[dartdev][flutter_tools] Rerunning commands needs to be a NOP for assets if hooks were cached

[dcharkes]

After the hooks are run in dartdev/flutter_tools, the assets might be copied/signed/install_name modified etc.

We'd like to ensure that we can cache those steps.

We cannot blindly rely on timestamps, as a hook might return a file with an older timestamp (on Windows downloaded files have their creation date as lastmodified, not the downloaded time). We could fix this by mandating that assets should have monotonically increasing timestamps, but that puts burden on the hook writer.

Maybe there are better solutions (using a hash, ...).

Also flutter run and flutter build --release copy files both to build/native_assets/* which also breaks the NOP-ness.

To investigate

• What does flutter_tools currently do? (Possibly content hashes https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/build_system/build_system.dart#L1174)

Context:

• [native_assets_builder] Set the last modified date of assets to the build output timestamp #1533 (comment)
• https://dart-review.googlesource.com/c/sdk/+/381580/5/pkg/dartdev/lib/src/native_assets_bundling.dart

This needs to play well with concurrent invocations as well:

• [native_assets_builder] Concurrency between Dart runs and asset edits #1534

Edit:

• We probably want to align the hook caching ([native_assets_builder] Caching strategy for hook invocations #1593) with whatever dartdev and flutter_tools are doing with the results of the hooks (this issue).

Thanks @mkustermann and @blaugold

[dcharkes]

This is working for flutter_tools, which consumes the BuildOutput.dependencies and does it's own file-content-hashing based tracking of whether those files change.

For Dartdev we have a tracking issue:

• [native assets] dartdev commands should be a nop if nothing changed sdk#59668

Since this is an implementation details that should work, I'm bumping this to after v1.0.