Handle SetLastError=true

[elinor-fung]

• Add functions for getting/setting system error (Get/SetLastSystemError) and setting P/Inovke error (SetLastWin32Error) to Ancillary.Interop
• Update generated stub code to clear the system error before invoke, get the system error after and update the stored P/Invoke error

cc @AaronRobinsonMSFT @jkoritzinsky

Examples:

SetLastError=true, blittable signature

public static partial int SetError(int error, byte shouldSetError)
{
    unsafe
    {
        int __retVal = default;
        int __lastError = default;
        //
        // Invoke
        //
        {
            System.Runtime.InteropServices.MarshalEx.SetLastSystemError(0);
            __retVal = SetError__PInvoke__(error, shouldSetError);
            __lastError = System.Runtime.InteropServices.MarshalEx.GetLastSystemError();
        }

        System.Runtime.InteropServices.MarshalEx.SetLastWin32Error(__lastError);
        return __retVal;
    }
}

[System.Runtime.InteropServices.DllImportAttribute("NativeExportsNE", EntryPoint = "set_error")]
extern private static unsafe int SetError__PInvoke__(int error, byte shouldSetError);

SetLastError=true, non-blittable signature

public static partial string SetError_NonBlittableSignature(int error, bool shouldSetError, string errorString)
{
    unsafe
    {
        byte __shouldSetError_gen_native = default;
        ushort *__errorString_gen_native = default;
        string __retVal = default;
        ushort *__retVal_gen_native = default;
        int __lastError = default;
        //
        // Marshal
        //
        __shouldSetError_gen_native = (byte)(shouldSetError ? 1 : 0);
        //
        // Invoke
        //
        fixed (char *__errorString_gen_native__pinned = errorString)
        {
            System.Runtime.InteropServices.MarshalEx.SetLastSystemError(0);
            __retVal_gen_native = SetError_NonBlittableSignature__PInvoke__(error, __shouldSetError_gen_native, (ushort *)__errorString_gen_native__pinned);
            __lastError = System.Runtime.InteropServices.MarshalEx.GetLastSystemError();
        }

        //
        // Unmarshal
        //
        __retVal = __retVal_gen_native == null ? null : new string ((char *)__retVal_gen_native);
        System.Runtime.InteropServices.MarshalEx.SetLastWin32Error(__lastError);
        return __retVal;
    }
}

[System.Runtime.InteropServices.DllImportAttribute("NativeExportsNE", EntryPoint = "set_error_return_string")]
extern private static unsafe ushort *SetError_NonBlittableSignature__PInvoke__(int error, byte shouldSetError, ushort *errorString);

[AaronRobinsonMSFT]

LGTM. Looks like we found the owner for proposing these new APIs :-)

I assume we need/want:

• SetLastSystemError(), even though we could P/Invoke into that ourselves - see example in PR.
• SetLastWin32Error(), even though we could use reflection to get at the call - see example in PR.

[jkotas]

        fixed (char *__errorString_gen_native__pinned = errorString)
        {
            System.Runtime.InteropServices.MarshalEx.SetLastSystemError(0);
            __retVal_gen_native = SetError_NonBlittableSignature__PInvoke__(error, __shouldSetError_gen_native, (ushort *)__errorString_gen_native__pinned);
            System.Runtime.InteropServices.MarshalEx.SetLastWin32Error(System.Runtime.InteropServices.MarshalEx.GetLastSystemError());
        }

        //
        // Unmarshal
        //
        __retVal = __retVal_gen_native == null ? null : new string ((char *)__retVal_gen_native);
        return __retVal;

Is the last error guaranteed to be left intact by all the unmarshalling code? Should SetLastWin32Error be called right before returning instead?

[elinor-fung]

Should SetLastWin32Error be called right before returning instead?

Ahh, yeah. Good call.

[elinor-fung]

Interestingly, the built-in system today can allow writing over the error code in unmarshalling (tried this quickly with a custom marshaler does a Console.WriteLine in its unmarashalling function).

[jkotas]

Yes, the built-in system has a patch work of workarounds like this that I am sure has many holes.