commit 69f32d6ad7e724e3148cb2eb6601e63165e76ad3
author Lasse R.H. Nielsen <lrn@google.com> Thu Jun 09 15:32:33 2022 +0000
committer Commit Bot <commit-bot@chromium.org> Thu Jun 09 15:32:33 2022 +0000
tree 910762a2fa408f5c96ac8e46a0b1431139580072
parent 5875a9ba9d7f84aec616ab89b4c29bae06d1587c

Refactor `_Future`.

This is a major rewrite of the `_Future` class,
which is the default implementation of the `Future` interface.

The main goal was to reduce the number of expensive type checks
in the internal passing around of data.
Expensive type checks are things like
* `is _Future<T>` (more expensive than just `is _Future`, the latter
  can be a single class-ID check.
* Covariant generic parameter checks (using `T` covariantly in a
  parameter forces a run-time type check).

Also removed some plain unnecessary casts and turned some
implicit casts from `dynamic` into `unsafeCast`s.

This seems to be an success, at least on very primitive benchmarks, according to Golem:
FutureCatchErrorTest    41.22% (1.9 noise)
FutureValueTest         46.51% (2.8 noise)
EmptyFutureTest         59.15% (3.1 noise)
FutureWhenCompleteTest  51.10% (3.2 noise)

A secondary goal was to clean up a very old and messy class,
and make it clearer for other `dart:async` how to interact
with the future.

The change has a memory cost: The `_FutureListener<S,T>` class,
which represents a `then`, `catchError` or `whenComplete`
call on a `_Future`, now contains a reference to its source future,
the one which provides the inputs to the callbacks,
as well as the result future returned by the call.
That's one extra memory slot per listener.

In return, the `_FutureListener` now does not need to
get its source future as an argument, which needs a covariant
generic type check, and the methods of `_Future` can be written
in a way which ignores the type parameters of both `_Future`
and `_FutureListener`, which reduces complex type checks
significantly.

In general, typed code is in `_FutureListener`, which knows both
the source and target types of the listener callbacks, and which
contains the futures already at that type, so no extra type checking
is needed.
The `_Future` class is mostly untyped, except for its "public"
API, called by other classes, which checks inputs,
and code interacting with non-native futures.
Invariants ensure that only correctly typed values
are stored in the untyped shared `_resultOrListeners` field
on `_Future`, as determined by its `_state` integer.
(This was already partially true, and has simply been made
more consistent.)

Further, we now throw an error in a situation that was previously
unhandled: When a `_Future` is completed with *itself*.
That would ensure that the future would never complete
(it waits for itself to complete before it can complete),
and may potentially have caused weird loops in the representation.
In practice, it probably never happens. Now it makes the error
fail with an error.
Currently a private `_FutureCyclicDependencyError` which presents
as an `UnsupportedError`.
That avoids code like
```dart
import "dart:async";
void main() {
  var c = Completer();
  c.complete(c.future); // bad.
  print("well!");
  var d = Completer();
  d.complete(c.future);
  print("shucks!");
}
```
from hanging the runtime by busily searching for the end of a cycle.

See https://github.com/dart-lang/sdk/issues/48225
Fixes #48225

TEST= refactoring covered by existing tests, few new tests.

Change-Id: Id9fc5af5fe011deb0af3e1e8a4ea3a91799f9da4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/244241
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>