Understanding dart build_runner scripts AssetNotFoundException between packages when using melos

Question

My question is about project configuration and generatting boilerplate code using build_runner: json_serializable, freezed. We are using melos.

The project is configured as a workspace. It has two packages let's name it:

• packages/sdk
• packages/app

app is using sdk.

In both app and sdk packages there are some boilerplate to generate using freezed and json_serializable. In our root pubspec.yaml we have:

name: workspace
publish_to: none
environment:
 sdk: ^3.8.1
workspace:
 - packages/sdk
 - packages/app
dev_dependencies:
 melos: 7.1.0
melos:
 scripts:
 build_runner:
 exec: dart run build_runner build --delete-conflicting-outputs
 description: Run code generation
 packageFilters:
 dependsOn: build_runner

When we execute: melos run build_runner and we select sdk package it ends with success.

After this we try to run this again and select app and we receive such errors:

app:
 Generating the build script.
 Reading the asset graph.
 Checking for updates.
 Updating the asset graph.
 Building, incremental build.
 0s freezed on 20 inputs; lib/feature/dashboard/domain/model/dashboard_tab.dart
 0s freezed on 20 inputs: 1 skipped; lib/feature/dashboard/domain/model/transaction.dart
 0s freezed on 20 inputs: 20 skipped
 0s json_serializable on 40 inputs; lib/feature/dashboard/domain/model/dashboard_tab.dart
 0s json_serializable on 40 inputs: 40 skipped
 0s source_gen:combining_builder on 40 inputs; lib/feature/dashboard/domain/model/dashboard_tab.dart
 0s source_gen:combining_builder on 40 inputs: 40 skipped
 0s flutter_gen_runner on 1 input; $package$
 0s flutter_gen_runner on 1 input: 1 skipped
 Running the post build.
 Writing the asset graph.
E json_serializable on lib/feature/dashboard/presentation/screen/dashboard_screen.dart:
 AssetNotFoundException: sdk|lib/feature/auth/domain/model/auth_result.freezed.dart (/Users/dev/project_root/packages/sdk/lib/feature/auth/domain/model/auth_result.freezed.dart)
E json_serializable on lib/feature/dashboard/presentation/widget/home_tab.dart:
 AssetNotFoundException: sdk|lib/feature/auth/domain/model/authenticated_user.g.dart (/Users/dev/project_root/packages/sdk/lib/feature/auth/domain/model/authenticated_user.g.dart)
E json_serializable on lib/feature/dashboard/presentation/widget/settings_tab.dart:
 AssetNotFoundException: sdk|lib/feature/auth/domain/model/authenticated_user.freezed.dart (/Users/dev/project_root/packages/sdk/lib/feature/auth/domain/model/authenticated_user.freezed.dart)
E json_serializable on lib/feature/splash/presentation/screen/splash_screen.dart:
 AssetNotFoundException: sdk|lib/feature/auth/domain/model/user.g.dart (/Users/dev/project_root/packages/sdk/lib/feature/auth/domain/model/user.g.dart)
E json_serializable on lib/main.dart:
 AssetNotFoundException: sdk|lib/feature/auth/domain/model/user.freezed.dart (/Users/dev/project_root/packages/sdk/lib/feature/auth/domain/model/user.freezed.dart)
E json_serializable on test/widget_test.dart:
 AssetNotFoundException: sdk|lib/feature/auth/domain/model/auth_token.g.dart (/Users/dev/project_root/packages/sdk/lib/feature/auth/domain/model/auth_token.g.dart)
 Failed to build with build_runner in 1s; wrote 0 outputs.

All generated files *.g.dart and *.freezed.dart under project_root exists but it reports AssetNotFoundException.

When I execute script like this:

#!/bin/bash
set -e
echo "Cleaning and building build_runner in sdk..."
melos exec --scope sdk -- flutter pub run build_runner clean
melos exec --scope sdk -- flutter pub run build_runner build --delete-conflicting-outputs
echo "Cleaning and building build_runner in app..."
melos exec --scope app -- flutter pub run build_runner clean
melos exec --scope app -- flutter pub run build_runner build --delete-conflicting-outputs

Then there is no errors like: AssetNotFoundException.

And after this also when I execute: melos run build_runner it finishes with success. I wonder what is the difference between this script above and the melos run build_runner which returns errors.

Can someone put more light on this. I tried to extract the problem to small one but I can't reproduce it in small codebase. Need some ideas where to look or what to check in our large codebase.

• David Morgan: build_runner maintainer here. That's surprising--usually there should be no way to cause an AssetNotFoundException. Since it only happens with melos, I guess it's something to do with how melos works. Maybe try asking them? I wonder if it runs multiple things in parallel; build_runner expects to only be run once in parallel.