Allow calculating cpp coverage

[louwers]

@TimSylvester Could you try this with

just coverage-cpp-html

I only tried on macOS.

You need to have lcov installed

brew install lcov

[Copilot]

Pull Request Overview

This PR adds C++ code coverage support using lcov and genhtml tools to generate coverage reports for the C++ codebase.

• Introduces CMake option MLT_WITH_COVERAGE to enable coverage instrumentation
• Adds two new justfile recipes: coverage-cpp for generating raw coverage data and coverage-cpp-html for HTML reports
• Configures compiler/linker flags for coverage in both the main library and test targets, with special handling for macOS static library linking

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
justfile	Adds coverage-cpp and coverage-cpp-html recipes that build with coverage flags, run tests, and generate lcov/HTML reports
cpp/CMakeLists.txt	Introduces MLT_WITH_COVERAGE option and applies coverage compiler/linker flags to the mlt-cpp library when enabled
cpp/test/CMakeLists.txt	Applies coverage flags to the test target and adds macOS-specific force_load linker option to include static library coverage symbols
cpp/.gitignore	Adds build-coverage directory to ignore list for coverage build artifacts

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The --coverage flag already includes -fprofile-arcs -ftest-coverage, making the explicit addition of these flags redundant. You can simplify this to just --coverage.

Suggested change

	target_compile_options(mlt-cpp PRIVATE --coverage -fprofile-arcs -ftest-coverage)
	target_compile_options(mlt-cpp PRIVATE --coverage)

[Copilot]

The --coverage flag already includes -fprofile-arcs -ftest-coverage, making the explicit addition of these flags redundant. You can simplify this to just --coverage.

Suggested change

	target_compile_options(mlt-cpp-test PRIVATE --coverage -fprofile-arcs -ftest-coverage)
	target_compile_options(mlt-cpp-test PRIVATE --coverage)

[Copilot]

Using the same file for both input (coverage/coverage.info) and output (--output-file coverage/coverage.info) in the lcov extract command is risky. If the extraction fails midway, you could lose the original data. Consider using a temporary file first (e.g., coverage/coverage.filtered.info) and then moving it, or at minimum using a different intermediate filename.

Suggested change

	lcov --extract coverage/coverage.info '*/cpp/src/mlt/*' '*/cpp/include/mlt/*' --output-file coverage/coverage.info --ignore-errors inconsistent
	lcov --extract coverage/coverage.info '*/cpp/src/mlt/*' '*/cpp/include/mlt/*' --output-file coverage/coverage.filtered.info --ignore-errors inconsistent
	mv coverage/coverage.filtered.info coverage/coverage.info

[TimSylvester]

I had to add additional ignores to make it work:

-    lcov --directory build-coverage --capture --output-file coverage/coverage.info --ignore-errors inconsistent
+    lcov --directory build-coverage --capture --output-file coverage/coverage.info --ignore-errors inconsistent,format

-    genhtml coverage/coverage.info --output-directory coverage/html --title "MLT C++ Coverage" --legend --demangle-cpp --show-details --branch-coverage --ignore-errors inconsistent,corrupt
+    genhtml coverage/coverage.info --output-directory coverage/html --title "MLT C++ Coverage" --legend --demangle-cpp --show-details --branch-coverage --ignore-errors inconsistent,corrupt,category