[GitHub] [druid] leventov opened a new issue #9482: An ExecutorService must not be created for each KerberosHttpClient; Don't assign ExecutorService into variable of Executor type

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leventov opened a new issue #9482: An ExecutorService must not be created for each KerberosHttpClient; Don't assign ExecutorService into variable of Executor type
URL: https://github.com/apache/druid/issues/9482
 
 
   Similarly to #9286, we should catch assignments of `ExecutorService` (within type hierarchy) into variables of `Executor` type.
   
   Two Structural search patterns are needed:
    - "Java" pattern: `$x$ = $y$;`, where the Type of `$x$` is `Executor` and the Type of `$y$` is `ExecutorService`.
    - "Java - Class Member" pattern: `$Type$ $x$ = $y$;`, where the Text of `$Type$` is `Executor` and the Type of `$y$` is `ExecutorService`.
   
   (BTW, there is only "Java" pattern suggested in #9286 and added in #9325; a similar "Java - Class Member" pattern should be added for `ExecutorService` - `ScheduledExecutorService` pair, too.)
   
   This is needed because [`ExecutorService` instances must be shut down explicitly](https://github.com/code-review-checklists/java-concurrency#explicit-shutdown) and `Executor` doesn't allow for this.
   
   Incidentally, the sole violation of this rule in Druid code, in `KerberosHttpClient`:
   
   https://github.com/apache/druid/blob/a6776648112917b72c077ba3ac0cb7f61993a2d0/extensions-core/druid-kerberos/src/main/java/org/apache/druid/security/kerberos/KerberosHttpClient.java#L44-L54
   
   Is actually an instance of another Java concurrency problem: [an `ExecutorService` must not be created for each instance of a short-lived object](https://github.com/code-review-checklists/java-concurrency#reuse-threads) like `KerberosHttpClient`. So the `exec` in `KerberosHttpClient` should properly made static, or cached on the DI level as a `@Named` `ExecutorService`.

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