ContentUnitSaver stage is vulnerable to race conditions.
The ContentUnitSaver stage is creating Content using save() and both ContentArtifact and RemoteArtifact using bulk_create() which will raise an IntegretyError on any constraint violation. The same content can be created concurrently during operations such as sync and upload when running more than 1 worker. The race condition exists between the QueryExistingContentUnits and ContentUnitSaver stages. As a result, the un-handled IntegretyError will cause one of the operations to fail.
The impact on users is that syncs will randomly fail with an IntegretyError which will be very concerning.
Problem: bulk_create can fail with IntegrityError
Solution: add a manager that provides a bulk_get_or_create() method
This patch introduces a BulkCreateManager that provides the bulk_get_or_create method. This method
handles IntegrityErrors encountered during bulk_create() by inserting each object into the database
serially. When an IntegrityError or ValueError is encountered during serial saving of objects, the
object being saved is replaced with an instance from the database.
This patch introduces a mixin used by Artifact, Content, ContentArtifact, and RemoteArtifact models.
The mixin provides the q() method which returns a Q object that can be used to retreive the database
instance of the model.
Problem: Artifact Saver fails to associate existing Artifacts
Solution: Update Declarative Artifacts with Artifacts returned by bulk_get_or_create()
The Content unit saver did not account for duplicate units at all. This patch also addresses
Problem: duplicate content can't by synced
Solution: fix the content unit saver stage
This patch addresses 2 problems:
1) Error because an exception was being caught during a transaction
2) The q() method did not provide the correct query for Content objects
#6 Updated by email@example.com about 1 year ago
This is totally different on the single-content branch which converts the last call to bulk_create() and all of them occur in the transaction. So I think this is maybe NOTABUG. I missed triage so I can only comment now.
What do you mean by "single-content branch" ?
#8 Updated by firstname.lastname@example.org about 1 year ago
There has been some confusion with this one and https://pulp.plan.io/issues/4060. 4060 is a problem within a single task (sync) of duplicates existing in a stream at the same time. This problem is related, but distinct. As noted in the description, this problem exists when 2 or more tasks are happening at once. If a unit passes through QueryExisting as a non-dupe, and while it is waiting in the Queue for the save stage, and another task creates that unit, it this one becomes a dupe and will fail when it is bulk_saved.
I think it is important to document these problems separately since they occur for different reasons. Some possible solutions might fix both at once. For instance, with docker sync we solve both problems by implementing our own save stage which does not use bulk writes. This allows us to catch and handle integrity errors as they happen. Adding this stage to pulpcore would solve both problems at the expense of database write performance. Since db performance could be a bottleneck, it is worth considering other options if we have any-- but those options would need to take into account both problems (dupes in stream, parallel tasks).
#9 Updated by email@example.com about 1 year ago
@bmbouter, What makes you think that this cannot happen?
worker 1: query content ABC (QueryStage) not-found worker 2: query content ABC (QueryStage) not-found worker 1: create content ABC (CreateStage) Inserted worker 2: create content ABC (CreateStage) IntegrityError
#10 Updated by daviddavis about 1 year ago
@jortel, I agree it could happen. We saw the same problem when we worked on base_paths. We thought running both the query and the create in a single transaction would solve it but it did not:
Recapping some of the discussion from last week, we do need to handle IntegrityErrors-with-requery support in the ContentUnitSaver stage. This ticket can track that. We also need to handle it in the ArtifactSaver stage too, which I believe is tracked as issue https://pulp.plan.io/issues/4060
Does that sound right ^?
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