perf(core): Remove executor prewarm

[runningcode]

Summary

prewarm() scheduled 40 dummy tasks far in the future, cancelled them, and purged the queue, in order to pre-grow the single-threaded ScheduledThreadPoolExecutor's work queue during Sentry.init. On-device measurement shows the cost it avoids is negligible-to-zero, so this removes it (and the now-unused constants) along with its two init call sites.

Why it's safe to remove

prewarm's only job was to pre-grow the executor's work queue so a later resize wouldn't happen "in an unexpected area of the SDK". Measured on a Galaxy A55 (Android 16), growing the queue the full 16→54 (all three array resizes) costs ~8µs total — and that is the ceiling: init only ever queues a handful of tasks, well under the default 16-slot capacity, so in practice no resize happens and the saving is 0µs.

PrewarmBenchmarkTest, n=1000 × 3 runs, median µs. Both paths schedule 40 tasks into an empty queue (identical per-insert work); they differ only in whether the backing array resizes:

	Run 1	Run 2	Run 3
grow 16→54	15	15	15
preGrown 54	7	6	7
resize cost (delta)	~8	~9	~8

It also can't help, and slightly regresses init

The executor is single-threaded and prewarm is submitted ahead of the first useful task (loadLazyFields, submitted right after in Sentry.init), so the 40-task loop can only delay that task, never speed it up. Thread creation and executor class-loading are paid by whichever task runs first, so loadLazyFields warms them identically with or without prewarm.

Measured end-to-end, the time until the first useful task finishes (from a fresh executor, n=400 × 3 runs) is ~100µs slower with prewarm than without, at every percentile; a plain submit(noop) is indistinguishable from doing nothing:

median µs	Run 1	Run 2	Run 3
prewarm	567	524	537
no warming	460	428	435

(PrewarmBenchmarkTest also records the per-op loop cost and the process-once class-loading cost, which is unavoidable regardless of prewarm; both confirm the above.)

Changes

• Remove prewarm() from ISentryExecutorService, SentryExecutorService (and the now-unused INITIAL_QUEUE_SIZE / dummyRunnable), and NoOpSentryExecutorService.
• Remove the two init call sites (Sentry.init re-init branch and SentryOptions.activate()).
• Drop the prewarm unit test and mock overrides; add PrewarmBenchmarkTest.

Original prewarm PR: #4606 — its only stated reason was the queue-growth cost measured above.

🤖 Generated with Claude Code

[sentry]

📲 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

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	344.23 ms	411.47 ms	67.24 ms
Size	0 B	0 B	0 B

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
abfcc92	304.04 ms	370.33 ms	66.29 ms
4e3e79d	365.83 ms	477.62 ms	111.79 ms
d8912da	329.94 ms	389.68 ms	59.74 ms
d15471f	294.13 ms	399.49 ms	105.36 ms
abf451a	332.82 ms	403.67 ms	70.85 ms
604a261	380.65 ms	451.27 ms	70.62 ms
806307f	357.85 ms	424.64 ms	66.79 ms
22f4345	307.87 ms	354.51 ms	46.64 ms
d217708	375.27 ms	415.68 ms	40.41 ms
c3ee041	310.64 ms	361.90 ms	51.26 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
d8912da	0 B	0 B	0 B
d15471f	1.58 MiB	2.13 MiB	559.54 KiB
abf451a	1.58 MiB	2.20 MiB	635.29 KiB
604a261	1.58 MiB	2.10 MiB	533.42 KiB
806307f	1.58 MiB	2.10 MiB	533.42 KiB
22f4345	1.58 MiB	2.29 MiB	719.83 KiB
d217708	1.58 MiB	2.10 MiB	532.97 KiB
c3ee041	0 B	0 B	0 B