[unified_analytics] Full log rewrite on every event once log is full #2611

Hi Keerti,

While looking at dart format performance with an agent, it suggested
what looks like a nice optimization: when the log is full, a tweak to how
it's handled could save 14ms on every dart command.

Please let me know if that makes sense and if you'd like me to propose a PR.
Or if not--I'll just drop it, this was just a side note to the actual work :)

Thanks :)

Morgan

Agent generated content follows.

LogHandler.save in lib/src/log_handler.dart reads the entire log file on
every event, and once it holds kLogFileLength = 2500 records it rewrites
the whole file:

records = logFile.readAsLinesSync();
final content = '${jsonEncode(data)}\n';
if (records.length < kLogFileLength) {
  logFile.writeAsStringSync(content, mode: FileMode.writeOnlyAppend);
} else {
  records.add(content);
  records = records.skip(records.length - kLogFileLength).toList();
  logFile.writeAsStringSync(records.join('\n'));
}

Any developer machine that has sent 2500 events is permanently in the second
branch. On mine the log is 1,669,466 bytes and exactly 2500 lines, so every
dart command reads and rewrites 1.6 MB.

Measured cost: about 14 ms per dart format invocation, local SSD. Two
independent derivations, released SDK 3.14.0-75.0.dev, 30 interleaved
repetitions each:

"Empty log" is a temporary HOME with the same consent state and client id
but a zero-length dart-flutter-telemetry.log.

Trimming in blocks rather than per record would remove it: for example,
append until the file holds 2 × kLogFileLength records, then trim to
kLogFileLength in one rewrite. That amortises the rewrite over 2500 events
and keeps the append path a single small write. Reading the whole file on
every event to count lines could be avoided the same way, by checking
stat.size against a byte threshold instead of counting records.