address unassigned task leak when service is removed (v17.06 backport)

[dani-docker]

Signed-off-by: Dani Louca dani.louca@docker.com

- What I did
BACKPORT v17.06
If a task is not yet assigned (for ex: noSuitableNode ) and its service has been removed, the task stays in the unassignedTasks map until a new leader election.

- How I did it

Before re-adding the task to unassignedTasks map, the fix checks if the task has a valid service in the store.

- How to test it

steps to repro:

• set daemon.log to debug
• create a service with a node constraint that does not exist:
  ex: docker service create -d --constraint 'node.labels.type == queue' alpine sleep 10000
• watch the logs, you should see an error
  no suitable node available for task
• delete the service and invoke the tick() by creating another service; watch the log and notice the same warning taskID in pops up after service is deleted

- Description for the changelog

Avoid a leak when a service with unassigned tasks is deleted

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "task_leak-v17.06" git@github.com:dani-docker/swarmkit.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov]

Codecov Report

Merging #2708 into bump_v17.06 will increase coverage by 3.75%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           bump_v17.06    #2708      +/-   ##
===============================================
+ Coverage        57.72%   61.47%   +3.75%     
===============================================
  Files               10      121     +111     
  Lines             1230    20203   +18973     
===============================================
+ Hits               710    12420   +11710     
- Misses             451     6423    +5972     
- Partials            69     1360    +1291

[thaJeztah]

@dani-docker was this cherry-picked or a different implementation as on master? If you did cherry-pick, could you use the -x option for future back ports? That makes it include a reference to the original commit in the commit-message.

[thaJeztah]

For future reference; PR against master was
#2706

[dani-docker]

@thaJeztah was cherry picked but had to resolve conflicts