Implement a more explicit object system

Make every dynamically allocated, opaque data structure inherit from
a common sqfs_object_t structure with common entry points (e.g. destroy).

This removes tons of public API functions and replaces them with a
simple sqfs_destroy instead. If semantics of the (until now implicit)
object system need to be extended, it can be much more conveniantely
done this way.

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

22 files changed, 213 insertions, 157 deletions

diff --git a/lib/sqfs/block_processor/internal.h b/lib/sqfs/block_processor/internal.h
index b3d76ba..71f8fab 100644
--- a/lib/sqfs/block_processor/internal.h
+++ b/lib/sqfs/block_processor/internal.h
@@ -50,6 +50,8 @@ typedef struct sqfs_block_t {
 } sqfs_block_t;
 
 struct sqfs_block_processor_t {
+	sqfs_object_t obj;
+
 	/* synchronization primitives */
 #ifdef WITH_PTHREAD
 	pthread_mutex_t mtx;
diff --git a/lib/sqfs/block_processor/serial.c b/lib/sqfs/block_processor/serial.c
index e18b67e..469388c 100644
--- a/lib/sqfs/block_processor/serial.c
+++ b/lib/sqfs/block_processor/serial.c
@@ -7,6 +7,11 @@
 #define SQFS_BUILDING_DLL
 #include "internal.h"
 
+static void block_processor_destroy(sqfs_object_t *obj)
+{
+	block_processor_cleanup((sqfs_block_processor_t *)obj);
+}
+
 sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 						    sqfs_compressor_t *cmp,
 						    unsigned int num_workers,
@@ -27,14 +32,10 @@ sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 		return NULL;
 	}
 
+	((sqfs_object_t *)proc)->destroy = block_processor_destroy;
 	return proc;
 }
 
-void sqfs_block_processor_destroy(sqfs_block_processor_t *proc)
-{
-	block_processor_cleanup(proc);
-}
-
 int test_and_set_status(sqfs_block_processor_t *proc, int status)
 {
 	if (proc->status == 0)
diff --git a/lib/sqfs/block_processor/winpthread.c b/lib/sqfs/block_processor/winpthread.c
index 85416ab..a91f001 100644
--- a/lib/sqfs/block_processor/winpthread.c
+++ b/lib/sqfs/block_processor/winpthread.c
@@ -94,6 +94,35 @@ static THREAD_TYPE worker_proc(THREAD_ARG arg)
 }
 
 #if defined(_WIN32) || defined(__WINDOWS__)
+static void block_processor_destroy(sqfs_object_t *obj)
+{
+	sqfs_block_processor_t *proc = (sqfs_block_processor_t *)obj;
+	unsigned int i;
+
+	EnterCriticalSection(&proc->mtx);
+	proc->status = -1;
+	WakeAllConditionVariable(&proc->queue_cond);
+	LeaveCriticalSection(&proc->mtx);
+
+	for (i = 0; i < proc->num_workers; ++i) {
+		if (proc->workers[i] == NULL)
+			continue;
+
+		if (proc->workers[i]->thread != NULL) {
+			WaitForSingleObject(proc->workers[i]->thread, INFINITE);
+			CloseHandle(proc->workers[i]->thread);
+		}
+
+		if (proc->workers[i]->cmp != NULL)
+			sqfs_destroy(proc->workers[i]->cmp);
+
+		free(proc->workers[i]);
+	}
+
+	DeleteCriticalSection(&proc->mtx);
+	block_processor_cleanup(proc);
+}
+
 sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 						    sqfs_compressor_t *cmp,
 						    unsigned int num_workers,
@@ -112,6 +141,8 @@ sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 	if (proc == NULL)
 		return NULL;
 
+	((sqfs_object_t *)obj)->destroy = block_processor_destroy;
+
 	InitializeCriticalSection(&proc->mtx);
 	InitializeConditionVariable(&proc->queue_cond);
 	InitializeConditionVariable(&proc->done_cond);
@@ -142,38 +173,34 @@ sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 
 	return proc;
 fail:
-	sqfs_block_processor_destroy(proc);
+	block_processor_destroy(proc);
 	return NULL;
 }
-
-void sqfs_block_processor_destroy(sqfs_block_processor_t *proc)
+#else
+static void block_processor_destroy(sqfs_object_t *obj)
 {
+	sqfs_block_processor_t *proc = (sqfs_block_processor_t *)obj;
 	unsigned int i;
 
