Few months ago, I've written about getting coverage information from many platforms into one report. That approach worked, but I've always felt guilty for pushing almost thousand of files to Codecov.
This week I finally found time to revisit this and make it work better and faster. Actually just uploading these files took about 30 minutes. Together with slow tests execution (which took about 30 minutes as well), we were reaching AppVeyor build time limits and builds did timeout quite often, what is not really nice result.
I've started with rewriting the wrapper used to execute OpenCppCoverage. I've originally used Python for that, which is nice, but I thought the overhead must be noticeable. As it is not possible to execute Python script directly from CTest, it was wrapped in simple bat file, adding another overhead. Reimplementing this in C showed that there is indeed overhead, but this is not going to save more time than few minutes.
Next obvious step was to look at uploading coverage files as this is really something what should be fast and not take such enormous time. When writing the original post, I've already tried to merge coverage data using OpenCppCoverage, but that showed to be too slow to make it actually work (the testsuite didn't complete in given 60 minutes).
I was also looking at existing solutions to merge Cobertura XML files, but I've found nothing working reasonably fast. The problem is that all of these always merge two files at one step, making merging thousand files really slow job as you're constantly generating, parsing and processing the resulting xml file for 1000 times. Also these solutions are probably more generic that what I needed.
In the end these files are just simple XML and merging them should not be hard. I was able to quickly write Python script to merge them. It does not support all of the Cobertura attributes, it just merges line based coverage (as this is the only thing which OpenCppCoverage generates), but works pretty fast and reliable.
Overall the build time went down from 60 minutes to 35 minutes and I don't see much space to improve besides improving OpenCppCoverage speed, what is really out of scope for me. Actually older version (0.9.5.3) performs way faster than current one (0.9.6), which is 2-3 times slower.