[Live Debugger] Add 10ms snapshot capture timeout circuit breaker

[watson]

Motivation

Snapshot capture in the Live Debugger serializes JavaScript values synchronously by walking object graphs recursively. For large or deeply nested objects, this can block the main thread for an unacceptable amount of time. We need a circuit breaker that aborts capture when it takes too long, preventing the debugger from degrading application performance.

Changes

Adds a 10ms cooperative timeout to the snapshot capture path in the Browser Debugger SDK.

capture.ts

• Introduces a CaptureContext interface (deadline + timedOut flag) threaded through capture(), captureFields(), and all internal recursive helpers.
• Both public functions now require a CaptureContext argument.
• isTimedOut() checks the flag first (fast path), then calls performance.now() against the deadline. Checks are placed at coarse logical boundaries: before each object property, array element, map entry, set item, and typed array element iteration.
• When timed out, captureValue returns { type: <real typeof>, notCapturedReason: 'timeout' }, consistent with how all other Datadog tracers handle notCapturedReason.

api.ts

• Each hook (onEntry, onReturn, onThrow) computes a single deadline before the probe loop and shares it across all probes. This means if one snapshot probe exhausts the budget, subsequent snapshot probes exit immediately, while non-snapshot probes are unaffected.
• When capture times out, the snapshot event is dropped entirely (not sent).

Test instructions

Run the debugger unit tests:

yarn test:unit packages/debugger

Key test scenarios:

• Entry/return/throw capture timeout drops the snapshot
• Non-snapshot probes still fire even after a timeout
• Shared deadline causes second snapshot probe to exit immediately
• No active entry leaks when entry capture times out
• capture() returns the real typeof value (including 'null' not 'object') on timeout

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

[watson]

• [Live Debugger] Add 10ms snapshot capture timeout circuit breaker #4512 👈 (View in Graphite)
• Add Live Debugger package #4449 : 4 other dependent PRs (#4480 , #4503 , #4508 and 1 other)
• Fix TanStack Router lint resolution in test app #4513
• main

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[datadog-datadog-prod-us1]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 67.39%
• Overall Coverage: 76.97% (-0.08%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: ec46917 | Docs | Datadog PR Page | Give us feedback!}

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	169.36 KiB	169.36 KiB	0 B	0.00%	✅
Rum Profiler	5.97 KiB	5.97 KiB	0 B	0.00%	✅
Rum Recorder	21.23 KiB	21.23 KiB	0 B	0.00%	✅
Logs	54.70 KiB	54.70 KiB	0 B	0.00%	✅
Rum Slim	127.69 KiB	127.69 KiB	0 B	0.00%	✅
Worker	22.99 KiB	22.99 KiB	0 B	0.00%	✅

🚀 CPU Performance

Action Name	Base CPU Time (ms)	Local CPU Time (ms)	𝚫%
RUM - add global context	0.0026	0.0021	-19.23%
RUM - add action	0.0127	0.0106	-16.54%
RUM - add error	0.0166	0.0106	-36.14%
RUM - add timing	0.0006	0.0005	-16.67%
RUM - start view	0.0144	0.0096	-33.33%
RUM - start/stop session replay recording	0.0012	0.0007	-41.67%
Logs - log message	0.0268	0.014	-47.76%

🧠 Memory Performance

Action Name	Base Memory Consumption	Local Memory Consumption	𝚫
RUM - add global context	38.44 KiB	38.08 KiB	-370 B
RUM - add action	69.11 KiB	63.60 KiB	-5.52 KiB
RUM - add timing	36.85 KiB	38.20 KiB	+1.35 KiB
RUM - add error	69.34 KiB	67.78 KiB	-1.56 KiB
RUM - start/stop session replay recording	41.08 KiB	40.85 KiB	-230 B
RUM - start view	484.32 KiB	481.97 KiB	-2.35 KiB
Logs - log message	54.85 KiB	54.18 KiB	-693 B

🔗 RealWorld

[chatgpt-codex-connector]

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