-	EnterCriticalSection(&proc->mtx);
+	pthread_mutex_lock(&proc->mtx);
 	proc->status = -1;
-	WakeAllConditionVariable(&proc->queue_cond);
-	LeaveCriticalSection(&proc->mtx);
+	pthread_cond_broadcast(&proc->queue_cond);
+	pthread_mutex_unlock(&proc->mtx);
 
 	for (i = 0; i < proc->num_workers; ++i) {
-		if (proc->workers[i] == NULL)
-			continue;
-
-		if (proc->workers[i]->thread != NULL) {
-			WaitForSingleObject(proc->workers[i]->thread, INFINITE);
-			CloseHandle(proc->workers[i]->thread);
-		}
-
-		if (proc->workers[i]->cmp != NULL)
-			proc->workers[i]->cmp->destroy(proc->workers[i]->cmp);
+		pthread_join(proc->workers[i]->thread, NULL);
 
+		sqfs_destroy(proc->workers[i]->cmp);
 		free(proc->workers[i]);
 	}
 
-	DeleteCriticalSection(&proc->mtx);
+	pthread_cond_destroy(&proc->done_cond);
+	pthread_cond_destroy(&proc->queue_cond);
+	pthread_mutex_destroy(&proc->mtx);
+
 	block_processor_cleanup(proc);
 }
-#else
+
 sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 						    sqfs_compressor_t *cmp,
 						    unsigned int num_workers,
@@ -194,6 +221,7 @@ sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 	if (proc == NULL)
 		return NULL;
 
+	((sqfs_object_t *)proc)->destroy = block_processor_destroy;
 	proc->mtx = (pthread_mutex_t)PTHREAD_MUTEX_INITIALIZER;
 	proc->queue_cond = (pthread_cond_t)PTHREAD_COND_INITIALIZER;
 	proc->done_cond = (pthread_cond_t)PTHREAD_COND_INITIALIZER;
@@ -246,10 +274,8 @@ fail_thread:
 fail_init:
 	for (i = 0; i < num_workers; ++i) {
 		if (proc->workers[i] != NULL) {
-			if (proc->workers[i]->cmp != NULL) {
-				proc->workers[i]->cmp->
-					destroy(proc->workers[i]->cmp);
-			}
+			if (proc->workers[i]->cmp != NULL)
+				sqfs_destroy(proc->workers[i]->cmp);
 
 			free(proc->workers[i]);
 		}
@@ -260,29 +286,6 @@ fail_init:
 	block_processor_cleanup(proc);
 	return NULL;
 }
-
-void sqfs_block_processor_destroy(sqfs_block_processor_t *proc)
-{
-	unsigned int i;
-
-	pthread_mutex_lock(&proc->mtx);
-	proc->status = -1;
-	pthread_cond_broadcast(&proc->queue_cond);
-	pthread_mutex_unlock(&proc->mtx);
-
-	for (i = 0; i < proc->num_workers; ++i) {
-		pthread_join(proc->workers[i]->thread, NULL);
-
-		proc->workers[i]->cmp->destroy(proc->workers[i]->cmp);
-		free(proc->workers[i]);
-	}
-
-	pthread_cond_destroy(&proc->done_cond);
-	pthread_cond_destroy(&proc->queue_cond);
-	pthread_mutex_destroy(&proc->mtx);
-
-	block_processor_cleanup(proc);
-}
 #endif
 
 int append_to_work_queue(sqfs_block_processor_t *proc, sqfs_block_t *block,
diff --git a/lib/sqfs/block_writer.c b/lib/sqfs/block_writer.c
index f77fcf8..63f8c61 100644
--- a/lib/sqfs/block_writer.c
+++ b/lib/sqfs/block_writer.c
@@ -26,6 +26,7 @@ typedef struct {
 } blk_info_t;
 
 struct sqfs_block_writer_t {
+	sqfs_object_t *base;
 	sqfs_file_t *file;
 
 	size_t num_blocks;
@@ -115,6 +116,12 @@ static int align_file(sqfs_block_writer_t *wr)
 	return store_block_location(wr, size, 0, 0);
 }
 
+static void block_writer_destroy(sqfs_object_t *wr)
+{
+	free(((sqfs_block_writer_t *)wr)->blocks);
+	free(wr);
+}
+
 sqfs_block_writer_t *sqfs_block_writer_create(sqfs_file_t *file,
 					      size_t devblksz, sqfs_u32 flags)
 {
@@ -127,6 +134,7 @@ sqfs_block_writer_t *sqfs_block_writer_create(sqfs_file_t *file,
 	if (wr == NULL)
 		return NULL;
 
+	((sqfs_object_t *)wr)->destroy = block_writer_destroy;
 	wr->file = file;
 	wr->devblksz = devblksz;
 	wr->max_blocks = INIT_BLOCK_COUNT;
@@ -153,12 +161,6 @@ int sqfs_block_writer_set_hooks(sqfs_block_writer_t *wr, void *user_ptr,
 	return 0;
 }
 
