Add gcov coverage collecting of C code (#387)

.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 -k);;
+    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