[I] [Bug] Subscribing to topic executes work on the single threaded Pulsar metadata thread in some cases [pulsar]

["lhotari (via GitHub)" <gi...@apache.org>]

lhotari opened a new issue, #23866:
URL: https://github.com/apache/pulsar/issues/23866

   ### Search before asking
   
   - [x] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.
   
   
   ### Read release policy
   
   - [x] I understand that unsupported versions don't get bug fixes. I will attempt to reproduce the issue on a supported version of Pulsar client and Pulsar broker.
   
   
   ### Version
   
   All released versions
   
   ### Minimal reproduce step
   
   Subscribe to a topic when metadata isn't yet cached.
   
   ### What did you expect to see?
   
   The topic ordered executor to be used.
   
   ### What did you see instead?
   
   configuration-metadata-store executor was used:
   ![Image](https://github.com/user-attachments/assets/600079dc-274b-4d23-808e-98dd90364e34)
   It's this executor: https://github.com/apache/pulsar/blob/80157955b69d97fa0568fad4127e34e30e9a2b85/pulsar-metadata/src/main/java/org/apache/pulsar/metadata/impl/AbstractMetadataStore.java#L93-L95
   
   ### Anything else?
   
   _No response_
   
   ### Are you willing to submit a PR?
   
   - [x] I'm willing to submit a PR!


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Re: [I] [Bug] Subscribing to topic executes work on the single threaded Pulsar metadata thread in some cases [pulsar]

["merlimat (via GitHub)" <gi...@apache.org>]

merlimat commented on issue #23866:
URL: https://github.com/apache/pulsar/issues/23866#issuecomment-6380955576

   Confirmed on current master. In `ServerCnx.handleSubscribe` (pulsar-broker/src/main/java/org/apache/pulsar/broker/service/ServerCnx.java:2383 on master), `topic.subscribe(option)` is chained with `thenCompose` onto `BrokerService.isAllowAutoSubscriptionCreationAsync()` (BrokerService.java:4903), which reads the namespace policies through `getPoliciesAsync`. On a cache miss that future completes on the configuration-metadata-store thread, so the subscribe and everything after it run on the single metadata thread. The likely fix is to switch to `thenComposeAsync` on the topic's ordered executor (or `ctx.executor()`) before calling `topic.subscribe`. ServerCnx.java:2837 (the producer's initial subscription) has the same pattern.


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