[NathanWalker]

Summary

windows-napi can now run in a host whose executable doesn't export Node-API. The embedder lists the modules that do in NAPI_HOST_MODULES, ;-separated and in priority order, and napi-sys takes each napi_* from the first module that exports it.

Why

React Native for Windows needs it (NativeScript/runtimes#112). @nativescript/react-native runs the addon in a Hermes runtime made through hermes.dll's jsr_* API:

• hermes.dll has the engine half of Node-API (js_native_api.h, 115 functions), but none of Node's (node_api.h). napi-rs calls napi_create_threadsafe_function and napi_add_env_cleanup_hook during registration (create_custom_gc), so the addon panicked with "Node-API symbol has not been loaded".
• The React Native module implements Node's half itself and sets NAPI_HOST_MODULES=<its DLL>;<hermes.dll>.

Change

• packages/vendor/napi-sys (load_all): with NAPI_HOST_MODULES set, load each listed module, and load the symbol groups from them in reverse order. A symbol a library lacks keeps its earlier value, so each one ends up from the first library that has it. Without the variable, nothing changes.
• The workspace patches napi-sys with the vendored copy ([patch.crates-io]), as windows-hermes already did, so windows-napi gets it.
• windows-napi/README.md: "Embedding outside Node".

Evidence

Built with cargo build -p windows-napi --release. The resulting windows_napi.dll ran in a react-native-windows 0.83.2 app through @nativescript/react-native's Windows module: sync WinRT, awaited async operations, console and errors all work (details in the runtimes PR).

Found along the way (not fixed here)

• GUID marshalling in the Node-API engine. NativeType::try_from("Guid") returns Pointer (runtime/src/value.rs). A string argument then can't be parsed as a pointer, so the callee gets a null or zero GUID: BitmapEncoder.CreateAsync(id, stream) fails with WINCODEC_ERR_COMPONENTNOTFOUND (0x88982F50) for any form of the id. On return, a 16-byte GUID is written into an 8-byte pointer slot and then read as an object pointer, which crashes (BitmapEncoder.JpegEncoderId, BitmapCodecInformation.CodecId). The classic engine special-cases Guid (guid_ptr_to_js_object, parse_guid_str); the Node-API engine needs the same for parameters (napi_parse_arg) and return slots (method_call.rs / property_call.rs size it as a pointer).
• XAML stops laying out after the last island closes. Once every XamlWindow on a thread has closed, new ones stay 0×0 (XamlRoot.IsHostVisible false). A hidden, never-closed island works around it.

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