Improve ABI backward & forward compatibillity

 - Padd the compressor config union
    - 128 bytes aught to be enough for everyone, i.e. future compressors.
    - Insist that the padding space is initialized to 0. If a field gets
      added to an existing compressor, it can test for 0 as a sentinel
      value.
 - Add a size field to the hook structure, aka "the Microsoft way".
    - The explanation is in the comment.
    - Don't make the Microsoft mistake of checking for >=, insist on *exact*
      size match. Future users will need a fallback if their hooks are
      rejected. But at least they will be rejected instead of silently not
      being used.
 - Add an unsupported flag check to the dir tree reader.
 - Add a basic abi unit test that, for now, checks the size of the compressor
   config struct fields.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

1 files changed, 25 insertions, 0 deletions

diff --git a/tests/abi.c b/tests/abi.c
new file mode 100644
index 0000000..1601e0a
--- /dev/null
+++ b/tests/abi.c
@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-3.0-or-later */
+/*
+ * abi.c
+ *
+ * Copyright (C) 2019 David Oberhollenzer <goliath@infraroot.at>
+ */
+#include "config.h"
+
+#include "sqfs/compressor.h"
+
+#include <assert.h>
+#include <stdlib.h>
+
+int main(void)
+{
+	sqfs_compressor_config_t cfg;
+
+	assert(sizeof(cfg.opt.gzip) == sizeof(cfg.opt));
+	assert(sizeof(cfg.opt.zstd) == sizeof(cfg.opt));
+	assert(sizeof(cfg.opt.lzo) == sizeof(cfg.opt));
+	assert(sizeof(cfg.opt.xz) == sizeof(cfg.opt));
+	assert(sizeof(cfg.opt.padd0) == sizeof(cfg.opt));
+
+	return EXIT_SUCCESS;
+}