tree-wide: reset the cleaned-up variable in cleanup functions

If the cleanup function returns the appropriate type, use that to reset the
variable. For other functions (usually the foreign ones which return void), add
an explicit value to reset to.

This causes a bit of code churn, but I think it might be worth it. In a
following patch static destructors will be called from a fuzzer, and this
change allows them to be called multiple times. But I think such a change might
help with detecting unitialized code reuse too. We hit various bugs like this,
and things are more obvious when a pointer has been set to NULL.

I was worried whether this change increases text size, but it doesn't seem to:

-Dbuildtype=debug:
before "tree-wide: return NULL from freeing functions":
-rwxrwxr-x 1 zbyszek zbyszek 4117672 Feb 16 14:36 build/libsystemd.so.0.30.0*
-rwxrwxr-x 1 zbyszek zbyszek 4494520 Feb 16 15:06 build/systemd*
after "tree-wide: return NULL from freeing functions":
-rwxrwxr-x 1 zbyszek zbyszek 4117672 Feb 16 14:36 build/libsystemd.so.0.30.0*
-rwxrwxr-x 1 zbyszek zbyszek 4494576 Feb 16 15:10 build/systemd*
now:
-rwxrwxr-x 1 zbyszek zbyszek 4117672 Feb 16 14:36 build/libsystemd.so.0.30.0*
-rwxrwxr-x 1 zbyszek zbyszek 4494640 Feb 16 15:15 build/systemd*

-Dbuildtype=release:
before "tree-wide: return NULL from freeing functions":
-rwxrwxr-x 1 zbyszek zbyszek 5252256 Feb 14 14:47 build-rawhide/libsystemd.so.0.30.0*
-rwxrwxr-x 1 zbyszek zbyszek 1834184 Feb 16 15:09 build-rawhide/systemd*
after "tree-wide: return NULL from freeing functions":
-rwxrwxr-x 1 zbyszek zbyszek 5252256 Feb 14 14:47 build-rawhide/libsystemd.so.0.30.0*
-rwxrwxr-x 1 zbyszek zbyszek 1834184 Feb 16 15:10 build-rawhide/systemd*
now:
-rwxrwxr-x 1 zbyszek zbyszek 5252256 Feb 14 14:47 build-rawhide/libsystemd.so.0.30.0*
-rwxrwxr-x 1 zbyszek zbyszek 1834184 Feb 16 15:16 build-rawhide/systemd*

I would expect that the compiler would be able to elide the setting of a
variable if the variable is never used again. And this seems to be the case:
in optimized builds there is no change in size whatsoever. And the change in
size in unoptimized build is negligible.

Something strange is happening with size of libsystemd: it's bigger in
optimized builds. Something to figure out, but unrelated to this patch.

src/basic/capability-util.h

@@ -25,7 +25,7 @@ int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities);
 
 int drop_capability(cap_value_t cv);
 
-DEFINE_TRIVIAL_CLEANUP_FUNC(cap_t, cap_free);
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(cap_t, cap_free, NULL);
 #define _cleanup_cap_free_ _cleanup_(cap_freep)
 
 static inline void cap_free_charpp(char **p) {

src/basic/dlfcn-util.h

@@ -5,7 +5,7 @@
 
 #include "macro.h"
 
-DEFINE_TRIVIAL_CLEANUP_FUNC(void*, dlclose);
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(void*, dlclose, NULL);
 
 int dlsym_many_and_warn(void *dl, int level, ...);
 

src/basic/fd-util.h

@@ -41,8 +41,8 @@ static inline void fclosep(FILE **f) {
         safe_fclose(*f);
 }
 
-DEFINE_TRIVIAL_CLEANUP_FUNC(FILE*, pclose);
-DEFINE_TRIVIAL_CLEANUP_FUNC(DIR*, closedir);
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(FILE*, pclose, NULL);
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(DIR*, closedir, NULL);
 
 #define _cleanup_close_ _cleanup_(closep)
 #define _cleanup_fclose_ _cleanup_(fclosep)

src/basic/fileio.c

@@ -1146,7 +1146,7 @@ static EndOfLineMarker categorize_eol(char c, ReadLineFlags flags) {
         return EOL_NONE;
 }
 
-DEFINE_TRIVIAL_CLEANUP_FUNC(FILE*, funlockfile);
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(FILE*, funlockfile, NULL);
 
 int read_line_full(FILE *f, size_t limit, ReadLineFlags flags, char **ret) {
         size_t n = 0, allocated = 0, count = 0;

src/basic/gcrypt-util.h

@@ -14,7 +14,7 @@
 void initialize_libgcrypt(bool secmem);
 int string_hashsum(const char *s, size_t len, int md_algorithm, char **out);
 
-DEFINE_TRIVIAL_CLEANUP_FUNC(gcry_md_hd_t, gcry_md_close);
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(gcry_md_hd_t, gcry_md_close, NULL);
 #endif
 
 static inline int string_hashsum_sha224(const char *s, size_t len, char **out) {

src/basic/macro.h

@@ -405,10 +405,20 @@ static inline int __coverity_check_and_return__(int condition) {
                 func(p);                                        \
         }
 
+/* When func() returns the void value (NULL, -1, …) of the appropriate type */
 #define DEFINE_TRIVIAL_CLEANUP_FUNC(type, func)                 \
         static inline void func##p(type *p) {                   \
                 if (*p)                                         \
+                        *p = func(*p);                          \
+        }
+
+/* When func() doesn't return the appropriate type, set variable to empty afterwards */
+#define DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(type, func, empty)     \
+        static inline void func##p(type *p) {                   \
+                if (*p != (empty)) {                            \
                         func(*p);                               \
+                        *p = (empty);                           \
+                }                                               \
         }
 
 #define _DEFINE_TRIVIAL_REF_FUNC(type, name, scope)             \

src/basic/selinux-util.c

@@ -29,7 +29,7 @@
 #include "time-util.h"
 
 #if HAVE_SELINUX
-DEFINE_TRIVIAL_CLEANUP_FUNC(context_t, context_free);
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(context_t, context_free, NULL);
 #define _cleanup_context_free_ _cleanup_(context_freep)
 
 static int mac_selinux_reload(int seqno);

src/basic/selinux-util.h

@@ -11,7 +11,7 @@
 #if HAVE_SELINUX
 #include <selinux/selinux.h>
 
-DEFINE_TRIVIAL_CLEANUP_FUNC(char*, freecon);
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(char*, freecon, NULL);
 #define _cleanup_freecon_ _cleanup_(freeconp)
 #endif