Fix for Uncaught Exception and Crash, Lost Messages in Unsynchronized Local-Only Folders

[dnicules]

These are our fixes to address issues #7688 and #823

Uncaught Exception, Crash: Issue #7688
Problem:

• Previously, if a user goes offline, moves a message into another folder, opens the message, and stars/unstars it, the app would consistently crash
  • This did NOT happen when a message was starred/unstarred from the List View.

Solution:

• Now we catch an exception in queueSetFlag() to avoid the crash; this is how it is handled in List View.
  • List View catches the exception in runInBackground()

"Lost" Messages in Unsynchronized, Local-Only Folders: Issue #823
Problem:

• A user using Thunderbird with an account having no archive folder reported that, on moving a message to archive, they were not able to move it elsewhere in any way; the message effectively became local-only and lost to the server.
• Potentially happens with other folders that only exist locally but not on the server

Solution:

• Online Fix: If a user has connectivity, refresh the folder list before making any move operations
• Offline Fix: A dialog/prompt shows up, informing the user that the operation might lead to data loss
  • The user is allowed to proceed anyways, cancel the operation, or to proceed and never show the warning again
• Applies to moving (to arbitrary folders, or also to drafts and spam), copying, archiving.

Authors: @dnicules @gkhourii

[cketti]

Thank you for working on fixing these issues ❤️

Issues #7688 and #823 are somewhat related because the underlying cause is that pending commands reference server IDs (UIDs) of messages, and sometimes those aren't known to the app yet. Unfortunately, this pull request is only addressing the symptoms, not the cause. Sometimes this can be a good first step, because often this can be done much faster than changing the architecture to avoid the bug. But that is exactly what the "Cannot copy or move message that is not synchronized with the server" message already is. It avoids performing an operation that can't be performed because of missing information. Adding a much more complicated mechanism (distinguishing between online and offline) to avoid showing that message some of the time, is not the solution. We want the user to be able to perform the operation in all cases.

As I mentioned before, a proper fix is quite a bit of work and requires a good understanding of the inner workings of the app. It's not a good project to get started as a new contributor.

What you could do, is avoid calling MessaginigController.setFlag() and displaying a toast when there's no server ID available to avoid the crash. Basically, using the same pattern that is used for moving/copying messages. It's not fixing the problem, but avoids the crash until a proper fix is in place.