list-objects: convert name_path to a strbuf

The "struct name_path" data is examined in only two places:
we generate it in process_tree(), and we convert it to a
single string in path_name(). Everyone else just passes it
through to those functions.

We can further note that process_tree() already keeps a
single strbuf with the leading tree path, for use with
tree_entry_interesting().

Instead of building a separate name_path linked list, let's
just use the one we already build in "base". This reduces
the amount of code (especially tricky code in path_name()
which did not check for integer overflows caused by deep
or large pathnames).

It is also more efficient in some instances.  Any time we
were using tree_entry_interesting, we were building up the
strbuf anyway, so this is an immediate and obvious win
there. In cases where we were not, we trade off storing
"pathname/" in a strbuf on the heap for each level of the
path, instead of two pointers and an int on the stack (with
one pointer into the tree object). On a 64-bit system, the
latter is 20 bytes; so if path components are less than that
on average, this has lower peak memory usage.  In practice
it probably doesn't matter either way; we are already
holding in memory all of the tree objects leading up to each
pathname, and for normal-depth pathnames, we are only
talking about hundreds of bytes.

This patch leaves "struct name_path" as a thin wrapper
around the strbuf, to avoid disrupting callbacks. We should
fix them, but leaving it out makes this diff easier to view.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -23,26 +23,11 @@ volatile show_early_output_fn_t show_early_output;
 
 char *path_name(const struct name_path *path, const char *name)
 {
-	const struct name_path *p;
-	char *n, *m;
-	int nlen = strlen(name);
-	int len = nlen + 1;
-
-	for (p = path; p; p = p->up) {
-		if (p->elem_len)
-			len += p->elem_len + 1;
-	}
-	n = xmalloc(len);
-	m = n + len - (nlen + 1);
-	strcpy(m, name);
-	for (p = path; p; p = p->up) {
-		if (p->elem_len) {
-			m -= p->elem_len + 1;
-			memcpy(m, p->elem, p->elem_len);
-			m[p->elem_len] = '/';
-		}
-	}
-	return n;
+	struct strbuf ret = STRBUF_INIT;
+	if (path)
+		strbuf_addbuf(&ret, path->base);
+	strbuf_addstr(&ret, name);
+	return strbuf_detach(&ret, NULL);
 }
 
 void show_object_with_name(FILE *out, struct object *obj,