-void sqfs_block_writer_destroy(sqfs_block_writer_t *wr)
-{
-	free(wr->blocks);
-	free(wr);
-}
-
 int sqfs_block_writer_write(sqfs_block_writer_t *wr, sqfs_u32 size,
 			    sqfs_u32 checksum, sqfs_u32 flags,
 			    const sqfs_u8 *data, sqfs_u64 *location)
diff --git a/lib/sqfs/comp/gzip.c b/lib/sqfs/comp/gzip.c
index 7183e5a..37491bb 100644
--- a/lib/sqfs/comp/gzip.c
+++ b/lib/sqfs/comp/gzip.c
@@ -31,7 +31,7 @@ typedef struct {
 	gzip_options_t opt;
 } gzip_compressor_t;
 
-static void gzip_destroy(sqfs_compressor_t *base)
+static void gzip_destroy(sqfs_object_t *base)
 {
 	gzip_compressor_t *gzip = (gzip_compressor_t *)base;
 
@@ -264,10 +264,10 @@ sqfs_compressor_t *gzip_compressor_create(const sqfs_compressor_config_t *cfg)
 	gzip->compress = (cfg->flags & SQFS_COMP_FLAG_UNCOMPRESS) == 0;
 	gzip->block_size = cfg->block_size;
 	base->do_block = gzip_do_block;
-	base->destroy = gzip_destroy;
 	base->write_options = gzip_write_options;
 	base->read_options = gzip_read_options;
 	base->create_copy = gzip_create_copy;
+	((sqfs_object_t *)base)->destroy = gzip_destroy;
 
 	if (gzip->compress) {
 		ret = deflateInit2(&gzip->strm, cfg->opt.gzip.level,
diff --git a/lib/sqfs/comp/lz4.c b/lib/sqfs/comp/lz4.c
index dd9b6ac..7656d67 100644
--- a/lib/sqfs/comp/lz4.c
+++ b/lib/sqfs/comp/lz4.c
@@ -110,7 +110,7 @@ static sqfs_compressor_t *lz4_create_copy(sqfs_compressor_t *cmp)
 	return (sqfs_compressor_t *)lz4;
 }
 
-static void lz4_destroy(sqfs_compressor_t *base)
+static void lz4_destroy(sqfs_object_t *base)
 {
 	free(base);
 }
@@ -132,11 +132,11 @@ sqfs_compressor_t *lz4_compressor_create(const sqfs_compressor_config_t *cfg)
 
 	lz4->high_compression = (cfg->flags & SQFS_COMP_FLAG_LZ4_HC) != 0;
 
-	base->destroy = lz4_destroy;
 	base->do_block = (cfg->flags & SQFS_COMP_FLAG_UNCOMPRESS) ?
 		lz4_uncomp_block : lz4_comp_block;
 	base->write_options = lz4_write_options;
 	base->read_options = lz4_read_options;
 	base->create_copy = lz4_create_copy;
+	((sqfs_object_t *)base)->destroy = lz4_destroy;
 	return base;
 }
diff --git a/lib/sqfs/comp/lzma.c b/lib/sqfs/comp/lzma.c
index f7ab428..900cc14 100644
--- a/lib/sqfs/comp/lzma.c
+++ b/lib/sqfs/comp/lzma.c
@@ -153,7 +153,7 @@ static sqfs_compressor_t *lzma_create_copy(sqfs_compressor_t *cmp)
 	return (sqfs_compressor_t *)copy;
 }
 
-static void lzma_destroy(sqfs_compressor_t *base)
+static void lzma_destroy(sqfs_object_t *base)
 {
 	free(base);
 }
@@ -176,11 +176,11 @@ sqfs_compressor_t *lzma_compressor_create(const sqfs_compressor_config_t *cfg)
 	if (lzma->block_size < SQFS_META_BLOCK_SIZE)
 		lzma->block_size = SQFS_META_BLOCK_SIZE;
 
-	base->destroy = lzma_destroy;
 	base->do_block = (cfg->flags & SQFS_COMP_FLAG_UNCOMPRESS) ?
 		lzma_uncomp_block : lzma_comp_block;
 	base->write_options = lzma_write_options;
 	base->read_options = lzma_read_options;
 	base->create_copy = lzma_create_copy;
+	((sqfs_object_t *)base)->destroy = lzma_destroy;
 	return base;
 }
diff --git a/lib/sqfs/comp/xz.c b/lib/sqfs/comp/xz.c
index 91d48da..d612729 100644
--- a/lib/sqfs/comp/xz.c
+++ b/lib/sqfs/comp/xz.c
@@ -207,7 +207,7 @@ static sqfs_compressor_t *xz_create_copy(sqfs_compressor_t *cmp)
 	return (sqfs_compressor_t *)xz;
 }
 
