perf(core): Reuse shared executor for transaction timeouts (SDK-1347)

[runningcode]

📜 Description

SentryTracer created a dedicated java.util.Timer — i.e. a new thread — for every transaction that had an idle or deadline timeout. This change schedules those timeouts on the SDK's existing shared SentryExecutorService (a single ScheduledThreadPoolExecutor) instead, so no per-transaction thread is created.

• Idle/deadline TimerTasks become Future<?>s scheduled via ISentryExecutorService.schedule(...).
• The shared executor is created with setRemoveOnCancelPolicy(true) so timeouts that are cancelled early (the common case — a transaction finishing before its idle/deadline) are evicted from the work queue immediately instead of lingering until their scheduled time.
• Reschedule-on-activity, cancel-on-finish, and the fallback that finishes immediately if scheduling is rejected are all preserved.

💡 Motivation and Context

Part of the Reduce SDK init time [Android] effort — specifically SDK-1347 (reduce the number of threads the SDK creates) and the thread/executor audit in JAVA-570. A per-transaction Timer thread is the SDK's largest source of thread churn at runtime; apps with many auto-instrumented transactions (activity/navigation/HTTP) paid a thread create+teardown each time. The shared ScheduledThreadPoolExecutor is alive for the whole transaction lifetime, so it is a safe host for these timeouts.

In production the shared executor is activated during Sentry.init, before any transaction is created. Tests that previously relied on the always-on per-transaction Timer now provide a real executor (or disable the transaction timeouts when isolating an unrelated scheduled task).

💚 How did you test it?

SentryTracerTest updated to assert the new future/scheduler state (incl. reschedule-on-child-finish and the closed-executor fallback); full :sentry:test, :sentry-android-core:testDebugUnitTest, and :sentry-android-navigation:testDebugUnitTest are green.

📝 Checklist

• I added GH Issue ID & Linear ID
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

Further thread consolidation candidates from the JAVA-570 audit: DefaultAndroidEventProcessor's throwaway init-path executor, and folding the remaining new Timer(true) sites (DefaultCompositePerformanceCollector, RateLimiter, LifecycleWatcher) onto the shared scheduler.

SDK-1347
JAVA-570

[linear-code]

SDK-1347

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.46.0 (1)	release

⚙️ sentry-android Build Distribution Settings

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	337.35 ms	398.32 ms	60.97 ms
Size	0 B	0 B	0 B

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
18c0bc2	306.73 ms	349.77 ms	43.03 ms
0eaac1e	316.82 ms	357.34 ms	40.52 ms
d15471f	303.49 ms	439.08 ms	135.59 ms
fc5ccaf	276.52 ms	370.46 ms	93.93 ms
e2dce0b	308.96 ms	360.10 ms	51.14 ms
5b1a06b	352.27 ms	413.70 ms	61.43 ms
37ec571	366.04 ms	424.28 ms	58.23 ms
9fbb112	361.43 ms	427.57 ms	66.14 ms
bbc35bb	324.88 ms	425.73 ms	100.85 ms
ff8eea4	313.42 ms	337.08 ms	23.66 ms

App size

Revision	Plain	With Sentry	Diff
18c0bc2	1.58 MiB	2.13 MiB	557.33 KiB
0eaac1e	1.58 MiB	2.19 MiB	619.17 KiB
d15471f	1.58 MiB	2.13 MiB	559.54 KiB
fc5ccaf	1.58 MiB	2.13 MiB	557.54 KiB
e2dce0b	0 B	0 B	0 B
5b1a06b	0 B	0 B	0 B
37ec571	0 B	0 B	0 B
9fbb112	1.58 MiB	2.11 MiB	539.18 KiB
bbc35bb	1.58 MiB	2.12 MiB	553.01 KiB
ff8eea4	1.58 MiB	2.28 MiB	718.64 KiB