[unified_analytics] Append to the log file instead of rewriting it

[davidmorgan]

LogHandler.save read the whole log file on every event and, once the log held kLogFileLength records, rewrote all of it. With a typical full log, about 1.7 MB, that costs about 16 ms of synchronous I/O per Analytics.send.

Long-lived tools feel this most: the analysis server sends about 130 events in a burst on its first periodic report and at shutdown, blocking its isolate for about 2.4 s. Processes rewriting the file concurrently also corrupt lines.

Now save stats the file and appends. When the file passes kLogFileTrimSize, 4 MiB, it is trimmed to the newest kLogFileLength records by writing a temporary file and renaming it over the log. logFileStats reads only the newest kLogFileLength records, so surveys see the same window as before.

Measured with a full log: send goes from 16.6 ms to 0.27 ms median; the analysis server burst from 2.42 s to 95 ms; a trim costs about 37 ms, once per roughly 3000 events.

Fixes #2611.

[github-actions]

Package publishing

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Package	Version	Status	Publish tag (post-merge)
package:test_reflective_loader	0.6.0	ready to publish	test_reflective_loader-v0.6.0
package:unified_analytics ⭐	8.0.19-wip	WIP (no publish necessary)

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• 21 WIP (no publish necessary).

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[gemini-code-assist]

Code Review

This pull request optimizes the log-saving mechanism in LogHandler by appending events directly to the log file instead of reading and rewriting it on every save. Trimming is now deferred until the log file exceeds 4 MiB, at which point it is trimmed to the newest records using a temporary file. The feedback suggests optimizing the trimming logic in _trim by using string length instead of utf8.encode to avoid unnecessary allocations in a loop.

[github-actions]

PR Health

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pkgs/unified_analytics/lib/src/log_handler.dart	💚 85 % ⬆️ 1 %

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
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unified_analytics	Condition	survey_handler.dart::Survey::conditionList survey_handler.dart::Survey::new::conditionList
unified_analytics	PersistedSurvey	survey_handler.dart::SurveyHandler::fetchPersistedSurveys
unified_analytics	GAClient	analytics.dart::Analytics::fake::gaClient analytics.dart::AnalyticsImpl::new::gaClient
unified_analytics	UserProperty	analytics.dart::FakeAnalytics::userProperty

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[keertip]

Overall looks good. I can't think of any major downsides to landing this, and if we do lose some entries in the local log file, that's the price for performance optimization. Thanks for doing this!

[davidmorgan]

Overall looks good. I can't think of any major downsides to landing this, and if we do lose some entries in the local log file, that's the price for performance optimization. Thanks for doing this!

Great--sent for your review, please.

[srawlins]

Nice tests