CompileStringAsyc() and race conditions when used in package build

[nshahan]

I'm working on implementing an AsyncImporter to use in package sass_builder. I've got a working synchronous importer by wrapping async calls with waitFor from dart:cli but I would like to compare the performance with an async version.

For a given build target I am finding that calls to canonicalize() continue after the call CompileStringAsyc() has returned a value. This causes an error in package build:

Concurrent modification during iteration: Instance of 'SplayTreeSet<AssetId>'

I don't have a proper stack trace to point to the call site in package sass because the error arises from the iteration site in package build. I do believe it is a call to canonicalize() that is causing the problem.

How can we investigate this more?

[nex3]

If a call to canonicalize() is the root cause, then there are two ways it could stay alive after the compilation has finished: either the compilation logic could fail to wait on the returned future, or the implementation of canonicalize() could have asynchronous logic that doesn't get piped into the returned future. It looks like the asynchronous _EvaluateVisitor awaits both of its calls to .canonicalize(), so it seems like the first place to look is probably your canonicalize() implementation.

[nshahan]

False alarm. The unawaited future in my code has been found. Now the question is why didn't the lint give any feedback for it.

dart-lang/sdk#57711