dependency resolution on a non-JS thread fails with 0x80131509 instead of falling back #504

Environment

• node-api-dotnet / Microsoft.JavaScript.NodeApi(.*): 0.9.27
• Node.js: v24.19.0 (Windows x64)
• .NET runtime loaded by the host: 8.0.29

Summary

When a dependency assembly is first resolved on a thread that has no JS value scope (a .NET thread-pool
thread in the example below), ManagedHost.OnResolvingAssembly throws JSInvalidThreadAccessException while
trying to emit the resolving event.

See https://github.com/microsoft/node-api-dotnet/blob/main/src/NodeApi.DotNetHost/ManagedHost.cs#L276-L291

The runtime exposes that as a FileLoadException:

System.IO.FileLoadException: Could not load file or assembly 'Newtonsoft.Json, Version=13.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed'. An operation is not legal in the current state. (0x80131509)
   ---> Microsoft.JavaScript.NodeApi.JSInvalidThreadAccessException: There is no active JS value scope.
        Current thread: #4 ".NET TP Worker". Consider using the synchronization context to switch to the JS thread.

Note this is 0x80131509 (COR_E_INVALIDOPERATION), not 0x80070002 (file not found): in the repro the assembly is next to the module, so the message points away from the cause.

It also throws before the loader's next step, which looks for the assembly in the same directory as any already-loaded assemblies: see https://github.com/microsoft/node-api-dotnet/blob/main/src/NodeApi.DotNetHost/ManagedHost.cs#L280-L316

try
{
    assembly = LoadAssembly(assemblyName, allowNativeLibrary: false);
}
catch (FileNotFoundException)
{
    // not found in the system directories.
    // Emit a resolving event to allow listeners to load the assembly.
    Emit( ResolvingEventName, assemblyName, assemblyVersion!, 
        new JSFunction(ResolveAssembly) ////////  may throw here: building the JSValue/JSFunction arguments requires a JS scope
    );   
    _loadedAssembliesByName.TryGetValue(assemblyName, out assembly);
}
/////////// not reached when the Emit above throws
if (assembly == null)
{
    // The dependency assembly was not resolved by an event-handler.
    // Look for it in the same directory as any already-loaded assemblies.
    // ...
}

Minimal reproduction

see the minimal repo that reproduces

Trace evidence (NODE_API_TRACE_HOST=1)

resolve assembly on .NET Task Pool:

> ManagedHost.OnResolvingAssembly(Newtonsoft.Json, 13.0.0.0)
> ManagedHost.LoadAssembly(Newtonsoft.Json)              <-- (1) framework dirs
[repro] ... FileLoadException 0x80131509 / JSInvalidThreadAccessException ".NET TP Worker"
<-- no `ManagedHost.LoadAssembly(<path>)` line, no `not found at ...` line: (3) never ran

resolve assembly on JS Scope:

> ManagedHost.OnResolvingAssembly(Newtonsoft.Json, 13.0.0.0)
> ManagedHost.LoadAssembly(Newtonsoft.Json)
> ManagedHost.LoadAssembly(<module dir>\Newtonsoft.Json.dll)     <-- (3) adjacent-directory fallback
< ManagedHost.OnResolvingAssembly(Newtonsoft.Json) => <module dir>\Newtonsoft.Json.dll

Impact

• the adjacent-directory fallback never runs when resolution happens off the JS thread, so a dependency that would have resolved fails
• The error message is misleading

Suggested

Skip the notification when there is no JS scope. Since there's no JSValueScope, there's no need to report to JS

catch (FileNotFoundException)
{
    // guard: use the existing internal api
    if (JSValueScope.CurrentOrNull is not null)   
    {
        Emit(ResolvingEventName, assemblyName, assemblyVersion!, new JSFunction(ResolveAssembly));
        _loadedAssembliesByName.TryGetValue(assemblyName, out assembly);
    }
}

With this, it will fallback to the next step.

Further Requests

Orthogonal to this report: the host's load context is private and its resolution policy is fixed, so a consumer
cannot use his own AssemblyLoadContext. It's better to allow developer to custom the ALC