ci: collect coverage on _exit() as well

_exit() skips the gcov hooks, so we lose all coverage collected up to
that point. Let's work around this by intercepting _exit() with our
wrapper that calls __gcov_dump() just before _exit().

Makefile.am

@@ -17,6 +17,11 @@ AM_CPPFLAGS += \
 endif
 endif
 
+if WITH_COVERAGE
+AM_CPPFLAGS += \
+	-include $(top_srcdir)/include/coverage.h
+endif
+
 AM_CFLAGS = -fsigned-char $(WARN_CFLAGS)
 AM_CXXFLAGS = $(AM_CFLAGS)
 AM_LDFLAGS = $(ASAN_LDFLAGS) $(UBSAN_LDFLAGS) $(FUZZING_ENGINE_LDFLAGS) $(COVERAGE_LDFLAGS)

include/Makemodule.am

@@ -13,6 +13,7 @@ dist_noinst_HEADERS += \
 	include/closestream.h \
 	include/colors.h \
 	include/color-names.h \
+	include/coverage.h \
 	include/cpuset.h \
 	include/crc32.h \
 	include/crc32c.h \

include/coverage.h

@@ -0,0 +1,25 @@
+/*
+ * No copyright is claimed.  This code is in the public domain; do with
+ * it what you wish.
+ */
+#ifndef UTIL_LINUX_COVERAGE_H
+#define UTIL_LINUX_COVERAGE_H
+
+/* When built with --coverage (gcov) we need to explicitly call __gcov_dump()
+ * in places where we use _exit(), since _exit() skips at-exit hooks resulting
+ * in lost coverage.
+ *
+ * To make sure we don't miss any _exit() calls, this header file is included
+ * explicitly on the compiler command line via the -include directive (only
+ * when built with --coverage/-Db_coverage=true)
+ */
+void __gcov_dump(void);
+void _exit(int);
+
+__attribute__((noreturn)) static inline void _coverage__exit(int status) {
+        __gcov_dump();
+        _exit(status);
+}
+#define _exit(x) _coverage__exit(x)
+
+#endif