Fix eager evaluation of fallback routing in resolveRouting

[zeckem19]

Summary

• Change Optional.orElse() to Optional.orElseGet() in RoutingTargetHandler.resolveRouting() to prevent eager evaluation of the fallback path

Problem

Optional.orElse(value) eagerly evaluates value , in resolveRouting(), this means getRoutingTargetResponse(request) is always called, even when the query ID is found in the cache and previousCluster is present.

For requests like GET /ui/query.html?queryId (e.g. Trino Web UI tracking links via Superset), the request has no routing headers (X-Trino-Routing-Group, etc.). When getRoutingTargetResponse() runs the routing rules and falls through to provideBackendConfiguration(), it throws IllegalStateException: Number of active backends found zero if no default backend is configured. This exception propagates before .orElse() can return the cached result, causing a 500 error even though the query ID was successfully resolved.

Fix

Replace .orElse(getRoutingTargetResponse(request)) with .orElseGet(() -> getRoutingTargetResponse(request)) so the fallback is only evaluated when previousCluster is empty.

Test plan

• Added testResolveRoutingWithKnownQueryIdAndFailingFallback which verifies that when a query ID is found in the cache but the fallback routing would throw, resolveRouting still returns the cached backend successfully
• The test fails with orElse() and passes with orElseGet()

Fixes #920

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[zeckem19]

@ebyhr i added the test, and have emailed the CLA (zeckem19)

[cla-bot]

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[cla-bot]

Thank you for your pull request and welcome to the Trino community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. Continue to work with us on the review and improvements in this PR, and submit the signed CLA to cla@trino.io. Photos, scans, or digitally-signed PDF files are all suitable. Processing may take a few days. The CLA needs to be on file before we merge your changes. For more information, see https://github.com/trinodb/cla

[ebyhr]

Please fix CI failure:

Error:  src/test/java/io/trino/gateway/ha/handler/TestRoutingTargetHandler.java:[310] (regexp) RegexpMultiline: Line has trailing whitespace

Also, please follow the PR template next time.

[cla-bot]

Thank you for your pull request and welcome to the Trino community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. Continue to work with us on the review and improvements in this PR, and submit the signed CLA to cla@trino.io. Photos, scans, or digitally-signed PDF files are all suitable. Processing may take a few days. The CLA needs to be on file before we merge your changes. For more information, see https://github.com/trinodb/cla

[vaultah]

Thanks for fixing this. Just two cents/impact statement:

In trino-gateway 16, QueryCountBasedRouter overrode provideBackendConfiguration and selected from its in-memory clusterStats via getBackendConfigurationForRoutingGroup.

In trino-gateway 17, QueryCountBasedRouter no longer overrides provideBackendConfiguration. The inherited BaseRoutingManager method always does a DB call.

Practically this means that when using QueryCountBasedRouter, due to the eager getRoutingTargetResponse evaluation, trino-gateway 17 performs one or two DB calls for every routing decision/request.