Code coverage for TimescaleDB

Code coverage can be enabled for TimescaleDB builds by setting the option -DCODECOVEAGE=ON when running CMake (it is off by default). This enables the necessary compiler option (--coverage) to generate code coverage statistics and should be enough for CI build reports using, e.g., codecov.io. In addition, local code coverage reports can be generated with the lcov tool, when this tool is installed on the build system.

Generating local code coverage data files

A code coverage report is generated in three steps using lcov:

1. A pre test baseline run to learn what zero coverage looks like (the coverage_base target).
2. A post test run to learn the test coverage (the coverage_test target).
3. A final run to combine the pre test and post test output files into a final data file (the coverage_final target).

Each of these steps can be run manually using the mentioned targets, but should happen automatically as part of regular build and test steps. Optionally, this process can be extended with a filtering step to ignore certain paths that shouldn't be included in the final report.

Producing a HTML-based code coverage report

Once the complete test suite has run (installcheck target), it is possible to produce a HTML-based code coverage report that can be viewed in a web browser. This is automated by the coverage target. Thus, the complete steps to produce a code coverage report are:

1. cmake --build
2. cmake --build --target installcheck
3. cmake --build --target coverage