Deleted files remain accessible to iOS viewers

[anmipo]

Hi everyone,

There seems to be an odd caching issue with Dropbox app on iOS. Specifically, files that have been deleted from Dropbox remain accessible to other iOS apps.

Steps to reproduce:

• Initial setup
  • Add a PDF file to Dropbox
  • Install Acrobat for iOS
• Open the PDF in Acrobat
  • Open the iOS Files app and navigate to Dropbox
  • Long-press the PDF file → Share → (scroll icons left) More → Open in Acrobat
    • The PDF opens in Acrobat
• Delete the PDF via Dropbox web interface
  • Then open "Recently deleted" and delete the file permanently
• Check whether the file is gone on the iOS device
  • Open Dropbox app → the PDF is no longer listed
  • Open the Files app → the PDF is no longer listed
  • Open Acrobat → Home → Recent, and tap on the PDF listed there 

Observed behavior:

• The permanently deleted PDF file opens just fine!

Expected behavior:

• The viewer should fail to open the deleted file, saying the file does not exist.

Other information:

• The file is not listed in the folder, as if it was actually gone
• I have tried to "permanently delete" the file via the web interface
• I have tried clearing the cache in Dropbox app
• I have tried restarting the device
• I have not tried logging out-and-back-in to Dropbox app (this would invalidate Acrobat's reference to the cached file and hide the symptom, but won't solve the underlying caching issue).

System information:

• iOS 13.7
• Dropbox Plus account
• Dropbox app version 202.3.2
• Acrobat version 20.08.00

[Walter]

Hi @anmipo; thank you for posting on our Community!

I appreciate the extensive report and thank you for providing the steps to reproduce the caching issue you've described. 

While I suppose this issue would be better investigated if we switched to email, may I ask for some additional information first?

For example, are you seeing this (and only) with PDF files or other file types as well?

Does this issue occur with any other connected apps to your account? 

Any additional information and/or screenshots would be much appreciated.

Thanks!

[anmipo]

Hi Walter,

Thank you for the prompt response.

> For example, are you seeing this (and only) with PDF files or other file types as well?

The issue has been initially reported to me by a user of my iOS app (KeePassium). The investigation led me to suspect a Dropbox caching issue, so I have published this post both as a bug report and an answer to my user  For the purpose of this discussion, the app can be seen as an editor of special type of files (KeePass databases, .kdbx).

So the issue affects other file types as well.

The common denominator between the two affected apps is that we seem to use the same approach for accessing files in Dropbox (document picker + security-scoped URL bookmarks). It's a kind of reference that allows an app to access files outside its sandbox (be it in other apps or in some storage provider integrated with the iOS Files app).

I used Acrobat mainly as an example which 1) covers a wide auditory, and 2) cannot be easily dismissed as "they don't know how to code".

> Does this issue occur with any other connected apps to your account?

Please note, this is not about apps connected to Dropbox account. This is about apps using the standard iOS mechanism for accessing remote files.

I believe the issue applies to all iOS apps that preserve references to the recently opened files (as URL bookmarks mentioned above). Such apps are rather difficult to find on purpose, though. In addition to KeePassium and Acrobat, I have just managed to reproduce the issue with "Documents by Readdle", both for PDF and TXT file (view a Dropbox file in Documents, then delete the file permanently — yet the file remains accessible via the "Recents" tab in Documents).

[Jay]

Since the files are being accessed via the iOS Files app, and not the Dropbox mobile app directly, does this behavior appear when using other cloud providers when accessed via the Files app?

[anmipo]

Thank you for looking into this, Jay. 

Just to clarify: my post is not a request for help, it's an FYI. The report provides all the details necessary to reproduce the issue. 

However, testing generic hypotheses (such as behavior of other cloud providers) can be easily done on your side. I am just notifying you there is an issue — debugging it is your (Dropbox) part of the job.

Should you have any specific questions about reproducing the issue, I would be happy to assist. 

[Daphne]

Hey there @anmipo!

When you're opening the file from the Dropbox app or Files app, it's essentially creating a copy in the 3rd party app you used.

In this case, a copy of the file is added to the Adobe app which is independent of the file in your Dropbox account.

When the file is then deleted from your Dropbox account:

• there are no links to copies of the file, and so Dropbox can't anything outside of the Dropbox apps.
• the file is removed from the Files app, as the Files app pulls the file info directly from the Dropbox mobile app

I hope this clarifies the matter!

[anmipo]

Hi @Daphne!

> When you're opening the file from the Dropbox app or Files app, it's essentially creating a copy in the 3rd party app you used.

This is the case for the Dropbox app, but not for the Files app.

• When the user exports a file from the Dropbox app, the final action looks like "Copy to Acrobat".
• When the user exports a file from Dropbox location in the Files app, the final action looks like "Open in Acrobat".

In the latter case, no copies are made. Instead, Acrobat receives a reference to the file managed by the Dropbox file provider (and stores that reference for future use). This is powered by the "open in place" feature introduced back in iOS 11.

This is easy to check:

1. On device A:
   1. Open a Dropbox file in Acrobat (Files → Dropbox → long-press a PDF → Share → More → Open in Acrobat)
   2. Close the Acrobat app.
2. On device B:
   1. Modify the PDF file. Say, highlight something.
3. Back on device A:
   1. Open Acrobat directly and tap on the PDF in Recent files.
   2. Observe the changes made on device B

> In this case, a copy of the file is added to the Adobe app which is independent of the file in your Dropbox account.

If this was the case, at step 3.2 above we would not see the changes made on the other device.