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, 17 insertions, 2 deletions

diff --git a/include/sqfs/data_writer.h b/include/sqfs/data_writer.h
index 210a08d..4623493 100644
--- a/include/sqfs/data_writer.h
+++ b/include/sqfs/data_writer.h
@@ -59,6 +59,18 @@
  */
 struct sqfs_block_hooks_t {
 	/**
+	 * @brief Set this to the size of the struct.
+	 *
+	 * This is required for future expandabillity while maintaining ABI
+	 * compatibillity. At the current time, the implementation of
+	 * @ref sqfs_data_writer_set_hooks rejects any hook struct where this
+	 * isn't the exact size. If new hooks are added in the future, the
+	 * struct grows and the future implementation can tell by the size
+	 * whether the application uses the new version or the old one.
+	 */
+	size_t size;
+
+	/**
 	 * @brief Gets called before writing a block to disk.
 	 *
 	 * If this is not NULL, it gets called before a block is written to
@@ -283,10 +295,13 @@ int sqfs_data_writer_write_fragment_table(sqfs_data_writer_t *proc,
  * @param proc A pointer to a data writer object.
  * @param user_ptr A user pointer to pass to the callbacks.
  * @param hooks A structure containing the hooks.
+ *
+ * @return Zero on success, @ref SQFS_ERROR_UNSUPPORTED if the size field of
+ *         the hooks doesn't match any size knwon to the library.
  */
 SQFS_API
-void sqfs_data_writer_set_hooks(sqfs_data_writer_t *proc, void *user_ptr,
-				const sqfs_block_hooks_t *hooks);
+int sqfs_data_writer_set_hooks(sqfs_data_writer_t *proc, void *user_ptr,
+			       const sqfs_block_hooks_t *hooks);
 
 #ifdef __cplusplus
 }