-static void xz_destroy(sqfs_compressor_t *base)
+static void xz_destroy(sqfs_object_t *base)
 {
 	free(base);
 }
@@ -233,11 +233,11 @@ sqfs_compressor_t *xz_compressor_create(const sqfs_compressor_config_t *cfg)
 	xz->flags = cfg->flags;
 	xz->dict_size = cfg->opt.xz.dict_size;
 	xz->block_size = cfg->block_size;
-	base->destroy = xz_destroy;
 	base->do_block = (cfg->flags & SQFS_COMP_FLAG_UNCOMPRESS) ?
 		xz_uncomp_block : xz_comp_block;
 	base->write_options = xz_write_options;
 	base->read_options = xz_read_options;
 	base->create_copy = xz_create_copy;
+	((sqfs_object_t *)base)->destroy = xz_destroy;
 	return base;
 }
diff --git a/lib/sqfs/comp/zstd.c b/lib/sqfs/comp/zstd.c
index a850033..4bd9456 100644
--- a/lib/sqfs/comp/zstd.c
+++ b/lib/sqfs/comp/zstd.c
@@ -110,7 +110,7 @@ static sqfs_compressor_t *zstd_create_copy(sqfs_compressor_t *cmp)
 	return (sqfs_compressor_t *)zstd;
 }
 
-static void zstd_destroy(sqfs_compressor_t *base)
+static void zstd_destroy(sqfs_object_t *base)
 {
 	zstd_compressor_t *zstd = (zstd_compressor_t *)base;
 
@@ -142,11 +142,11 @@ sqfs_compressor_t *zstd_compressor_create(const sqfs_compressor_config_t *cfg)
 		return NULL;
 	}
 
-	base->destroy = zstd_destroy;
 	base->do_block = cfg->flags & SQFS_COMP_FLAG_UNCOMPRESS ?
 		zstd_uncomp_block : zstd_comp_block;
 	base->write_options = zstd_write_options;
 	base->read_options = zstd_read_options;
 	base->create_copy = zstd_create_copy;
+	((sqfs_object_t *)base)->destroy = zstd_destroy;
 	return base;
 }
diff --git a/lib/sqfs/data_reader.c b/lib/sqfs/data_reader.c
index c8886c5..8c9bb2e 100644
--- a/lib/sqfs/data_reader.c
+++ b/lib/sqfs/data_reader.c
@@ -21,6 +21,8 @@
 #include <string.h>
 
 struct sqfs_data_reader_t {
+	sqfs_object_t obj;
+
 	sqfs_frag_table_t *frag_tbl;
 	sqfs_compressor_t *cmp;
 	sqfs_file_t *file;
@@ -125,6 +127,16 @@ static int precache_fragment_block(sqfs_data_reader_t *data, size_t idx)
 			 &data->frag_blk_size, &data->frag_block);
 }
 
+static void data_reader_destroy(sqfs_object_t *obj)
+{
+	sqfs_data_reader_t *data = (sqfs_data_reader_t *)obj;
+
+	sqfs_destroy(data->frag_tbl);
+	free(data->data_block);
+	free(data->frag_block);
+	free(data);
+}
+
 sqfs_data_reader_t *sqfs_data_reader_create(sqfs_file_t *file,
 					    size_t block_size,
 					    sqfs_compressor_t *cmp)
@@ -140,6 +152,7 @@ sqfs_data_reader_t *sqfs_data_reader_create(sqfs_file_t *file,
 		return NULL;
 	}
 
+	((sqfs_object_t *)data)->destroy = data_reader_destroy;
 	data->file = file;
 	data->block_size = block_size;
 	data->cmp = cmp;
@@ -164,14 +177,6 @@ int sqfs_data_reader_load_fragment_table(sqfs_data_reader_t *data,
 	return 0;
 }
 
-void sqfs_data_reader_destroy(sqfs_data_reader_t *data)
-{
-	sqfs_frag_table_destroy(data->frag_tbl);
-	free(data->data_block);
-	free(data->frag_block);
-	free(data);
-}
-
 int sqfs_data_reader_get_block(sqfs_data_reader_t *data,
 			       const sqfs_inode_generic_t *inode,
 			       size_t index, size_t *size, sqfs_u8 **out)
diff --git a/lib/sqfs/dir_reader.c b/lib/sqfs/dir_reader.c
index 8eae5bd..39e3e57 100644
--- a/lib/sqfs/dir_reader.c
+++ b/lib/sqfs/dir_reader.c
@@ -20,6 +20,8 @@
 #include <stdlib.h>
 
 struct sqfs_dir_reader_t {
+	sqfs_object_t base;
+
 	sqfs_meta_reader_t *meta_dir;
 	sqfs_meta_reader_t *meta_inode;
 	const sqfs_super_t *super;
@@ -34,6 +36,15 @@ struct sqfs_dir_reader_t {
 	sqfs_u16 inode_offset;
 };
 
