worker(dm): isolate source errors in query status snapshots (#12883)

[ti-chi-bot]

This is an automated cherry-pick of #12883

What problem does this PR solve?

Issue Number: close #12882

When a worker already has a saved source error and the upstream connection remains
unavailable, each QueryStatus call appends another error to the saved result.
Repeated polling can grow the response beyond the gRPC receive limit, making even
queries for a nonexistent task fail.

What is changed and how it works?

• Keep getSourceStatus unchanged. In QueryStatus, clone an existing
  ProcessResult only when appending the current upstream error. This follows
  the result-copying pattern already used by getAllSubTaskStatus and avoids
  changing the saved result or an earlier response.
• Add a regression test for both nil and existing saved results. It exercises
  repeated and concurrent queries, checks stable response content/size, and
  verifies that the saved status and earlier response remain unchanged.
• Keep the wire format, existing error reporting, and gRPC receive limit
  unchanged. This PR fixes the source-error accumulation; it does not change the
  OpenAPI handling of unrelated worker RPC failures.

Check List

Tests

• Unit test
  • make dm_unit_test_pkg PKG=github.com/pingcap/tiflow/dm/worker (full package, race detector and failpoints enabled)
  • go test -race ./dm/worker -run '^TestQueryStatusDoesNotAccumulateSourceErrors$' -count=10
  • The regression test fails on the original implementation: the second query
    contains three errors instead of the expected two.
• Manual test
  • go vet ./dm/worker
  • make fmt
  • git diff --check

Questions

Will it cause performance regression or break compatibility?

No wire-format or configuration compatibility change. Only queries that append
an upstream error to an existing result copy that small result. The normal path
and the nil-result path remain unchanged, while error-list growth and shared
mutation are avoided.

Do you need to update user documentation, design documentation or monitoring documentation?

No.

Release note

Fix an issue where repeated DM status queries could accumulate source errors and exceed the gRPC message size limit when the upstream connection was unavailable.

Summary by CodeRabbit

• Bug Fixes
  • Fixed query status reporting for missing tasks so repeated and concurrent checks return consistent source-error information. Existing errors are preserved without being duplicated, and checking status no longer changes previously returned results or the stored status. This makes error details more reliable across repeated checks and avoids unexpected changes in reported task status.

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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1. It must be LGTMed and approved by the reviewers firstly.
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ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: Organization UI
• Review profile: CHILL
• Plan: Advanced
• Run ID: a7cc9dfe-4868-4821-b490-8cfaa76b4fb3

📥 Commits

Reviewing files that changed from the base of the PR and between 22d515b and ae06c6e.

📒 Files selected for processing (2)

• dm/worker/server.go
• dm/worker/server_test.go

Included review availability: This review used your included allowance. Your plan provides up to 2 included reviews per hour; 0 remain after this review.

📝 Walkthrough

QueryStatus now clones an existing saved source result before appending the current source error. Tests verify that repeated and concurrent queries do not change saved status or earlier responses.

Changes: Status query error handling

Layer / File(s)	Summary
Clone the saved result before appending errors dm/worker/server.go, dm/worker/server_test.go	QueryStatus clones an existing source result before appending the current source error. Tests check repeated and concurrent responses and verify that saved status and earlier responses remain unchanged.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to ae06c

The change appears ready to merge after normal checks; no actionable issue remains identified.

Security architecture risk: ⚪ Minimal · up to ae06c

The change prevents status polling from repeatedly adding errors to saved worker state. It preserves the existing endpoint, response format and access paths. No introduced or worsened security risk was identified within the changed behavior.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The affected availability scope is status reporting for the worker's handled source and task queries. The fix removes polling-induced growth in shared source-error state without adding tenant, service, credential or upstream access.

Trust Boundaries and Controls

• observed — The caller-supplied task name follows the same worker QueryStatus path as before. The changed branch operates only on returned error snapshots; it introduces no new caller, authorization bypass, sensitive sink or authority transition. This does not establish the security of unchanged access controls.

Pre-merge checks · 4 · 1

Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks (4 passed)

Check name	Status	Explanation
Title check		The title clearly identifies the DM worker change: isolating source errors in QueryStatus snapshots. It is concise and directly related to the changes.
Description check		The description covers the problem, issue reference, implementation, regression tests, compatibility impact, documentation impact, and release note. It matches the repository template and explains the…
Linked Issues check		Issue #12882 is closed. It provides historical context only. No active directly linked issue supplies coding requirements for this pull request.
Out of Scope Changes check		The changes stay within the stated QueryStatus fix. The server change clones the saved ProcessResult before appending the current source error. The added test covers repeated, concurrent, nil-result, …

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_{A rabbit checks the status stream,
No growing errors crowd the screen.
Each saved result stays as it was,
New errors join a copy because—
Then quiet queries hop along.}

_{Comment @coderabbitai help to get the list of available commands.}

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: GMHDBJD

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