Ensure all events sent via package:unified_analytics are received

[christopherfujino]

From the discussion in #136647 (comment), since the public API to send analytics from package:unified_analytics returns a Future<http.Response> (https://github.com/dart-lang/tools/blob/92c5c15e3eb713b39779f4545bfa207ccdfeb1af/pkgs/unified_analytics/lib/src/analytics.dart#L562), callers need to decide to either:

1. await this future, or
2. ignore the future, with the unawaited() function

With option 1, however, this would mean that future tool work may not happen until AFTER the HTTP request has finished. And with option 2, if the dart isolate finishes its other work, it appears to exit, cancelling the HTTP request to the analytics server.

With the existing GA3 implementation, before exiting the tool we call globals.flutterUsage.ensureAnalyticsSent(); https://github.com/flutter/flutter/blob/main/packages/flutter_tools/lib/src/base/process.dart#L617 which ends up calling the waitForLastPing() method in package:usage: https://github.com/dart-lang/usage/blob/master/lib/src/usage_impl.dart#L224. package:usage maintains its own list of futures (https://github.com/dart-lang/usage/blob/master/lib/src/usage_impl.dart#L77) that waitForLastPing() will ensure are completed (although we provide that method with a 250ms timeout to avoid extended delays of the tool exiting).

I believe the simplest solution for the flutter tool would be to create a global class that wraps all calls to package:unified_analytics' .send() method, and collects the futures. Then we can await all these futures at the same place we do for GA3 events (https://github.com/flutter/flutter/blob/main/packages/flutter_tools/lib/src/base/process.dart#L617), with a 250ms timeout.

[christopherfujino]

FYI @eliasyishak & @andrewkolos

[christopherfujino]

Actually, what would be easier than wrapping, is to just have class Analytics from package:unified_analytics do what package:usage did, and keep track of all these futures, and expose a public .waitForLastPing({Duration? timeout}) method that takes an optional timeout.

[eliasyishak]

Related to what I said in this comment, whatever we decide on for sending events, I don't think we can reliably check BigQuery exports to confirm our approach is working.

I am exploring now how it would look if we kept the futures for each send internal to package:unified_analytics now

[eliasyishak]

Captured the issue in the dart-lang/tools repo as well, I will create a separate PR

[eliasyishak]

Looping back on this issue, we made progress on this by implementing an internal futures list within the Analytics instance so that we don't have to await send calls anymore.

There was also additional functionality added to the existing close method to flush out any remaining send events in the futures list (if there are any), and similar to package:usage, we also provide a default value of 250 ms before the close method timeouts and exits.

PR below is where these updates were made:

• Use futures list internally to manage send events dart-lang/tools#184

[eliasyishak]

Closing since I believe we addressed this in dart-lang/tools#184, feel free to reopen if needed

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