Once I got CI working on multiple platforms the obvious next step was to be able to aggregate coverage reports across them. This should not be that hard, right? Well I've spent couple of hours on that during last few days.
On Linux and OSX it was pretty much straightforward. Both GCC and Clang do support coverage, so it's just matter of configuring them properly and collect the coverage reports. I've used own solution for that in past and that was really far from working well (somehow I never managed to get coverage fully uploaded to Codecov). Fortunately there exists CMake script called CMake-codecov which does all needed work and works out of the box on GCC and Clang (even on OSX). Well it works on Travis only once you update the compilers and install llvm-cov tool.
The Windows part on AppVeyor was much harder for me. This can be heavily accounted to lack of my experience with Windows and especially development on Windows in past ten years (probably even more). First challenge was to find something what can generate code coverage there.
After lot of googling I've settled down on OpenCppCoverage what seems to be the only free solution I was able to find. The good thing is that it can generate coverage in Cobertura format that Codecov undestands. There are also bad things that I've learned. First of all it's quite hard to integrate this with CTest. There is no support for wrapping test calls in custom commands, so I've misused the memory checks for that purpose. I've written small python script which pretends the valgrind interface and does call OpenCppCoverage in the background.
Now I had around 800 coverage files (one for each test case) and we need to deal with them somehow. The Codeconv command line client doesn't deal wit this out of the box so the obvious choice was to merge them before upload. There even seems to be script doing that, but unfortunately trying that on our coverage data make it nowhere near completion within hour, so that's not really good choice. Second thing I've tried was merging binary coverage in OpenCppCoverage and then exporting to Cobertura format. Obviously Gammu is special project as all I got from this attempt was crashing OpenCppCoverage (it did merge some of the coverages, but it failed in the end without indicating any error).
In the end I've settled down to uploading files in chunks to Codecov. This seems to work quite okay, though is a bit slow, mostly due to way how Codecov bash uploader prepares data to upload (but this will be hopefully fixed soon).
Anyway the goal has been reached, both Windows and Linux code shows in coverage reports.