Add coverage_report target (#1058)

README.md

@@ -292,6 +292,55 @@ E.g.:
 export CLIO_CLANG_TIDY_BIN=/opt/homebrew/opt/llvm@17/bin/clang-tidy
 ```
 
+## Coverage report
+
+Coverage report is intended for the developers using compilers GCC
+or Clang (including Apple Clang). It is generated by the build target `coverage_report`,
+which is only enabled when both `tests` and `coverage` options are set, e.g. with
+`-o coverage=True -o tests=True` in `conan`
+
+Prerequisites for the coverage report:
+
+- [gcovr tool](https://gcovr.com/en/stable/getting-started.html) (can be installed e.g. with `pip install gcovr`)
+- `gcov` for GCC (installed with the compiler by default) or
+- `llvm-cov` for Clang (installed with the compiler by default, also on Apple)
+- `Debug` build type
+
+Coverage report is created when the following steps are completed, in order:
+
+1. `clio_tests` binary built with the instrumentation data, enabled by the `coverage`
+   option mentioned above
+2. completed run of unit tests, which populates coverage capture data
+3. completed run of `gcovr` tool (which internally invokes either `gcov` or `llvm-cov`)
+   to assemble both instrumentation data and coverage capture data into a coverage report
+
+Above steps are automated into a single target `coverage_report`. The instrumented
+`clio_tests` binary can be also used for running regular unit tests. In case of a
+spurious failure of unit tests, it is possile to re-run `coverage_report` target without
+rebuilding the `clio_tests` binary (since it is simply a dependency of the coverage report target).
+
+The default coverage report format is `html-details`, but the developers
+can override it to any of the formats listed in `CMake/CodeCoverage.cmake`
+by setting `CODE_COVERAGE_REPORT_FORMAT` variable in `cmake`. For example, CI
+is setting this parameter to `xml` for the [codecov](codecov.io) integration.
+
+In case if some unit tests predictably fail e.g. due to absence of a Cassandra database, it is possible
+to set unit tests options in `CODE_COVERAGE_TESTS_ARGS` cmake variable, as demonstrated below:
+
+```
+cd .build
+conan install .. --output-folder . --build missing --settings build_type=Debug -o tests=True -o coverage=True
+cmake -DCODE_COVERAGE_REPORT_FORMAT=json-details -DCMAKE_BUILD_TYPE=Debug -DCODE_COVERAGE_TESTS_ARGS="--gtest_filter=-BackendCassandra*" -DCMAKE_TOOLCHAIN_FILE:FILEPATH=build/generators/conan_toolchain.cmake ..
+cmake --build . --target coverage_report
+```
+
+After `coverage_report` target is completed, the generated coverage report will be
+stored inside the build directory, as either of:
+
+- file named `coverage_report.*`, with a suitable extension for the report format, or
+- directory named `coverage_report`, with `index.html` and other files inside, for `html-details` or `html-nested` report formats.
+
+
 ## Developing against `rippled` in standalone mode
 
 If you wish you develop against a `rippled` instance running in standalone