Save operation completes before all files are uploaded in Multiple File Attachments control

[avernet]

To reproduce:

• In Form Builder, create a form with a Multiple File Attachments control and publish it.
• Open /new, attach several files, and click Save while the files are still uploading (network throttling helps).
• Once the upload is complete, reload the page.

Result:

• Observed: only some of the uploaded files have been saved.
• Expected: the save operation should wait for all files to finish uploading before saving the data.

+1 from customer

[ebruchez]

Right now, the XFCD keeps a pendingUploads set . This is:

• added to when getting xxforms-upload-start -removed from when
  • getting xxforms-upload-cancel|done|error
  • the control is destroyed
• checked with the pending-uploads action

Now, in the case of a multiple file upload, each upload is sent separately on the client. We have a processFileList() function
based on Future which sends each file at at time using addFile().

So it is possible that the save-final() process, which uses the pending-uploads, catches pendingUploads just at a time between two uploads of a multiple-upload sequence. Ideally, this shouldn't happen, and save-final() should not be allowed to proceed.

Note that this could also happen in even with separate upload controls. So the client should somehow be able to tell the server whether there are pending uploads on the client.

[ebruchez]

The client keeps a pending uploads queue in UploaderClient. Sequence of events:

• upload(s) triggered
• client sends xxforms-upload-start
• client initiates background file upload
• when done
  • server
    • responds with "server events" containing xxforms-upload-done or xxforms-upload-error
  • client, upon response to background file upload
    • if got a proper response, sends back to server xxforms-upload-done or xxforms-upload-error (within "server events")
    • if got an error, sends xxforms-upload-error explicitly
    • in case of successful upload, client continues with remaining events in pending uploads queue, which will cause xxforms-upload-start

One difficulty is to make sure the client and the server are synchronized. You don't want, for example, the client to tell the server that 3 uploads are expected, 1 upload completes, the client somehow is interrupted, and the server forever expects 2 more uploads, while the client no longer knows about it.

Adding files to the queue, even though it is based on Future, allows, at least in "online" mode, to fill the pending uploads queue as uploads are added. So at the end of an upload, we know if there are pending uploads on the client. Could we, at the same time we send xxforms-upload-done or xxforms-upload-error to the server, send a new xxforms-upload-start just before that?

Q: What happens with xxforms-upload-start/xxforms-upload-done in the case of multiple file uploads? Are they dispatched to the same XForms control, or different ones?

[ebruchez]

So, attachment.xbl has a single xf:upload, which uploads to the local instance. Upon xxforms-upload-done, it copies over the the data as appropriate.

So, in the case of a multiple attachments control, it doesn't quite make sense from a sequence point of view to dispatch xxforms-upload-start before xxforms-upload-done or xxforms-upload-error, does it? We'd like the start/done|error to be paired. In addition, the id will be added to the list of pending uploads, and the subsequent done|error will remove it. So It think that option will not work.