grpc-labview v1.3.0.1+ shared library does not work on LVLRT

[jplotzke]

This is a branch of the issue discovered in #439.

In LVLRT 24.8, the grpc-labview shared library does not function in LabVIEW real-time, starting in release v1.3.0.1. Release v1.2.7.1 does function OK.

In LabVIEW, I get:

Error 13 occurred at Call Library Function Node in grpc-lvsupport-release.lvlib:Create Client.vi:560001->test_service_client.lvlib:Create Client.vi

Possible reason(s):

LabVIEW: (Hex 0xD) Failed to load dynamic library because of missing external symbols or dependencies, or because of an invalid file format.
=========================
Shareable board exclusively owned.

It appears that there are undefined symbols in the Linux RT shared library starting in v1.3.0.1:

undefined symbol: grpc_labview::wellknown::I2DArray::_rowsIndex     (./liblabview_grpc_server_1.5.0.1.so)
undefined symbol: grpc_labview::wellknown::I2DArray::_dataIndex     (./liblabview_grpc_server_1.5.0.1.so)
undefined symbol: grpc_labview::wellknown::I2DArray::_columnsIndex  (./liblabview_grpc_server_1.5.0.1.so)

AB#3122435

[jplotzke]

I can confirm that updating these const int variables to constexpr int as suggested by @bkeryan does fix the issue and after re-building I am able to use the shared library on NILRT 24.8 without an issue.

grpc-labview/src/well_known_messages.h

Lines 52 to 54 in 95d9675

	static const int _rowsIndex = 1;
	static const int _columnsIndex = 2;
	static const int _dataIndex = 3;

I will submit a PR with this update, but will admit that I'm not a C++ guru, so please double-check if this change causes any separate issues.