shallow: add setup_temporary_shallow()

This function is like setup_alternate_shallow() except that it does
not lock $GIT_DIR/shallow.  It is supposed to be used when a program
generates temporary shallow for use by another program, then throw
the shallow file away.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -201,6 +201,7 @@ extern void set_alternate_shallow_file(const char *path);
 extern int write_shallow_commits(struct strbuf *out, int use_pack_protocol);
 extern void setup_alternate_shallow(struct lock_file *shallow_lock,
 				    const char **alternate_shallow_file);
+extern char *setup_temporary_shallow(void);
 
 int is_descendant_of(struct commit *, struct commit_list *);
 int in_merge_bases(struct commit *, struct commit *);

shallow.c

@@ -175,6 +175,29 @@ int write_shallow_commits(struct strbuf *out, int use_pack_protocol)
 	return data.count;
 }
 
+char *setup_temporary_shallow(void)
+{
+	struct strbuf sb = STRBUF_INIT;
+	int fd;
+
+	if (write_shallow_commits(&sb, 0)) {
+		struct strbuf path = STRBUF_INIT;
+		strbuf_addstr(&path, git_path("shallow_XXXXXX"));
+		fd = xmkstemp(path.buf);
+		if (write_in_full(fd, sb.buf, sb.len) != sb.len)
+			die_errno("failed to write to %s",
+				  path.buf);
+		close(fd);
+		strbuf_release(&sb);
+		return strbuf_detach(&path, NULL);
+	}
+	/*
+	 * is_repository_shallow() sees empty string as "no shallow
+	 * file".
+	 */
+	return xstrdup("");
+}
+
 void setup_alternate_shallow(struct lock_file *shallow_lock,
 			     const char **alternate_shallow_file)
 {