Replace ResourceWatcher with SnapshotWatcher

[jrhee17]

Motivation:

Simplify the internal xDS resource watcher contract from three callbacks (onChanged, onError, onResourceDoesNotExist) to a single onUpdate(@Nullable T, @Nullable Throwable) by replacing ResourceWatcher/XdsStreamSubscriber with the existing SnapshotWatcher/new CompositeSnapshotWatcher.

Modifications:

• Deleted ResourceWatcher, XdsStreamSubscriber, and DummyResourceWatcher
• Added CompositeSnapshotWatcher as a drop-in replacement for XdsStreamSubscriber
• Updated SubscriberStorage, StateCoordinator, ConfigSourceHandler, ResourceNode, and ResourceNodeAdapter to use the new types
• Updated StateCoordinatorTest and SubscriberStorageTest to use SnapshotWatcher lambdas

Result:

• No user-facing API changes — all modified types are package-private.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 891457e7-fe75-4311-a66a-bd11a29fe501

📥 Commits

Reviewing files that changed from the base of the PR and between 7dc92fc and eda0cf0.

📒 Files selected for processing (2)

• xds/src/main/java/com/linecorp/armeria/xds/ConfigSourceHandler.java
• xds/src/main/java/com/linecorp/armeria/xds/StateCoordinator.java

💤 Files with no reviewable changes (1)

• xds/src/main/java/com/linecorp/armeria/xds/StateCoordinator.java

🚧 Files skipped from review as they are similar to previous changes (1)

• xds/src/main/java/com/linecorp/armeria/xds/ConfigSourceHandler.java

---

📝 Walkthrough

Walkthrough

Migrates xDS subscription from stream-style callbacks (ResourceWatcher, XdsStreamSubscriber) to snapshot-style SnapshotWatcher with a CompositeSnapshotWatcher fan-out and updates storage, coordinator, adapters, and tests to use onUpdate(value, error).

Changes

XDS Watcher Architecture Refactor

Layer / File(s)	Summary
Composite snapshot watcher implementation xds/src/main/java/com/linecorp/armeria/xds/CompositeSnapshotWatcher.java	Introduces CompositeSnapshotWatcher<T> that schedules optional absent-on-timeout, cancels timers on update, manages downstream SnapshotWatcher registration, fans out onUpdate(value,error) to all watchers, and logs individual watcher exceptions.
Watcher interface contract migration xds/src/main/java/com/linecorp/armeria/xds/ResourceNode.java, xds/src/main/java/com/linecorp/armeria/xds/ResourceNodeAdapter.java, xds/src/main/java/com/linecorp/armeria/xds/ConfigSourceHandler.java	ResourceNode now extends SnapshotWatcher<T>; ResourceNodeAdapter implements single onUpdate(@nullableT value,@nullable Throwable error) with conditional metric routing; ConfigSourceHandler methods accept SnapshotWatcher<? extends XdsResource> instead of ResourceWatcher.
Subscriber storage and coordination xds/src/main/java/com/linecorp/armeria/xds/SubscriberStorage.java, xds/src/main/java/com/linecorp/armeria/xds/StateCoordinator.java	SubscriberStorage stores CompositeSnapshotWatcher instances and wires per-subscriber addWatcher/removeWatcher. StateCoordinator updates register/unregister signatures to SnapshotWatcher<T> and rewrites delivery/replay to call onUpdate(resource, null) or onUpdate(null, exception). XdsStreamSubscriber is removed.
Test updates for snapshot watcher API xds/src/test/java/com/linecorp/armeria/xds/StateCoordinatorTest.java, xds/src/test/java/com/linecorp/armeria/xds/SubscriberStorageTest.java	Replaces bespoke watcher helpers with inline SnapshotWatcher lambdas and AtomicReference captures; reorganizes tests to validate immediate delivery, cached replay on late registration, absence-after-missing behavior, duplicate-register semantics, and timeout error capture. The DummyResourceWatcher helper is removed.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Possibly related PRs

• line/armeria#6608: Refactors SnapshotWatcher to use unified onUpdate(snapshot, error) callback.
• line/armeria#6747: Changes ConfigSourceHandler-based subscription wiring that overlaps with this PR's subscription boundary updates.
• line/armeria#6709: Related StateCoordinator/subscriber plumbing changes that touch similar watcher integration points.

Suggested reviewers

• trustin
• ikhoon
• minwoox

"From my rabbit burrow I peep and say with cheer,
Watchers gathered closely now — no streams to steer.
One snapshot voice for many, small errors ignored,
Composite fans the signal — the meadow sings toward! 🐇✨"

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 5.56% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Replace ResourceWatcher with SnapshotWatcher' accurately and concisely summarizes the primary change: migrating from one watcher interface to another.
Description check	✅ Passed	The description comprehensively explains the motivation, modifications, and results, clearly relating to all aspects of the changeset.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[github-actions]

🔍 Build Scan® (commit: eda0cf0)

Job name	Status	Build Scan®
build-ubicloud-standard-16-jdk-8	✅	https://ge.armeria.dev/s/52radrie7kubu
build-ubicloud-standard-16-jdk-25	✅	https://ge.armeria.dev/s/df2gb7mkeljl6
build-ubicloud-standard-16-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/rzkiptqjp6bqw
build-ubicloud-standard-16-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/cjpudarlkureg
build-ubicloud-standard-16-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/qg77qnzyaqgji
build-ubicloud-standard-16-jdk-17-leak	✅	https://ge.armeria.dev/s/f64htitxdwpis
build-ubicloud-standard-16-jdk-11	✅	https://ge.armeria.dev/s/edkffkcors2ju
build-macos-latest-jdk-25	✅	https://ge.armeria.dev/s/3fagkotoixxbq

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.00%. Comparing base (8150425) to head (eda0cf0).
⚠️ Report is 482 commits behind head on main.

Files with missing lines	Patch %	Lines
...linecorp/armeria/xds/CompositeSnapshotWatcher.java	90.00%	2 Missing and 1 partial ⚠️
.../com/linecorp/armeria/xds/ResourceNodeAdapter.java	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6796      +/-   ##
============================================
+ Coverage     74.46%   75.00%   +0.54%     
- Complexity    22234    24914    +2680     
============================================
  Files          1963     2214     +251     
  Lines         82437    92667   +10230     
  Branches      10764    12106    +1342     
============================================
+ Hits          61385    69509    +8124     
- Misses        15918    17370    +1452     
- Partials       5134     5788     +654     

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