feat(profiling): Add 30-second timeout for chunk downloads

[sentry]

Fixes VROOM-56. The issue was that: Handler blocks on channel receive without checking request context, leading to 'context canceled' errors when client disconnects.

• Introduced a 30-second timeout for chunk download operations in the postProfileFromChunkIDs endpoint.
• Updated chunk.ReadJob to use the new context with the timeout.
• Added error handling for context.Canceled errors, returning HTTP 503 Service Unavailable.

This fix was generated by Seer in Sentry, triggered by txiao@sentry.io. 👁️ Run ID: 12113277

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Timeout produces DeadlineExceeded, not Canceled error

High Severity

The downloadContext is created with context.WithTimeout, which produces context.DeadlineExceeded when the 30-second timeout fires. However, the error check only matches context.Canceled. This means timeout errors will bypass the 503 handler and fall through to the generic 500 path, getting reported to Sentry — the exact problem this PR aims to fix. The existing flamegraph code correctly checks for context.DeadlineExceeded in the same scenario.

 

[sentry]

Bug: The new timeout error handling only checks for context.Canceled and misses context.DeadlineExceeded, causing timeouts to return an incorrect HTTP 500 error instead of 503.
_{Severity: MEDIUM}

Suggested Fix

Add a check for context.DeadlineExceeded alongside the existing check for context.Canceled. If the error is context.DeadlineExceeded, write the http.StatusServiceUnavailable header and return, mirroring the behavior for a canceled context. This should be applied in both postProfileFromChunkIDs() and getRawChunk().

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: cmd/vroom/chunk.go#L134-L137

Potential issue: The error handling for the newly introduced 30-second chunk download
timeout is incomplete. When the timeout is exceeded, `context.WithTimeout` generates a
`context.DeadlineExceeded` error. However, the code only checks for `context.Canceled`.
This causes the timeout error to be missed, leading the request to fall through to the
generic error handler which returns an HTTP 500 Internal Server Error instead of the
intended HTTP 503 Service Unavailable. This same logical error exists in both
`postProfileFromChunkIDs()` and `getRawChunk()`.

_{Did we get this right? 👍 / 👎 to inform future reviews.}