+static void dir_reader_destroy(sqfs_object_t *obj)
+{
+	sqfs_dir_reader_t *rd = (sqfs_dir_reader_t *)obj;
+
+	sqfs_destroy(rd->meta_inode);
+	sqfs_destroy(rd->meta_dir);
+	free(rd);
+}
+
 sqfs_dir_reader_t *sqfs_dir_reader_create(const sqfs_super_t *super,
 					  sqfs_compressor_t *cmp,
 					  sqfs_file_t *file)
@@ -66,22 +77,16 @@ sqfs_dir_reader_t *sqfs_dir_reader_create(const sqfs_super_t *super,
 	rd->meta_dir = sqfs_meta_reader_create(file, cmp, start, limit);
 
 	if (rd->meta_dir == NULL) {
-		sqfs_meta_reader_destroy(rd->meta_inode);
+		sqfs_destroy(rd->meta_inode);
 		free(rd);
 		return NULL;
 	}
 
+	((sqfs_object_t *)rd)->destroy = dir_reader_destroy;
 	rd->super = super;
 	return rd;
 }
 
-void sqfs_dir_reader_destroy(sqfs_dir_reader_t *rd)
-{
-	sqfs_meta_reader_destroy(rd->meta_inode);
-	sqfs_meta_reader_destroy(rd->meta_dir);
-	free(rd);
-}
-
 int sqfs_dir_reader_open_dir(sqfs_dir_reader_t *rd,
 			     const sqfs_inode_generic_t *inode)
 {
diff --git a/lib/sqfs/dir_writer.c b/lib/sqfs/dir_writer.c
index 53e5800..9dd7ad2 100644
--- a/lib/sqfs/dir_writer.c
+++ b/lib/sqfs/dir_writer.c
@@ -37,6 +37,8 @@ typedef struct index_ent_t {
 } index_ent_t;
 
 struct sqfs_dir_writer_t {
+	sqfs_object_t base;
+
 	dir_entry_t *list;
 	dir_entry_t *list_end;
 
@@ -136,6 +138,15 @@ static int add_export_table_entry(sqfs_dir_writer_t *writer,
 	return 0;
 }
 
+static void dir_writer_destroy(sqfs_object_t *obj)
+{
+	sqfs_dir_writer_t *writer = (sqfs_dir_writer_t *)obj;
+
+	writer_reset(writer);
+	free(writer->export_tbl);
+	free(writer);
+}
+
 sqfs_dir_writer_t *sqfs_dir_writer_create(sqfs_meta_writer_t *dm,
 					  sqfs_u32 flags)
 {
@@ -162,17 +173,11 @@ sqfs_dir_writer_t *sqfs_dir_writer_create(sqfs_meta_writer_t *dm,
 		       sizeof(writer->export_tbl[0]) * writer->export_tbl_max);
 	}
 
+	((sqfs_object_t *)writer)->destroy = dir_writer_destroy;
 	writer->dm = dm;
 	return writer;
 }
 
-void sqfs_dir_writer_destroy(sqfs_dir_writer_t *writer)
-{
-	writer_reset(writer);
-	free(writer->export_tbl);
-	free(writer);
-}
-
 int sqfs_dir_writer_begin(sqfs_dir_writer_t *writer, sqfs_u32 flags)
 {
 	sqfs_u32 offset;
diff --git a/lib/sqfs/frag_table.c b/lib/sqfs/frag_table.c
index 200f3d5..58851c5 100644
--- a/lib/sqfs/frag_table.c
+++ b/lib/sqfs/frag_table.c
@@ -27,6 +27,8 @@ typedef struct {
 
 
 struct sqfs_frag_table_t {
+	sqfs_object_t base;
+
 	size_t capacity;
 	size_t used;
 	sqfs_fragment_t *table;
@@ -37,6 +39,15 @@ struct sqfs_frag_table_t {
 	chunk_info_t *chunk_list;
 };
 
+static void frag_table_destroy(sqfs_object_t *obj)
+{
+	sqfs_frag_table_t *tbl = (sqfs_frag_table_t *)obj;
+
+	free(tbl->chunk_list);
+	free(tbl->table);
+	free(tbl);
+}
+
 sqfs_frag_table_t *sqfs_frag_table_create(sqfs_u32 flags)
 {
 	sqfs_frag_table_t *tbl;
@@ -48,16 +59,10 @@ sqfs_frag_table_t *sqfs_frag_table_create(sqfs_u32 flags)
 	if (tbl == NULL)
 		return NULL;
 
