[native_assets] Hot restart native libs

[dcharkes]

We should consider trying to support "hot restart" for native code.

The native code would need to be bundled with dylibs (not statically linked).

We could unload all dylibs loaded by @Natives and purging caches for resolved natives.

For Dart in JIT mode this should be enough, as the dylibs will just be on disk. (Windows might be problematic if as dylibs cannot be deleted while in use.)

For Flutter in JIT mode we'd need some Flutter specific support for copying the dylibs to the right place (and possibly signing them).

Hot reload doesn't make sense for dylibs, because the memory of the dylib cannot be migrated (the native languages don't have the same support as Dart).
Hot restarting native libs when doing a Dart hot reload seems to be a bad idea, due to the Dart and native state getting out of sync.

We'd need to decide whether we'd always want hot restart native libs on hot restarting Dart, or whether it should be a different command. I think hot restarting both together would be better than hot restarting one. Hot restarting only native code means things like native finalizers in the Dart heap would still exist. (Due to native finalizers being run on isolate shutdown, we should probably shut down Dart isolates first, then unload the dylibs and then start a Dart isolate again.)

Thanks for the suggestion @fzyzcjy! 🙏

• [native_toolchain_rust] Introduce a package:native_toolchain_rust native#883 (comment)

Related issues:

• Hot restart in Flutter not running native finalizers before starting a new isolate.
• Add hot restart hooks to support dart:ffi flutter/flutter#75528
• Long Running Isolates / FFI Calls Break Hot Reloading -> willReassemble Method Needed flutter/flutter#59061

[fzyzcjy]

You are welcome and thanks for the issue!

Hot reload doesn't make sense for dylibs, because the memory of the dylib cannot be migrated (the native languages don't have the same support as Dart).

For Rust, there are some (very hacky) approaches: https://github.com/rksm/hot-lib-reloader-rs. This is super hacky, but if the user just modifies some logic without changing the state, hopefully that will work.

[dcharkes]

If we do it for native assets with the build and link hooks. We also need to think about when to rerun the native builds. That actually advocates for a new letter in flutter run:

• r Dart hot reload (quick)
• R Dart hot restart (fairly quick)
• T [TBD what letter] native+Dart hot restart (slower, because it will rerun build / link hooks, which could be fast but could also be fairly slow)

Open to alternative suggestions on what the user experience should be.