writer: Don't grow children array by one each time

Each time a child is added we grow by 1, which causes a lot of
reallocs when adding children. Growing capacity by doubling saves us
about 10% of the time of a mkcomposefs --from-image run.

Signed-off-by: Alexander Larsson <[email protected]>

libcomposefs/lcfs-internal.h

@@ -140,6 +140,7 @@ struct lcfs_node_s {
 	struct lcfs_node_s *parent;
 
 	struct lcfs_node_s **children; /* Owns refs */
+	size_t children_capacity;
 	size_t children_size;
 
 	/* Used to create hard links.  */

libcomposefs/lcfs-writer.c

@@ -906,7 +906,7 @@ int lcfs_node_add_child(struct lcfs_node_s *parent, struct lcfs_node_s *child,
 			const char *name)
 {
 	struct lcfs_node_s **new_children;
-	size_t new_size;
+	size_t new_capacity;
 	char *name_copy;
 
 	if ((parent->inode.st_mode & S_IFMT) != S_IFDIR) {
@@ -936,20 +936,26 @@ int lcfs_node_add_child(struct lcfs_node_s *parent, struct lcfs_node_s *child,
 		return -1;
 	}
 
-	new_size = parent->children_size + 1;
+	if (parent->children_capacity == parent->children_size) {
+		if (parent->children_size == 0)
+			new_capacity = 16;
+		else
+			new_capacity = parent->children_capacity * 2;
 
-	new_children = reallocarray(parent->children, sizeof(*parent->children),
-				    new_size);
-	if (new_children == NULL) {
-		errno = ENOMEM;
-		free(name_copy);
-		return -1;
-	}
+		new_children = reallocarray(parent->children,
+					    sizeof(*parent->children), new_capacity);
+		if (new_children == NULL) {
+			errno = ENOMEM;
+			free(name_copy);
+			return -1;
+		}
 
-	parent->children = new_children;
+		parent->children = new_children;
+		parent->children_capacity = new_capacity;
+	}
 
 	parent->children[parent->children_size] = child;
-	parent->children_size = new_size;
+	parent->children_size += 1;
 	child->parent = parent;
 	child->name = name_copy;