ci: add code coverage via Codecov

Add a simple code coverage pipeline based on the CodeQL one.

Should give us pretty reports in the PRs and web UI with the results. We
can consider having HTML reports on our website (once it's up that is)
in addition or instead of at a future point.

Closes: #61
Signed-off-by: Emil Velikov <[email protected]>

.github/actions/setup-ubuntu/action.yml

@@ -18,6 +18,7 @@ runs:
           build-essential \
           clang \
           gcc-multilib \
+          gcovr \
           git \
           gtk-doc-tools \
           liblzma-dev \

.github/workflows/coverage.yml

@@ -0,0 +1,65 @@
+# SPDX-FileCopyrightText: 2024 Emil Velikov <[email protected]>
+# SPDX-FileCopyrightText: 2024 Lucas De Marchi <[email protected]>
+#
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+name: Code Coverage
+
+on:
+  push:
+    branches: [master, ci-test]
+  pull_request:
+    branches: [master]
+  schedule:
+    - cron: "30 2 * * 0"
+
+permissions:
+  contents: read
+
+jobs:
+  analyze:
+    name: Analyze
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        container:
+          - name: 'ubuntu:24.04'
+            meson_setup: '-D b_sanitize=none -D b_coverage=true'
+
+    container:
+      image: ${{ matrix.container.name }}
+
+    steps:
+      - name: Sparse checkout the local actions
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+        with:
+          sparse-checkout: .github
+
+      - uses: ./.github/actions/setup-ubuntu
+        if: ${{ startsWith(matrix.container.name, 'ubuntu') }}
+
+      - name: Checkout the whole project
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+
+      - name: Set the environment
+        run: |
+          # The second checkout above claims to set safe.directory, yet it
+          # doesn't quite work. Perhaps our double/sparse checkout is to blame?
+          git config --global --add safe.directory '*'
+
+          .github/print-kdir.sh >> "$GITHUB_ENV"
+
+      - name: Build
+        run: |
+          mkdir build && cd build
+          meson setup --native-file ../build-dev.ini ${{ matrix.container.meson_setup }} . ..
+          meson compile
+          meson test
+          ninja coverage-xml
+
+      - name: Upload Coverage
+        uses: codecov/codecov-action@b9fd7d16f6d7d1b5d2bec1a2887e65ceed900238 # v4.6.0
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
+          file: build/meson-logs/coverage.xml