,7 @@ func Create(ctx context.Context, cfg Config, rt http.RoundTripper) (restic.Backe
 	}
 	found, err := be.client.BucketExists(ctx, cfg.Bucket)
 
-	if err != nil && be.IsAccessDenied(err) {
+	if err != nil && isAccessDenied(err) {
 		err = nil
 		found = true
 	}
@@ -158,29 +158,23 @@ func Create(ctx context.Context, cfg Config, rt http.RoundTripper) (restic.Backe
 	return be, nil
 }
 
-// IsAccessDenied returns true if the error is caused by Access Denied.
-func (be *Backend) IsAccessDenied(err error) bool {
-	debug.Log("IsAccessDenied(%T, %#v)", err, err)
+// isAccessDenied returns true if the error is caused by Access Denied.
+func isAccessDenied(err error) bool {
+	debug.Log("isAccessDenied(%T, %#v)", err, err)
 
-	if e, ok := errors.Cause(err).(minio.ErrorResponse); ok && e.Code == "AccessDenied" {
-		return true
-	}
-
-	return false
+	var e minio.ErrorResponse
+	return errors.As(err, &e) && e.Code == "Access Denied"
 }
 
 // IsNotExist returns true if the error is caused by a not existing file.
 func (be *Backend) IsNotExist(err error) bool {
 	debug.Log("IsNotExist(%T, %#v)", err, err)
-	if os.IsNotExist(errors.Cause(err)) {
+	if errors.Is(err, os.ErrNotExist) {
 		return true
 	}
 
-	if e, ok := errors.Cause(err).(minio.ErrorResponse); ok && e.Code == "NoSuchKey" {
-		return true
-	}
-
-	return false
+	var e minio.ErrorResponse
+	return errors.As(err, &e) && e.Code == "NoSuchKey"
 }
 
 // Join combines path components with slashes.

internal/backend/sftp/sftp.go

@@ -183,7 +183,6 @@ func (r *SFTP) ReadDir(ctx context.Context, dir string) ([]os.FileInfo, error) {
 
 // IsNotExist returns true if the error is caused by a not existing file.
 func (r *SFTP) IsNotExist(err error) bool {
-	err = errors.Cause(err)
 	return errors.Is(err, os.ErrNotExist)
 }
 
@@ -496,7 +495,7 @@ func (r *SFTP) Test(ctx context.Context, h restic.Handle) (bool, error) {
 	defer r.sem.ReleaseToken()
 
 	_, err := r.c.Lstat(r.Filename(h))
-	if os.IsNotExist(errors.Cause(err)) {
+	if errors.Is(err, os.ErrNotExist) {
 		return false, nil
 	}
 

internal/backend/sftp/sftp_test.go

@@ -20,7 +20,7 @@ func findSFTPServerBinary() string {
 	for _, dir := range strings.Split(rtest.TestSFTPPath, ":") {
 		testpath := filepath.Join(dir, "sftp-server")
 		_, err := os.Stat(testpath)
-		if !os.IsNotExist(errors.Cause(err)) {
+		if !errors.Is(err, os.ErrNotExist) {
 			return testpath
 		}
 	}

internal/backend/swift/swift.go

@@ -311,11 +311,8 @@ func (be *beSwift) removeKeys(ctx context.Context, t restic.FileType) error {
 
 // IsNotExist returns true if the error is caused by a not existing file.
 func (be *beSwift) IsNotExist(err error) bool {
-	if e, ok := errors.Cause(err).(*swift.Error); ok {
-		return e.StatusCode == http.StatusNotFound
-	}
-
-	return false
+	var e *swift.Error
+	return errors.As(err, &e) && e.StatusCode == http.StatusNotFound
 }
 
 // Delete removes all restic objects in the container.

internal/backend/test/tests.go

@@ -361,8 +361,8 @@ func (s *Suite) TestListCancel(t *testing.T) {
 			return nil
 		})
 
-		if errors.Cause(err) != context.Canceled {
-			t.Fatalf("expected error not found, want %v, got %v", context.Canceled, errors.Cause(err))
+		if !errors.Is(err, context.Canceled) {
+			t.Fatalf("expected error not found, want %v, got %v", context.Canceled, err)
 		}
 	})
 
@@ -380,7 +380,7 @@ func (s *Suite) TestListCancel(t *testing.T) {
 			return nil
 		})
 
-		if errors.Cause(err) != context.Canceled {
+		if !errors.Is(err, context.Canceled) {
 			t.Fatalf("expected error not found, want %v, got %v", context.Canceled, err)
 		}
 
@@ -403,7 +403,7 @@ func (s *Suite) TestListCancel(t *testing.T) {
 			return nil
 		})
 
-		if errors.Cause(err) != context.Canceled {
+		if !errors.Is(err, context.Canceled) {
 			t.Fatalf("expected error not found, want %v, got %v", context.Canceled, err)
 		}
 
@@ -429,7 +429,7 @@ func (s *Suite) TestListCancel(t *testing.T) {
 			return nil
 		})
 
-		if errors.Cause(err) != context.DeadlineExceeded {
+		if !errors.Is(err, context.DeadlineExceeded) {
 			t.Fatalf("expected error not found, want %#v, got %#v", context.DeadlineExceeded, err)
 		}