[native_assets_builder] Concurrency between Dart runs and asset edits

[dcharkes]

For v1.0, we'll say that this is not a supported use case.

If we have a long running Dart program that's running in the background and using assets, and at the same time we try to modify asset files (either by hand, or by rerunning a hook), we run into file locking issues on Windows.

• Modifying a asset/my_data.txt while a Dart program has it already open.
• Modifying a C program while the Dart program is already running, and then running the same program in another terminal concurrently, triggering the hook again.

We either need to:

1. Ensure we copy files out, so that after the native_assets_builder is done running, Dart/Flutter only use copies of the files produced by hooks. Originally posted by @dcharkes in [native_assets_builder] Set the last modified date of assets to the build output timestamp #1533 (comment)
2. Or, we should provide some reasonable error messages. But how do we do that? Locking the asset build directories and asset files while running?

Thanks @blaugold for bringing this up!

[dcharkes]

This requires a multi part fix I believe:

1. A unique directory per dart run instead of reusing .dart_tool/lib/.
2. Locking .dart_tool/build_runner/ until after copying out to wherever the embedder wants the file to be copied out to. To achieve that, we'll need a targetDirectory parameter for NativeAssetsBuildRunner that it will eagerly copy to per package. And then the HookResult should only contain assets nested in that directory.
3. And all assets flowing between different hooks also need to be copied to that output directory. Otherwise, another invocation could start running the build hook for a package while the first one is trying to use it in a link hook. (The locking is per package per hook. We could potentially lock for the duration of all hooks, but that would fully serialize any startup of all concurrent dart run commands in a pub workspace.)

(Note: Flutter doesn't have this issue by itself due to the global lock. But we could still have a race between a Flutter command and a Dart command both running the hook_runner to run all the hooks and one overwriting the outputs while the other is copying them out. So also for Flutter we need part 2 and 3.)