+	((sqfs_object_t *)tbl)->destroy = frag_table_destroy;
 	return tbl;
 }
 
-void sqfs_frag_table_destroy(sqfs_frag_table_t *tbl)
-{
-	free(tbl->chunk_list);
-	free(tbl->table);
-	free(tbl);
-}
-
 int sqfs_frag_table_read(sqfs_frag_table_t *tbl, sqfs_file_t *file,
 			 const sqfs_super_t *super, sqfs_compressor_t *cmp)
 {
diff --git a/lib/sqfs/id_table.c b/lib/sqfs/id_table.c
index 8c30964..86f2809 100644
--- a/lib/sqfs/id_table.c
+++ b/lib/sqfs/id_table.c
@@ -17,23 +17,35 @@
 #include <string.h>
 
 struct sqfs_id_table_t {
+	sqfs_object_t base;
+
 	sqfs_u32 *ids;
 	size_t num_ids;
 	size_t max_ids;
 };
 
+static void id_table_destroy(sqfs_object_t *obj)
+{
+	sqfs_id_table_t *tbl = (sqfs_id_table_t *)obj;
+
+	free(tbl->ids);
+	free(tbl);
+}
+
 sqfs_id_table_t *sqfs_id_table_create(sqfs_u32 flags)
 {
+	sqfs_id_table_t *tbl;
+
 	if (flags != 0)
 		return NULL;
 
-	return calloc(1, sizeof(sqfs_id_table_t));
-}
+	tbl = calloc(1, sizeof(sqfs_id_table_t));
 
-void sqfs_id_table_destroy(sqfs_id_table_t *tbl)
-{
-	free(tbl->ids);
-	free(tbl);
+	if (tbl != NULL) {
+		((sqfs_object_t *)tbl)->destroy = id_table_destroy;
+	}
+
+	return tbl;
 }
 
 int sqfs_id_table_id_to_index(sqfs_id_table_t *tbl, sqfs_u32 id, sqfs_u16 *out)
diff --git a/lib/sqfs/meta_reader.c b/lib/sqfs/meta_reader.c
index 19e856a..8fd7f21 100644
--- a/lib/sqfs/meta_reader.c
+++ b/lib/sqfs/meta_reader.c
@@ -18,6 +18,8 @@
 #include <string.h>
 
 struct sqfs_meta_reader_t {
+	sqfs_object_t base;
+
 	sqfs_u64 start;
 	sqfs_u64 limit;
 	size_t data_used;
@@ -44,6 +46,11 @@ struct sqfs_meta_reader_t {
 	sqfs_u8 scratch[SQFS_META_BLOCK_SIZE];
 };
 
+static void meta_reader_destroy(sqfs_object_t *m)
+{
+	free(m);
+}
+
 sqfs_meta_reader_t *sqfs_meta_reader_create(sqfs_file_t *file,
 					    sqfs_compressor_t *cmp,
 					    sqfs_u64 start, sqfs_u64 limit)
@@ -53,6 +60,7 @@ sqfs_meta_reader_t *sqfs_meta_reader_create(sqfs_file_t *file,
 	if (m == NULL)
 		return NULL;
 
+	((sqfs_object_t *)m)->destroy = meta_reader_destroy;
 	m->start = start;
 	m->limit = limit;
 	m->file = file;
@@ -60,11 +68,6 @@ sqfs_meta_reader_t *sqfs_meta_reader_create(sqfs_file_t *file,
 	return m;
 }
 
-void sqfs_meta_reader_destroy(sqfs_meta_reader_t *m)
-{
-	free(m);
-}
-
 int sqfs_meta_reader_seek(sqfs_meta_reader_t *m, sqfs_u64 block_start,
 			  size_t offset)
 {
diff --git a/lib/sqfs/meta_writer.c b/lib/sqfs/meta_writer.c
index acb2235..46a67cd 100644
--- a/lib/sqfs/meta_writer.c
+++ b/lib/sqfs/meta_writer.c
@@ -25,6 +25,8 @@ typedef struct meta_block_t {
 } meta_block_t;
 
 struct sqfs_meta_writer_t {
+	sqfs_object_t base;
+
 	/* A byte offset into the uncompressed data of the current block */
 	size_t offset;
 
@@ -53,6 +55,20 @@ static int write_block(sqfs_file_t *file, meta_block_t *outblk)
 	return file->write_at(file, off, outblk->data, count + 2);
 }
 
+static void meta_writer_destroy(sqfs_object_t *obj)
+{
+	sqfs_meta_writer_t *m = (sqfs_meta_writer_t *)obj;
+	meta_block_t *blk;
+
+	while (m->list != NULL) {
+		blk = m->list;
+		m->list = blk->next;
+		free(blk);
+	}
+
+	free(m);
+}
+
 sqfs_meta_writer_t *sqfs_meta_writer_create(sqfs_file_t *file,
 					    sqfs_compressor_t *cmp,
 					    sqfs_u32 flags)
@@ -66,25 +82,13 @@ sqfs_meta_writer_t *sqfs_meta_writer_create(sqfs_file_t *file,
 	if (m == NULL)
 		return NULL;
 
