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DRA depends on the assume cache having invoked all event handlers before
Assume() returns, because DRA maintains state that is relevant for scheduling
through those event handlers.
This log snippet shows how this went wrong during PreBind:
dynamicresources.go:1150: I0115 10:35:29.264437] scheduler: Claim stored in assume cache pod="testdra-all-usesallresources-kqjpj/my-pod-0091" claim="testdra-all-usesallresources-kqjpj/claim-0091" uid=<types.UID>: 516f274f-e1a9-4a4b-b7d2-bb86138e4240 resourceVersion="5636"
dra_manager.go:198: I0115 10:35:29.264448] scheduler: Removed in-flight claim claim="testdra-all-usesallresources-kqjpj/claim-0091" uid=<types.UID>: 516f274f-e1a9-4a4b-b7d2-bb86138e4240 version="287"
dynamicresources.go:1157: I0115 10:35:29.264463] scheduler: Removed claim from in-flight claims pod="testdra-all-usesallresources-kqjpj/my-pod-0091" claim="testdra-all-usesallresources-kqjpj/claim-0091" uid=<types.UID>: 516f274f-e1a9-4a4b-b7d2-bb86138e4240 resourceVersion="5636" allocation=<
...
allocateddevices.go:189: I0115 10:35:29.267315] scheduler: Observed device allocation device="testdra-all-usesallresources-kqjpj.driver/worker-1/worker-1-device-096" claim="testdra-all-usesallresources-kqjpj/claim-0091"
- goroutine #1: UpdateStatus result delivered via informer.
AssumeCache updates cache, pushes event A, emitEvents pulls event A from queue.
*Not* done with delivering it yet!
- goroutine #2: AssumeCache.Assume called. Updates cache, pushes event B, emits it.
Old and new claim have allocation, so no "Observed device allocation".
- goroutine #3: Schedules next pod, without considering device as allocated (not in the log snippet).
- goroutine #1: Finally delivers event A: "Observed device allocation", but too late.
Also, events are delivered out-of-order.
The fix is to let emitEvents when called by Assume wait for a potentially
running emitEvents in some other goroutine, thus ensuring that an event pulled
out of the queue by that other goroutine got delivered before Assume itself
checks the queue one more time and then returns.
The time window were things go wrong is small. An E2E test covering this only
flaked rarely, and only in the CI. An integration test (separate commit) with
higher number of pods finally made it possible to reproduce locally. It also
uncovered a second race (fix in separate commit).
The unit test fails without the fix:
=== RUN TestAssumeConcurrency
assume_cache_test.go:311: FATAL ERROR:
Assume should have blocked and didn't.
--- FAIL: TestAssumeConcurrency (0.00s)