fix: fixes race conditions in lock acquisition in PG backend

[JonathanFejtek]

I seem to have found a race condition in the PG backend which leads to the following log:

orchestration-processor: failed to complete work item: instance 'e8007fa4-dad3-48a4-90d7-1844c5631685' no longer exists or was locked by a different worker

I've traced the issue to GetOrchestrationWorkItem and GetActivityWorkItem which both acquire leases on NewEvents and NewTasks, respectively. It appears that orchestration workers are able to simultaneously acquire the same lock only to fail later in CompleteOrchestrationWorkItem where its determined that only one worker has acquired a lock.

I'm not sure if this is meant to be an optimistic concurrency control, but the pessimistic 'lock semantics' and apparent double work being done by workers suggests to me this may be a bug.

I would very much appreciate a review! I'm just digging into this project so I may be totally off base here, but I figured I'd throw this up and get some opinions at least.

(I also fixed a few lint errors in the PG backend along the way)

[JonathanFejtek]

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[cgillum]

Thanks for this PR! I think your changes are doing what the original code was meaning to. The original query, which contained both an UPDATE and SELECT in a single query, was supposed to ensure that only one caller would successfully acquire the lock. That probably worked fine with SQLite but I'm guessing the internal DB locking differences of Postgres might not have allowed this to work correctly. I'm not a PG expert, but would a transaction around the original UPDATE/SELECT query have solved this?

In any case, if your current PR is fixing the issue you're seeing, then I'm more than happy to merge it as-is.

[cgillum]

Hi @JonathanFejtek, just checking in to see if you saw my message above.

[mrjmarcelo]

@JonathanFejtek , your adjstments will be helpful.

[JonathanFejtek]

Hey @cgillum @mrjmarcelo . My apologies for radio silence, its been a busy couple of weeks and I haven't had much time outside of work to commit to this.

As far as I can tell, the issue with the current lock acquisition code is just that its not concurrency safe for the default Postgres transaction isolation (READ COMMITTED). Basically, two concurrent transactions can get the same result for the sub-select

 SELECT SequenceNumber FROM Instances I
    WHERE (I.LockExpiration IS NULL OR I.LockExpiration < $3) AND EXISTS (
        SELECT 1 FROM NewEvents E
        WHERE E.InstanceID = I.InstanceID AND (E.VisibleTime IS NULL OR E.VisibleTime < $4)
    )
    LIMIT 1

which is evaluated before the UPDATE statement and operates on two separate "views" of only previously-committed changes to the database. This approach can lead to lost update anomalies. The sub-select is equivalent to selecting the data upfront in one query (without locking for update) and then executing the update in another - theres no particular concurrency control happening that will prevent concurrent updates from overwriting each other. Adding a SELECT FOR UPDATE in the sub-select is another way to fix this, and the SKIPPED LOCKED is also essential to prevent blocking worker goroutines.

At a higher transaction isolation (SERIALIZABLE, for instance, which conveniently is the SQLite default), the existing code would theoretically work without issue but I haven't tested this yet.

[JonathanFejtek]

I'd be happy to merge this as-is if y'all are good with the change. I've also taken the liberty to add PG testcontainers to this project which can make controlled testing of specific PG behaviours a bit easier. If I have some time this week, I'll try to whip up a test that covers this particular race condition

[cgillum]

I'm good with this current approach. Thanks for the explanation about the race condition!

[cgillum]

Actually, @JonathanFejtek can you add another commit to update the CHANGELOG.md file and list this PR? Something along the lines of:

[vNext]

### Fixed

* Fixed race conditions in lock acquisition in PG backend ([#108](https://github.com/microsoft/durabletask-go/pull/108)) - contributed by [@JonathanFejtek](https://github.com/JonathanFejtek)

I'd do it myself but then I'd need to find another approver in order for the PR to be merged.

[cgillum]

@JonathanFejtek the v0.6.0 tag has already been published. Can you add a new section above the v0.6.0 section called [vNext] and add your entry there?

[cgillum]

Also, @JonathanFejtek looks like there is a new conflict that needs to be resolved due to changes by @mrjmarcelo.

[youngbupark]

@JonathanFejtek Thanks for this fix. I am waiting for this change :)