+	((sqfs_object_t *)m)->destroy = meta_writer_destroy;
 	m->cmp = cmp;
 	m->file = file;
 	m->flags = flags;
 	return m;
 }
 
-void sqfs_meta_writer_destroy(sqfs_meta_writer_t *m)
-{
-	meta_block_t *blk;
-
-	while (m->list != NULL) {
-		blk = m->list;
-		m->list = blk->next;
-		free(blk);
-	}
-
-	free(m);
-}
-
 int sqfs_meta_writer_flush(sqfs_meta_writer_t *m)
 {
 	meta_block_t *outblk;
diff --git a/lib/sqfs/read_table.c b/lib/sqfs/read_table.c
index 096ac1a..1c9c8ad 100644
--- a/lib/sqfs/read_table.c
+++ b/lib/sqfs/read_table.c
@@ -76,12 +76,12 @@ int sqfs_read_table(sqfs_file_t *file, sqfs_compressor_t *cmp,
 		table_size -= diff;
 	}
 
-	sqfs_meta_reader_destroy(m);
+	sqfs_destroy(m);
 	free(locations);
 	*out = data;
 	return 0;
 fail:
-	sqfs_meta_reader_destroy(m);
+	sqfs_destroy(m);
 fail_idx:
 	free(locations);
 fail_data:
diff --git a/lib/sqfs/unix/io_file.c b/lib/sqfs/unix/io_file.c
index f93af4d..1c181ab 100644
--- a/lib/sqfs/unix/io_file.c
+++ b/lib/sqfs/unix/io_file.c
@@ -25,7 +25,7 @@ typedef struct {
 } sqfs_file_stdio_t;
 
 
-static void stdio_destroy(sqfs_file_t *base)
+static void stdio_destroy(sqfs_object_t *base)
 {
 	sqfs_file_stdio_t *file = (sqfs_file_stdio_t *)base;
 
@@ -154,10 +154,10 @@ sqfs_file_t *sqfs_open_file(const char *filename, sqfs_u32 flags)
 
 	file->size = sb.st_size;
 
-	base->destroy = stdio_destroy;
 	base->read_at = stdio_read_at;
 	base->write_at = stdio_write_at;
 	base->get_size = stdio_get_size;
 	base->truncate = stdio_truncate;
+	((sqfs_object_t *)base)->destroy = stdio_destroy;
 	return base;
 }
diff --git a/lib/sqfs/win32/io_file.c b/lib/sqfs/win32/io_file.c
index 951a0fe..c1849a8 100644
--- a/lib/sqfs/win32/io_file.c
+++ b/lib/sqfs/win32/io_file.c
@@ -24,7 +24,7 @@ typedef struct {
 } sqfs_file_stdio_t;
 
 
-static void stdio_destroy(sqfs_file_t *base)
+static void stdio_destroy(sqfs_object_t *base)
 {
 	sqfs_file_stdio_t *file = (sqfs_file_stdio_t *)base;
 
@@ -161,10 +161,10 @@ sqfs_file_t *sqfs_open_file(const char *filename, sqfs_u32 flags)
 	}
 
 	file->size = size.QuadPart;
-	base->destroy = stdio_destroy;
 	base->read_at = stdio_read_at;
 	base->write_at = stdio_write_at;
 	base->get_size = stdio_get_size;
 	base->truncate = stdio_truncate;
+	((sqfs_object_t *)base)->destroy = stdio_destroy;
 	return base;
 }
diff --git a/lib/sqfs/write_table.c b/lib/sqfs/write_table.c
index 1931b83..b198941 100644
--- a/lib/sqfs/write_table.c
+++ b/lib/sqfs/write_table.c
@@ -74,7 +74,7 @@ int sqfs_write_table(sqfs_file_t *file, sqfs_compressor_t *cmp,
 	/* cleanup */
 	ret = 0;
 out:
-	sqfs_meta_writer_destroy(m);
+	sqfs_destroy(m);
 out_idx:
 	free(locations);
 	return ret;
diff --git a/lib/sqfs/xattr_reader.c b/lib/sqfs/xattr_reader.c
index d5b003b..97cf821 100644
--- a/lib/sqfs/xattr_reader.c
+++ b/lib/sqfs/xattr_reader.c
@@ -21,6 +21,8 @@
 #include <errno.h>
 
 struct sqfs_xattr_reader_t {
+	sqfs_object_t base;
+
 	sqfs_u64 xattr_start;
 
 	size_t num_id_blocks;
@@ -34,6 +36,20 @@ struct sqfs_xattr_reader_t {
 	sqfs_file_t *file;
 };
 
