dropbox: don't retry chunked upload requests when the upload has been cancelled

The append loop retries everything once the upload session has
started, so a cancelled context error was retried through all the low
level retries with exponential backoff before the upload gave up.

backend/dropbox/dropbox.go

@@ -2106,6 +2106,11 @@ func (o *Object) uploadChunked(ctx context.Context, in0 io.Reader, commitInfo *f
 					}
 				}
 			}
+			// Don't waste the low level retries if the context has
+			// been cancelled
+			if fserrors.ContextError(ctx, &err) {
+				return false, err
+			}
 			return err != nil, err
 		})
 		if err != nil {

backend/dropbox/dropbox_internal_test.go

@@ -151,6 +151,7 @@ type uploadSessionClient struct {
 	maxAppends   int    // fail the append after this many calls to stop runaway loops
 	bytesWritten int64  // bytes received by UploadSessionAppendV2Context
 	finishCalled bool   // set if UploadSessionFinishContext was called
+	appended     func() // if set, called after each successful append
 }
 
 var errTooManyAppends = errors.New("too many appends - upload looping?")
@@ -173,6 +174,9 @@ func (c *uploadSessionClient) UploadSessionAppendV2Context(ctx context.Context,
 		return err
 	}
 	c.bytesWritten += n
+	if c.appended != nil {
+		c.appended()
+	}
 	return nil
 }
 
@@ -197,6 +201,16 @@ func newUploadTestFs(t *testing.T, srv files.ContextClient, chunkSize fs.SizeSuf
 	return f
 }
 
+// endlessReader supplies bytes forever
+type endlessReader struct{}
+
+func (endlessReader) Read(p []byte) (int, error) {
+	for i := range p {
+		p[i] = 'x'
+	}
+	return len(p), nil
+}
+
 func TestUploadChunkedEarlyEOF(t *testing.T) {
 	ctx := context.Background()
 
@@ -235,6 +249,25 @@ func TestUploadChunkedEarlyEOF(t *testing.T) {
 	})
 }
 
+func TestUploadChunkedCancel(t *testing.T) {
+	// Cancelling the context must stop the upload even though every
+	// append is succeeding
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	client := &uploadSessionClient{maxAppends: 8}
+	client.appended = func() {
+		if client.appends == 2 {
+			cancel()
+		}
+	}
+	f := newUploadTestFs(t, client, 100)
+	o := &Object{fs: f, remote: "test.bin"}
+	_, err := o.uploadChunked(ctx, endlessReader{}, files.NewCommitInfo("/test.bin"), -1)
+	require.Error(t, err)
+	assert.ErrorIs(t, err, context.Canceled)
+	assert.False(t, client.finishCalled)
+}
+
 func (f *Fs) importPaperForTest(t *testing.T) {
 	content := `# test doc