tag: try not to reallocate tag.items in every add() call

If many tag_items are added at once while the tag cache is being
loaded, manage these items in a static fixed list, instead of
reallocating the list with every newly created item.  This reduces
heap fragmentation.

Massif results again:

 mk before:  total 12,837,632; useful 10,626,383; extra 2,211,249
 mk now:     total 12,736,720; useful 10,626,383; extra 2,110,337

The "useful" value is the same since this patch only changes the way
we allocate the same amount of memory, but heap fragmentation was
reduced by 5%.

1 files changed, 15 insertions, 1 deletions

diff --git a/src/tag.h b/src/tag.h
index 21dbfa48..11a12308 100644
--- a/src/tag.h
+++ b/src/tag.h
@@ -61,8 +61,22 @@ void tag_clear_items_by_type(struct tag *tag, enum tag_type itemType);
 
 void tag_free(struct tag *tag);
 
+/**
+ * Gives an optional hint to the tag library that we will now add
+ * several tag items; this is used by the library to optimize memory
+ * allocation.  Only one tag may be in this state, and this tag must
+ * not have any items yet.  You must call tag_end_add() when you are
+ * done.
+ */
+void tag_begin_add(struct tag *tag);
+
+/**
+ * Finishes the operation started with tag_begin_add().
+ */
+void tag_end_add(struct tag *tag);
+
 void tag_add_item_n(struct tag *tag, enum tag_type itemType,
-			    const char *value, size_t len);
+		    const char *value, size_t len);
 
 static inline void tag_add_item(struct tag *tag, enum tag_type itemType,
 				const char *value)