Add gcov coverage testing for C code

.github/workflows/test.yml

@@ -49,3 +49,15 @@ jobs:
       - name: Tests
         run: |
           pytest
+
+      - name: Test with coverage
+        if: ${{ startsWith(matrix.os, 'ubuntu') && matrix.python-version == '3.9' }}
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+        run: |
+          pip install -e .
+          pip install coverage
+          CFLAGS="--coverage -O0" python setup.py -q build_ext --inplace -f
+          coverage run -m pytest
+          ./scripts/coverage.sh
+          bash <(curl -s https://codecov.io/bash) -X gcov

.gitignore

@@ -141,3 +141,9 @@ python/version.h
 # Docker wheel build
 pip-cache/
 temp-wheels/
+
+# gcov coverage files
+cov
+*.gcov
+*.gcda
+*.gcno

scripts/coverage.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+# Coverage for ultrajson's C code.
+# Usage:
+#   CFLAGS="--coverage -O0" python setup.py -q build_ext --inplace -f
+#   pytest
+#   ./scripts/coverage.sh
+# Then inspect the files in the `cov` folder.
+
+# The exact arguments depend on whether we're using LLVM's gcov or GNU's.
+unameOut="$(uname -s)"
+case "${unameOut}" in
+    Linux*)     gcov_options=(--relative-only);;
+    Darwin*)    gcov_options=(--color);;
+    *)          echo "Unsupported OS ${unameOut}"; exit 1;;
+esac
+
+# The actual gcov instructions:
+gcov "${gcov_options[@]}" python/**.c -o build/temp.*/python
+gcov "${gcov_options[@]}" lib/**.c -o build/temp.*/lib
+
+# gcov dumps everything in the cwd without any option to change this.
+# Manually move the .gcov files to a `cov` folder.
+mkdir -p cov
+rm -rf cov/*
+mv ./**.gcov cov || exit 1
+
+echo Written gcov files to ./cov