perf(core): Batch and coalesce scope-persistence disk writes

[runningcode]

Summary

Scope persistence wrote to disk on every scope mutation, which a customer cold-start trace (JAVA-628) showed to be the single largest SDK cost during startup — larger than init itself:

• FileObjectQueue#add ×107 = 460 ms — every breadcrumb was a synchronous, fsync'd QueueFile append (the file was opened in rwd mode).
• PersistingScopeObserver#store ×310 = 376 ms — setContexts/setTrace/setUser/setTags/… each rewrote their whole file on every change, even though only the latest value matters.

What changed

Instead of writing eagerly on each mutation, mutations are coalesced and flushed together on a short debounce window (100 ms) on the Sentry executor, mirroring the existing LoggerBatchProcessor pattern:

• Latest-wins per file — each scope field keeps only its most recent pending value, collapsing the many redundant rewrites into a handful.
• Batched breadcrumbs — breadcrumbs are buffered and appended together behind a single fsync. QueueFile gains an opt-in buffered-write mode (synchronousWrites(false)) plus a sync() method; only the breadcrumb queue opts in, the ANR-profile queues keep synchronous writes.
• Mutations no longer run inline on the caller/main thread.

Tradeoff

Persistence exists to enrich crash/ANR events on the next launch and was already asynchronous, so nothing changes about the "data is only needed if the process dies" contract — the debounce just widens the potential loss window to ~100 ms of the most recent mutations before a crash. On-disk format is unchanged; restore is unaffected.

Notes

• Reuses the existing enableScopePersistence option; the debounce interval is an internal constant.
• The customer trace also flagged a 24 ms main-thread addBreadcrumb — the code path is (and remains) async; that number still needs to be verified against the trace and is intentionally not addressed here.

🤖 Generated with Claude Code

[linear-code]

JAVA-628

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.49.0 (1)	release

⚙️ sentry-android Build Distribution Settings

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	334.74 ms	380.29 ms	45.55 ms
Size	0 B	0 B	0 B

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
5b1a06b	310.56 ms	362.79 ms	52.22 ms
ad8da22	339.92 ms	407.37 ms	67.45 ms
d15471f	307.28 ms	381.85 ms	74.57 ms
2195398	345.88 ms	411.71 ms	65.82 ms
ee747ae	396.82 ms	441.67 ms	44.86 ms
abfcc92	304.04 ms	370.33 ms	66.29 ms
22f4345	312.78 ms	347.40 ms	34.62 ms
d15471f	304.55 ms	408.43 ms	103.87 ms
ad8da22	362.98 ms	453.94 ms	90.96 ms
bbc35bb	298.53 ms	372.17 ms	73.64 ms

App size

Revision	Plain	With Sentry	Diff
5b1a06b	0 B	0 B	0 B
ad8da22	1.58 MiB	2.29 MiB	719.83 KiB
d15471f	1.58 MiB	2.13 MiB	559.54 KiB
2195398	0 B	0 B	0 B
ee747ae	1.58 MiB	2.10 MiB	530.95 KiB
abfcc92	1.58 MiB	2.13 MiB	557.31 KiB
22f4345	1.58 MiB	2.29 MiB	719.83 KiB
d15471f	1.58 MiB	2.13 MiB	559.54 KiB
ad8da22	1.58 MiB	2.29 MiB	719.83 KiB
bbc35bb	1.58 MiB	2.12 MiB	553.01 KiB