Have set_try_to_free_routine return the previous routine

This effectively requires from the callers of set_try_to_free_routine to
treat the try-to-free-routines as a stack.

We will need this for the next patch where the only current caller cannot
depend on that the previously set routine was the default routine.
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-pack-objects.c

@@ -1556,6 +1556,8 @@ static void try_to_free_from_threads(size_t size)
 	read_unlock();
 }
 
+try_to_free_t old_try_to_free_routine;
+
 /*
  * The main thread waits on the condition that (at least) one of the workers
  * has stopped working (which is indicated in the .working member of
@@ -1590,12 +1592,12 @@ static void init_threaded_search(void)
 	pthread_mutex_init(&cache_mutex, NULL);
 	pthread_mutex_init(&progress_mutex, NULL);
 	pthread_cond_init(&progress_cond, NULL);
-	set_try_to_free_routine(try_to_free_from_threads);
+	old_try_to_free_routine = set_try_to_free_routine(try_to_free_from_threads);
 }
 
 static void cleanup_threaded_search(void)
 {
-	set_try_to_free_routine(NULL);
+	set_try_to_free_routine(old_try_to_free_routine);
 	pthread_cond_destroy(&progress_cond);
 	pthread_mutex_destroy(&read_mutex);
 	pthread_mutex_destroy(&cache_mutex);

git-compat-util.h

@@ -346,7 +346,8 @@ static inline char *gitstrchrnul(const char *s, int c)
 
 extern void release_pack_memory(size_t, int);
 
-extern void set_try_to_free_routine(void (*routine)(size_t));
+typedef void (*try_to_free_t)(size_t);
+extern try_to_free_t set_try_to_free_routine(try_to_free_t);
 
 extern char *xstrdup(const char *str);
 extern void *xmalloc(size_t size);

wrapper.c

@@ -10,9 +10,11 @@ static void try_to_free_builtin(size_t size)
 
 static void (*try_to_free_routine)(size_t size) = try_to_free_builtin;
 
-void set_try_to_free_routine(void (*routine)(size_t))
+try_to_free_t set_try_to_free_routine(try_to_free_t routine)
 {
-	try_to_free_routine = (routine) ? routine : try_to_free_builtin;
+	try_to_free_t old = try_to_free_routine;
+	try_to_free_routine = routine;
+	return old;
 }
 
 char *xstrdup(const char *str)