feat(core): Replace SentryExecutorService with pre-allocated queue

[romtsn]

Problem

ScheduledThreadPoolExecutor's internal DelayedWorkQueue is a min-heap that grows dynamically (50% per resize, starting from 16). prewarm() was introduced to pre-grow that array during Sentry.init — but init runs on the main thread, which is exactly when ANR risk is highest. The resize cost it avoids is only ~8µs; the prewarm itself adds ~100µs and is the more expensive operation.

More fundamentally, ScheduledThreadPoolExecutor's queue isn't replaceable, so there was no clean way to fix this without a custom implementation.

Solution

Replace SentryExecutorService with a custom implementation backed by:

• PriorityQueue<ScheduledTask<?>> pre-allocated to INITIAL_QUEUE_CAPACITY = 64 slots at construction — the backing array never resizes during normal SDK operation
• Single daemon worker thread that uses Object.wait(millis) / notifyAll() — sleeps precisely until the next task is due, wakes immediately when an earlier task is enqueued
• ScheduledTask<T> extends FutureTask<T> — gets the full Future<T> contract for free

prewarm() is now a documented no-op. The MAX_QUEUE_SIZE = 271 hard limit and purge-on-overflow behaviour are preserved unchanged.

What changed

	Before	After
Queue impl	ScheduledThreadPoolExecutor.DelayedWorkQueue (dynamic, unreplaceable)	PriorityQueue pre-allocated to 64 slots
Resize on init	Yes (prewarm triggers 3 resizes)	Never
prewarm()	Submits 40 far-future tasks + purge	No-op
@TestOnly constructor	Injects ScheduledThreadPoolExecutor	Removed (nothing to inject)
Tests	Mock-based delegation tests	Behaviour tests (execution, scheduling, close, ordering)

Notes

• ./gradlew spotlessApply apiDump must be run locally before merging — sandbox can't run the JVM
• prewarm() is kept in the interface as @Deprecated for API compatibility; can be removed from ISentryExecutorService in a follow-up
• Supersedes / makes unnecessary: perf(core): Remove executor prewarm #5681

cc @romtsn

--

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## Unreleased

### Features

- Replace SentryExecutorService with pre-allocated queue ([#5717](https://github.com/getsentry/sentry-java/pull/5717))

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📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.47.0 (1)	release

⚙️ sentry-android Build Distribution Settings

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Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	300.96 ms	360.30 ms	59.34 ms
Size	0 B	0 B	0 B

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
abfcc92	309.54 ms	380.32 ms	70.78 ms
4e3e79d	328.10 ms	395.64 ms	67.54 ms
8c1fb22	316.62 ms	352.78 ms	36.16 ms
b67bb28	307.59 ms	341.24 ms	33.65 ms
d15471f	361.89 ms	378.07 ms	16.18 ms
22f4345	313.52 ms	364.96 ms	51.44 ms
6727e14	337.22 ms	373.94 ms	36.71 ms
fc5ccaf	322.49 ms	405.25 ms	82.76 ms
22f4345	314.79 ms	375.02 ms	60.23 ms
2195398	319.02 ms	342.38 ms	23.36 ms

App size

Revision	Plain	With Sentry	Diff
abfcc92	1.58 MiB	2.13 MiB	557.31 KiB
4e3e79d	0 B	0 B	0 B
8c1fb22	0 B	0 B	0 B
b67bb28	0 B	0 B	0 B
d15471f	1.58 MiB	2.13 MiB	559.54 KiB
22f4345	1.58 MiB	2.29 MiB	719.83 KiB
6727e14	1.58 MiB	2.28 MiB	718.64 KiB
fc5ccaf	1.58 MiB	2.13 MiB	557.54 KiB
22f4345	1.58 MiB	2.29 MiB	719.83 KiB
2195398	0 B	0 B	0 B