MEDIUM: proxy: add lock for global accesses during default free

This patch is similar to the previous one, but this time it deals with
functions related to defaults proxies instances. Lock PROXIES_DEL_LOCK
is used to protect accesses on global collections.

This patch will be necessary to implement dynamic backend deletion, even
if defaults won't be use as direct target of a "del backend" CLI.
However, a backend may have a reference on a default instance. When the
backend is freed, this references is released, which can in turn cause
the freeing of the default proxy instance. All of this will occur at
runtime, outside of thread isolation.

Author: a-denoyelle

src/proxy.c

@@ -2980,21 +2980,27 @@ static void defaults_px_free(struct proxy *defproxy)
 
 /* Removes <px> defaults instance from the name tree, free its content and
  * storage. This must only be used if <px> is unreferenced.
+ *
+ * Uses PROXIES_DEL_LOCK to protect global tree/list accesses.
  */
 void defaults_px_destroy(struct proxy *px)
 {
 	BUG_ON(!(px->cap & PR_CAP_DEF));
 	BUG_ON(px->conf.def_ref != 0);
 
+	HA_SPIN_LOCK(PROXIES_DEL_LOCK, &proxies_del_lock);
 	cebis_item_delete(&defproxy_by_name, conf.name_node, id, px);
 	LIST_DELETE(&px->el);
+	HA_SPIN_UNLOCK(PROXIES_DEL_LOCK, &proxies_del_lock);
 
 	defaults_px_free(px);
 	free(px);
 }
 
 /* delete all unreferenced default proxies. A default proxy is unreferenced if
  * its <def_ref> count is equal to zero.
+ *
+ * Not thread safe - currently only used during init.
  */
 void defaults_px_destroy_all_unref(void)
 {
@@ -3011,6 +3017,8 @@ void defaults_px_destroy_all_unref(void)
 /* Removes <px> defaults from the name tree. This operation is useful when a
  * section is made invisible by a newer instance with the same name. If <px> is
  * not referenced it is freed immediately, else it is kept in defaults_list.
+ *
+ * Not thread safe - currently only used during parsing.
  */
 void defaults_px_detach(struct proxy *px)
 {
@@ -3021,7 +3029,10 @@ void defaults_px_detach(struct proxy *px)
 	/* If not destroyed, <px> can still be accessed in <defaults_list>. */
 }
 
-/* Increments by one defaults proxy reference of all defaults stored in tree name. */
+/* Increments by one defaults proxy reference of all defaults stored in tree name.
+ *
+ * Not thread safe - currently only used during init.
+ */
 void defaults_px_ref_all(void)
 {
 	struct proxy *px;
@@ -3033,7 +3044,11 @@ void defaults_px_ref_all(void)
 	}
 }
 
-/* Decrements defaults proxy ref of all defaults. This is the reverse of defaults_px_ref_all(). */
+/* Decrements defaults proxy ref of all defaults. This is the reverse of
+ * defaults_px_ref_all().
+ *
+ * Not thread safe - currently only used during deinit.
+ */
 void defaults_px_unref_all(void)
 {
 	struct proxy *px, *nx;