pack-objects: move read mutex to packing_data struct

ac77d0c37 ("pack-objects: shrink size field in struct object_entry",
2018-04-14) added an extra usage of read_lock/read_unlock in the newly
introduced oe_get_size_slow for thread safety in parallel calls to
try_delta(). Unfortunately oe_get_size_slow is also used in serial
code, some of which is called before the first invocation of
ll_find_deltas. As such the read mutex is not guaranteed to be
initialized.

Resolve this by moving the read mutex to packing_data and initializing
it in prepare_packing_data which is initialized in cmd_pack_objects.

Signed-off-by: Patrick Hogg <phogg@novamoon.net>
Reviewed-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -1954,9 +1954,8 @@ static int delta_cacheable(unsigned long src_size, unsigned long trg_size,
 }
 
 /* Protect access to object database */
-static pthread_mutex_t read_mutex;
-#define read_lock()		pthread_mutex_lock(&read_mutex)
-#define read_unlock()		pthread_mutex_unlock(&read_mutex)
+#define read_lock()		packing_data_read_lock(&to_pack)
+#define read_unlock()		packing_data_read_unlock(&to_pack)
 
 /* Protect delta_cache_size */
 static pthread_mutex_t cache_mutex;
@@ -2381,7 +2380,6 @@ static pthread_cond_t progress_cond;
  */
 static void init_threaded_search(void)
 {
-	init_recursive_mutex(&read_mutex);
 	pthread_mutex_init(&cache_mutex, NULL);
 	pthread_mutex_init(&progress_mutex, NULL);
 	pthread_cond_init(&progress_cond, NULL);
@@ -2392,7 +2390,6 @@ static void cleanup_threaded_search(void)
 {
 	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);
 	pthread_mutex_destroy(&progress_mutex);
 }

pack-objects.c

@@ -150,6 +150,7 @@ void prepare_packing_data(struct packing_data *pdata)
 						   1UL << OE_DELTA_SIZE_BITS);
 #ifndef NO_PTHREADS
 	pthread_mutex_init(&pdata->lock, NULL);
+	init_recursive_mutex(&pdata->read_lock);
 #endif
 }
 

pack-objects.h

@@ -146,6 +146,7 @@ struct packing_data {
 	struct packed_git **in_pack;
 
 	pthread_mutex_t lock;
+	pthread_mutex_t read_lock;
 
 	/*
 	 * This list contains entries for bases which we know the other side
@@ -174,6 +175,15 @@ static inline void packing_data_unlock(struct packing_data *pdata)
 	pthread_mutex_unlock(&pdata->lock);
 }
 
+static inline void packing_data_read_lock(struct packing_data *pdata)
+{
+	pthread_mutex_lock(&pdata->read_lock);
+}
+static inline void packing_data_read_unlock(struct packing_data *pdata)
+{
+	pthread_mutex_unlock(&pdata->read_lock);
+}
+
 struct object_entry *packlist_alloc(struct packing_data *pdata,
 				    const unsigned char *sha1,
 				    uint32_t index_pos);