t-reftable-stack: use Git's tempfile API instead of mkstemp()

Git's tempfile API defined by $GIT_DIR/tempfile.{c,h} provides
a unified interface for tempfile operations. Since reftable/stack.c
uses this API for all its tempfile needs instead of raw functions
like mkstemp(), make the ported stack test strictly use Git's
tempfile API as well.

A bigger benefit is the fact that we know to clean up the tempfile
in case the test fails because it gets registered and pruned via a
signal handler.

Mentored-by: Patrick Steinhardt <[email protected]>
Mentored-by: Christian Couder <[email protected]>
Signed-off-by: Chandra Pratap <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Chandra Pratap

t/unit-tests/t-reftable-stack.c

@@ -76,7 +76,8 @@ static char *get_tmp_dir(int linenumber)
 static void t_read_file(void)
 {
 	char *fn = get_tmp_template(__LINE__);
-	int fd = mkstemp(fn);
+	struct tempfile *tmp = mks_tempfile(fn);
+	int fd = get_tempfile_fd(tmp);
 	char out[1024] = "line1\n\nline2\nline3";
 	int n, err;
 	char **names = NULL;
@@ -95,6 +96,7 @@ static void t_read_file(void)
 		check_str(want[i], names[i]);
 	free_names(names);
 	(void) remove(fn);
+	delete_tempfile(&tmp);
 }
 
 static int write_test_ref(struct reftable_writer *wr, void *arg)