fast-export: Fix output order of D/F changes

The fast-import stream format requires incremental changes which take place
immediately, meaning that for D->F conversions all files below the relevant
directory must be deleted before the resulting file of the same name is
created.  Reversing the order can result in fast-import silently deleting
the file right after creating it, resulting in the file missing from the
resulting repository.

We correct this by first sorting the diff_queue_struct in depth-first
order.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fast-export.c

@@ -147,10 +147,39 @@ static void handle_object(const unsigned char *sha1)
 	free(buf);
 }
 
+static int depth_first(const void *a_, const void *b_)
+{
+	const struct diff_filepair *a = *((const struct diff_filepair **)a_);
+	const struct diff_filepair *b = *((const struct diff_filepair **)b_);
+	const char *name_a, *name_b;
+	int len_a, len_b, len;
+	int cmp;
+
+	name_a = a->one ? a->one->path : a->two->path;
+	name_b = b->one ? b->one->path : b->two->path;
+
+	len_a = strlen(name_a);
+	len_b = strlen(name_b);
+	len = (len_a < len_b) ? len_a : len_b;
+
+	/* strcmp will sort 'd' before 'd/e', we want 'd/e' before 'd' */
+	cmp = memcmp(name_a, name_b, len);
+	if (cmp)
+		return cmp;
+	return (len_b - len_a);
+}
+
 static void show_filemodify(struct diff_queue_struct *q,
 			    struct diff_options *options, void *data)
 {
 	int i;
+
+	/*
+	 * Handle files below a directory first, in case they are all deleted
+	 * and the directory changes to a file or symlink.
+	 */
+	qsort(q->queue, q->nr, sizeof(q->queue[0]), depth_first);
+
 	for (i = 0; i < q->nr; i++) {
 		struct diff_filespec *ospec = q->queue[i]->one;
 		struct diff_filespec *spec = q->queue[i]->two;

t/t9350-fast-export.sh

@@ -376,7 +376,7 @@ test_expect_success 'tree_tag-obj'    'git fast-export tree_tag-obj'
 test_expect_success 'tag-obj_tag'     'git fast-export tag-obj_tag'
 test_expect_success 'tag-obj_tag-obj' 'git fast-export tag-obj_tag-obj'
 
-test_expect_failure 'directory becomes symlink'        '
+test_expect_success 'directory becomes symlink'        '
 	git init dirtosymlink &&
 	git init result &&
 	(