Reindex Relocation: Prevent source task override task result

[samxbr]

This is a follow-up of #145488. This prevents the source reindex task overriding the result written by the destination task, effectively ensuring the destination always wins if there's a race. This is to ensure the relocation chain is preserved in the case where the source task observes another error (eg. network error if the resume response from destination to source is lost).

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[Copilot]

Pull request overview

This PR hardens reindex relocation task-result persistence by ensuring the relocation destination’s write to .tasks cannot be overwritten by a racing source-node write, preserving the relocation chain even under partial failures (e.g., lost resume response).

Changes:

• Add create-if-absent result storage (opType(CREATE)) for tasks that must not overwrite an existing .tasks document, treating version conflicts as success.
• Wire task-specific “create semantics” into TaskManager and enable it for reindex source tasks once relocation handoff begins.
• Expand unit + integration tests to cover race orderings (destination-first and source-first) and validate stored content/chain behavior.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
server/src/main/java/org/elasticsearch/tasks/TaskResultsService.java	Adds storeResultIfAbsent() using CREATE semantics and version-conflict-as-success handling.
server/src/main/java/org/elasticsearch/tasks/TaskManager.java	Routes result persistence through new task-level create/overwrite decision.
server/src/main/java/org/elasticsearch/tasks/Task.java	Introduces overridable hook to request create-if-absent result storage.
server/src/main/java/org/elasticsearch/index/reindex/BulkByScrollTask.java	Tracks relocation handoff initiation and opts into create-if-absent result storage afterwards.
modules/reindex/src/main/java/org/elasticsearch/reindex/Reindexer.java	Sets the handoff flag prior to sending resume request; adds assertion around when source-task results are stored.
server/src/main/java/org/elasticsearch/tasks/TaskResult.java	Enforces invariant: cannot have both error and response bytes.
server/src/test/java/org/elasticsearch/tasks/TaskResultTests.java	Adds a test validating the new TaskResult invariant.
server/src/main/java/org/elasticsearch/index/reindex/ResumeInfo.java	Updates documentation around source task result propagation.
modules/reindex/src/test/java/org/elasticsearch/reindex/ReindexerTests.java	Adds coverage ensuring relocation handoff flips the create-semantics flag; strengthens source-task-result assertions.
modules/reindex-management/src/test/java/org/elasticsearch/reindex/management/TransportGetReindexActionTests.java	Adjusts relocation-chain test setup consistent with updated chain preservation semantics.
modules/reindex-management/src/internalClusterTest/java/org/elasticsearch/reindex/management/ReindexRelocationIT.java	Adds IT coverage for destination-first/source-first write races; extends test plugin to block/defer .tasks writes and capture sources.

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[szybia]

LGTM!

only thing, copilot might be cooking with suggestion to use generic threadpool rather than stranded thread