+static void xattr_reader_destroy(sqfs_object_t *obj)
+{
+	sqfs_xattr_reader_t *xr = (sqfs_xattr_reader_t *)obj;
+
+	if (xr->kvrd != NULL)
+		sqfs_destroy(xr->kvrd);
+
+	if (xr->idrd != NULL)
+		sqfs_destroy(xr->idrd);
+
+	free(xr->id_block_starts);
+	free(xr);
+}
+
 int sqfs_xattr_reader_load_locations(sqfs_xattr_reader_t *xr)
 {
 	sqfs_xattr_id_table_t idtbl;
@@ -242,18 +258,6 @@ int sqfs_xattr_reader_get_desc(sqfs_xattr_reader_t *xr, sqfs_u32 idx,
 	return 0;
 }
 
-void sqfs_xattr_reader_destroy(sqfs_xattr_reader_t *xr)
-{
-	if (xr->kvrd != NULL)
-		sqfs_meta_reader_destroy(xr->kvrd);
-
-	if (xr->idrd != NULL)
-		sqfs_meta_reader_destroy(xr->idrd);
-
-	free(xr->id_block_starts);
-	free(xr);
-}
-
 sqfs_xattr_reader_t *sqfs_xattr_reader_create(sqfs_file_t *file,
 					      sqfs_super_t *super,
 					      sqfs_compressor_t *cmp)
@@ -263,6 +267,7 @@ sqfs_xattr_reader_t *sqfs_xattr_reader_create(sqfs_file_t *file,
 	if (xr == NULL)
 		return NULL;
 
+	((sqfs_object_t *)xr)->destroy = xattr_reader_destroy;
 	xr->file = file;
 	xr->super = super;
 
@@ -286,6 +291,6 @@ sqfs_xattr_reader_t *sqfs_xattr_reader_create(sqfs_file_t *file,
 
 	return xr;
 fail:
-	sqfs_xattr_reader_destroy(xr);
+	sqfs_destroy(xr);
 	return NULL;
 }
diff --git a/lib/sqfs/xattr_writer.c b/lib/sqfs/xattr_writer.c
index 047e411..827d0c5 100644
--- a/lib/sqfs/xattr_writer.c
+++ b/lib/sqfs/xattr_writer.c
@@ -99,6 +99,8 @@ typedef struct kv_block_desc_t {
 } kv_block_desc_t;
 
 struct sqfs_xattr_writer_t {
+	sqfs_object_t base;
+
 	str_table_t keys;
 	str_table_t values;
 
@@ -113,6 +115,23 @@ struct sqfs_xattr_writer_t {
 };
 
 
+static void xattr_writer_destroy(sqfs_object_t *obj)
+{
+	sqfs_xattr_writer_t *xwr = (sqfs_xattr_writer_t *)obj;
+	kv_block_desc_t *blk;
+
+	while (xwr->kv_blocks != NULL) {
+		blk = xwr->kv_blocks;
+		xwr->kv_blocks = xwr->kv_blocks->next;
+		free(blk);
+	}
+
+	free(xwr->kv_pairs);
+	str_table_cleanup(&xwr->values);
+	str_table_cleanup(&xwr->keys);
+	free(xwr);
+}
+
 sqfs_xattr_writer_t *sqfs_xattr_writer_create(void)
 {
 	sqfs_xattr_writer_t *xwr = calloc(1, sizeof(*xwr));
@@ -129,6 +148,7 @@ sqfs_xattr_writer_t *sqfs_xattr_writer_create(void)
 	if (xwr->kv_pairs == NULL)
 		goto fail_pairs;
 
+	((sqfs_object_t *)xwr)->destroy = xattr_writer_destroy;
 	return xwr;
 fail_pairs:
 	str_table_cleanup(&xwr->values);
@@ -139,22 +159,6 @@ fail_keys:
 	return NULL;
 }
 
-void sqfs_xattr_writer_destroy(sqfs_xattr_writer_t *xwr)
-{
-	kv_block_desc_t *blk;
-
-	while (xwr->kv_blocks != NULL) {
-		blk = xwr->kv_blocks;
-		xwr->kv_blocks = xwr->kv_blocks->next;
-		free(blk);
-	}
-
-	free(xwr->kv_pairs);
-	str_table_cleanup(&xwr->values);
-	str_table_cleanup(&xwr->keys);
-	free(xwr);
-}
-
 int sqfs_xattr_writer_begin(sqfs_xattr_writer_t *xwr)
 {
 	xwr->kv_start = xwr->num_pairs;
@@ -603,6 +607,6 @@ int sqfs_xattr_writer_flush(sqfs_xattr_writer_t *xwr, sqfs_file_t *file,
 				   locations, count);
 out:
 	free(locations);
-	sqfs_meta_writer_destroy(mw);
+	sqfs_destroy(mw);
 	return err;
 }