[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 : 5 other dependent PRs (#4480 , #4503 , #4508 and 2 others)
• 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: 76.62% (-0.09%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 77b798f | Docs | Datadog PR Page | Give us feedback!}

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	179.56 KiB	179.56 KiB	0 B	0.00%	✅
Rum Profiler	6.04 KiB	6.04 KiB	0 B	0.00%	✅
Rum Recorder	27.03 KiB	27.03 KiB	0 B	0.00%	✅
Logs	56.69 KiB	56.69 KiB	0 B	0.00%	✅
Rum Slim	135.41 KiB	135.41 KiB	0 B	0.00%	✅
Worker	23.63 KiB	23.63 KiB	0 B	0.00%	✅

🚀 CPU Performance

Action Name	Base CPU Time (ms)	Local CPU Time (ms)	𝚫%
RUM - add global context	0.005	0.004	-20.00%
RUM - add action	0.014133333333333333	0.0126	-10.85%
RUM - add error	0.013566666666666666	0.0118	-13.02%
RUM - add timing	0.0028333333333333335	0.0025	-11.76%
RUM - start view	0.0131	0.0113	-13.74%
RUM - start/stop session replay recording	0.0007333333333333333	0.0006	-18.18%
Logs - log message	0.014933333333333333	0.0135	-9.60%

🧠 Memory Performance

Action Name	Base Memory Consumption	Local Memory Consumption	𝚫
RUM - add global context	31.23 KiB	31.24 KiB	+10 B
RUM - add action	54.86 KiB	54.79 KiB	-73 B
RUM - add timing	32.96 KiB	32.41 KiB	-564 B
RUM - add error	60.66 KiB	61.05 KiB	+401 B
RUM - start/stop session replay recording	32.15 KiB	32.37 KiB	+227 B
RUM - start view	483.79 KiB	484.10 KiB	+317 B
Logs - log message	48.88 KiB	49.90 KiB	+1.02 KiB

🔗 RealWorld

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Start the snapshot deadline when capture begins

Because this deadline is created before iterating probes and before ENTRY condition/template evaluation, time spent in an earlier probe can consume the entire 10 ms budget before any snapshot capture runs. For example, if a non-snapshot probe on the same method has an expensive condition or template, the following snapshot probe will see performance.now() >= deadline on its first capture() call and be dropped even though its own capture did not exceed the timeout. The circuit breaker should measure snapshot capture work rather than unrelated probe evaluation.

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