[Bug 1925677] [NEW] Thunderbird: Recurring tasks on CalDAV account can no longer be set to completed

[Michael von Glasow]

Public bug reported:

Steps to reproduce:

    Set up calendar and tasks with a CalDav server and an old Thunderbird version (up to ca. December 2020).
    Set up a recurring task.
    Whenever an instance of the recurring task becomes due, mark it as completed.
    Repeat this for a few instances.
    Upgrade to Thunderbird 78.7.1.
    Examine the list of tasks.

Actual results:

Previous instances of the recurring tasks (which had already been marked
as completed before the update) now reappear and show as incomplete. If
I manually set them to Completed, the status will be reset upon sync.

Expected results:

Completion status (as previously set) is honored and the tasks no longer
show as open.

Additional information:

The server is running DAViCal.

I just ran a few extra tests:

* Recurring tasks in local storage (create a weekly task, starting a few weeks in the past and ending a few weeks into the future, then mark some of the past instances as complete)
* Same on the CalDAV calendar, sync after marking a few past instances complete
* Repeat the CalDAV tests with variations: complete the second instance but leave the first one uncompleted; complete the third instance, sync, then complete the second one
* Non-recurring task on CalDAV

Results:

* Non-recurring tasks are not affected; I can mark the task completed and it will keep its status.
* Recurring tasks in local storage are also not affected; completed instances will keep their status.
* Instances of recurring tasks on CalDAV can be marked completed but will lose their status upon synchronization. Exception: one instance maintains its completion status.
* The instance to maintain its completion status is the first one that got marked complete (i.e. based on the time the completion flag got set, not based on start date).

This bug began showing with an update deployed sometime between
January–March 2021, though I failed to record the exact version numbers.

I also have an Android app accessing the same DAViCal instance, where
the completed instance have not been reset.

In conclusion, the issue seems to occur when reading tasks from a CalDAV
server (tasks on the server are not altered). The first instance which
got set to Completed retains its completion status, whereas the flag is
ignored for all later ones.

I have filed this bug upstream at
https://bugzilla.mozilla.org/show_bug.cgi?id=1707032 and was told that
this issue should be fixed as of Thunderbird 78.8.0 (current being
78.10.0). So the main issue is that the Ubuntu repos have an outdated
Thunderbird version with a known issue.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: thunderbird 1:78.7.1+build1-0ubuntu0.20.04.1
ProcVersionSignature: Ubuntu 5.4.0-72.80-generic 5.4.101
Uname: Linux 5.4.0-72-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
BuildID: 20210203182138
CasperMD5CheckResult: skip
CurrentDesktop: MATE
Date: Thu Apr 22 22:31:01 2021
EcryptfsInUse: Yes
InstallationDate: Installed on 2014-12-09 (2325 days ago)
InstallationMedia: Ubuntu MATE 14.10 "Utopic Unicorn" - amd64 (20141023)
SourcePackage: thunderbird
UpgradeStatus: Upgraded to focal on 2020-11-17 (156 days ago)

** Affects: thunderbird (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug focal

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Title:
  Thunderbird: Recurring tasks on CalDAV account can no longer be set to
  completed

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