cloudstorage: fix a bug causing context cancelled errors and stuck sink

[zurakutsia]

What problem does this PR solve?

Issue Number: close #12277

Fixes TiCDC cloud storage sink flapping (start/stop spam) on Azure Blob caused by premature context cancellation during reads.

Symptoms included:

• “Failed to read data from azure blob, data info: pos='0', count='1': context canceled”
• “failed to generate data file path”
• “dead dmlSink”

The regression seems to be introduced by 0e6782b71. Switching to GetExternalStorageWithDefaultTimeout wrapped Open with a timeout and canceled it immediately, breaking subsequent Read() calls that reuse the Open() context.

What is changed and how it works?

Before: Open wrapped the passed context with context.WithTimeout and deferred cancel(). Since many backends bind the reader’s lifetime to the Open context, the deferred cancel immediately invalidated the reader’s future Read() calls, causing “context canceled” errors.

Now: Open passes the caller's context through without wrapping or canceling it. This prevents premature cancellation while keeping existing timeouts for other APIs. extStorageWithTimeout.Open returns a readerWithCancel wrapper struct which calls cancel() on Close()

Tests

• Unit tests

Questions

Will it cause performance regression or break compatibility?

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

Release note

Please refer to [Release Notes Language Style Guide](https://pingcap.github.io/tidb-dev-guide/contribute-to-tidb/release-notes-style-guide.html) to write a quality release note.

If you don't think this PR needs a release note then fill it with `None`.

[ti-chi-bot]

Hi @zurakutsia. Thanks for your PR.

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[pingcap-cla-assistant]

All committers have signed the CLA.

[gemini-code-assist]

Summary of Changes

Hello @zurakutsia, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

I've addressed a critical bug in the TiCDC cloud storage sink, specifically impacting Azure Blob storage, which was causing frequent "context cancelled" errors and leading to a "stuck sink" state. The core issue stemmed from an incorrect context cancellation mechanism within the Open method of extStorageWithTimeout, where the context was prematurely canceled, invalidating subsequent read operations. My fix refactors this by ensuring the context associated with the reader is only canceled when the reader itself is explicitly closed, preventing these disruptive errors and stabilizing the sink.

Highlights

• Bug Fix for Cloud Storage Sink: Resolved an issue where the TiCDC cloud storage sink, particularly on Azure Blob, experienced "context cancelled" errors and "stuck sink" behavior due to premature context cancellation during file Open operations.
• Context Handling Refinement: Modified the extStorageWithTimeout.Open method to no longer immediately cancel the context. Instead, a new readerWithCancel wrapper is introduced, which defers the context cancellation until the reader's Close() method is invoked, ensuring the reader's lifetime is respected.
• Enhanced Test Coverage: Added new unit tests (TestExtStorageOpenDoesNotCancelReaderContext and TestExtStorageOpenReaderRespectsCallerCancel) to specifically validate the corrected context behavior, ensuring the reader's context remains valid for its duration and properly responds to external cancellations.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a critical bug causing context cancellation errors by ensuring the context for a storage reader remains valid throughout its lifecycle. The approach of wrapping the reader to manage the context's cancellation in the Close method is sound. The accompanying tests are well-designed and validate the fix effectively. I have one suggestion to enhance the robustness of the new readerWithCancel.Close() method.

[wk989898]

/ok-to-test

[wk989898]

Can you create an issue to describe what you encountered and how to reproduce this bug? @zurakutsia

[hongyunyan]

/retest

[hongyunyan]

/retest

[OliverS929]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: asddongmen, hongyunyan

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-08-12 10:00:17.033807939 +0000 UTC m=+348016.068391167: ☑️ agreed by hongyunyan.
• 2025-08-12 10:16:22.005500243 +0000 UTC m=+348981.040083470: ☑️ agreed by asddongmen.

[OliverS929]

/retest

[OliverS929]

DM_DropAddColumn This case is a bit flaky. The dm-master restart took slightly longer, so the retry loop did not catch the expected conflict.

[OliverS929]

/retest

[OliverS929]

/retest

[hongyunyan]

/merge

[ti-chi-bot]

@hongyunyan: We have migrated to builtin LGTM and approve plugins for reviewing.

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[hongyunyan]

/run-check-issue-triage-complete

[purelind]

/check-issue-triage-complete

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #12280.
But this PR has conflicts, please resolve them!

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #12281.
But this PR has conflicts, please resolve them!

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #12282.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #12283.

[hongyunyan]

/cherrypick release-7.5-20250617-v7.5.6

[ti-chi-bot]

@hongyunyan: new pull request created to branch release-7.5-20250617-v7.5.6: #12285.

In response to this:

/cherrypick release-7.5-20250617-v7.5.6

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