--- a/src/lib-fs/Makefile.am	Wed Jul 06 13:15:22 2016 +0300
+++ b/src/lib-fs/Makefile.am	Tue Jul 05 17:52:00 2016 +0300
@@ -18,6 +18,7 @@
 	fs-sis.c \
 	fs-sis-common.c \
 	fs-sis-queue.c \
+	fs-wrapper.c \
 	istream-fs-file.c \
 	istream-fs-stats.c \
 	istream-metawrap.c \
@@ -28,6 +29,7 @@
 	fs-api.h \
 	fs-api-private.h \
 	fs-sis-common.h \
+	fs-wrapper.h \
 	fs-test.h \
 	istream-fs-file.h \
 	istream-fs-stats.h \

--- a/src/lib-fs/fs-api-private.h	Wed Jul 06 13:15:22 2016 +0300
+++ b/src/lib-fs/fs-api-private.h	Tue Jul 05 17:52:00 2016 +0300
@@ -2,6 +2,7 @@
 #define FS_API_PRIVATE_H
 
 #include "fs-api.h"
+#include "fs-wrapper.h"
 #include "module-context.h"
 
 #include <sys/time.h>
@@ -98,6 +99,7 @@
 	/* linked list of all files */
 	struct fs_file *prev, *next;
 
+	struct fs_file *parent; /* for wrapper filesystems */
 	struct fs *fs;
 	struct ostream *output;
 	char *path;

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/lib-fs/fs-wrapper.c	Tue Jul 05 17:52:00 2016 +0300
@@ -0,0 +1,163 @@
+/* Copyright (c) 2016 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "fs-api-private.h"
+
+struct wrapper_fs_iter {
+	struct fs_iter iter;
+	struct fs_iter *parent;
+};
+
+enum fs_properties fs_wrapper_get_properties(struct fs *fs)
+{
+	return fs_get_properties(fs->parent);
+}
+
+void fs_wrapper_file_close(struct fs_file *file)
+{
+	fs_file_close(file->parent);
+}
+
+const char *fs_wrapper_file_get_path(struct fs_file *file)
+{
+	return fs_file_path(file->parent);
+}
+
+void fs_wrapper_set_async_callback(struct fs_file *file,
+				   fs_file_async_callback_t *callback,
+				   void *context)
+{
+	fs_file_set_async_callback(file->parent, callback, context);
+}
+
+int fs_wrapper_wait_async(struct fs *fs)
+{
+	return fs_wait_async(fs->parent);
+}
+
+void fs_wrapper_set_metadata(struct fs_file *file, const char *key,
+			     const char *value)
+{
+	fs_set_metadata(file->parent, key, value);
+}
+
+int fs_wrapper_get_metadata(struct fs_file *file,
+			    const ARRAY_TYPE(fs_metadata) **metadata_r)
+{
+	return fs_get_metadata(file->parent, metadata_r);
+}
+
+bool fs_wrapper_prefetch(struct fs_file *file, uoff_t length)
+{
+	return fs_prefetch(file->parent, length);
+}
+
+ssize_t fs_wrapper_read(struct fs_file *file, void *buf, size_t size)
+{
+	return fs_read(file->parent, buf, size);
+}
+
+struct istream *
+fs_wrapper_read_stream(struct fs_file *file, size_t max_buffer_size)
+{
+	return fs_read_stream(file->parent, max_buffer_size);
+}
+
+int fs_wrapper_write(struct fs_file *file, const void *data, size_t size)
+{
+	return fs_write(file->parent, data, size);
+}
+
+void fs_wrapper_write_stream(struct fs_file *file)
+{
+	i_assert(file->output == NULL);
+
+	file->output = fs_write_stream(file->parent);
+}
+
+int fs_wrapper_write_stream_finish(struct fs_file *file, bool success)
+{
+	if (!success) {
+		fs_write_stream_abort(file->parent, &file->output);
+		return -1;
+	}
+
+	if (fs_write_stream_finish(file->parent, &file->output) < 0)
+		return -1;
+	return 1;
+}
+
+int fs_wrapper_lock(struct fs_file *file, unsigned int secs,
+		    struct fs_lock **lock_r)
+{
+	return fs_lock(file->parent, secs, lock_r);
+}
+
+void fs_wrapper_unlock(struct fs_lock *_lock ATTR_UNUSED)
+{
+	i_unreached();
+}
+
+int fs_wrapper_exists(struct fs_file *file)
+{
+	return fs_exists(file->parent);
+}
+
+int fs_wrapper_stat(struct fs_file *file, struct stat *st_r)
+{
+	return fs_stat(file->parent, st_r);
+}
+
+int fs_wrapper_copy(struct fs_file *src, struct fs_file *dest)
+{
+	if (src != NULL)
+		return fs_copy(src->parent, dest->parent);
+	else
+		return fs_copy_finish_async(dest->parent);
+}
+
+int fs_wrapper_rename(struct fs_file *src, struct fs_file *dest)
+{
+	return fs_rename(src->parent, dest->parent);
+}
+
+int fs_wrapper_delete(struct fs_file *file)
+{
+	return fs_delete(file->parent);
+}
+
+struct fs_iter *
+fs_wrapper_iter_init(struct fs *fs, const char *path,
+		     enum fs_iter_flags flags)
+{
+	struct wrapper_fs_iter *iter;
+
+	iter = i_new(struct wrapper_fs_iter, 1);
+	iter->iter.fs = fs;
+	iter->iter.flags = flags;
+	iter->parent = fs_iter_init(fs->parent, path, flags);
+	return &iter->iter;
+}
+
+const char *fs_wrapper_iter_next(struct fs_iter *_iter)
+{
+	struct wrapper_fs_iter *iter = (struct wrapper_fs_iter *)_iter;
+	const char *fname;
+
+	iter->parent->async_callback = _iter->async_callback;
+	iter->parent->async_context = _iter->async_context;
+
+	fname = fs_iter_next(iter->parent);
+	_iter->async_have_more = iter->parent->async_have_more;
+	return fname;
+}
+
+int fs_wrapper_iter_deinit(struct fs_iter *_iter)
+{
+	struct wrapper_fs_iter *iter = (struct wrapper_fs_iter *)_iter;
+	int ret;
+
+	ret = fs_iter_deinit(&iter->parent);
+	i_free(iter);
+	return ret;
+}

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/lib-fs/fs-wrapper.h	Tue Jul 05 17:52:00 2016 +0300
@@ -0,0 +1,36 @@
+#ifndef FS_WRAPPER_H
+#define FS_WRAPPER_H
+
+enum fs_properties fs_wrapper_get_properties(struct fs *fs);
+void fs_wrapper_file_close(struct fs_file *file);
+const char *fs_wrapper_file_get_path(struct fs_file *file);
+void fs_wrapper_set_async_callback(struct fs_file *file,
+				   fs_file_async_callback_t *callback,
+				   void *context);
+int fs_wrapper_wait_async(struct fs *fs);
+void fs_wrapper_set_metadata(struct fs_file *file, const char *key,
+			     const char *value);
+int fs_wrapper_get_metadata(struct fs_file *file,
+			    const ARRAY_TYPE(fs_metadata) **metadata_r);
+bool fs_wrapper_prefetch(struct fs_file *file, uoff_t length);
+ssize_t fs_wrapper_read(struct fs_file *file, void *buf, size_t size);
+struct istream *
+fs_wrapper_read_stream(struct fs_file *file, size_t max_buffer_size);
+int fs_wrapper_write(struct fs_file *file, const void *data, size_t size);
+void fs_wrapper_write_stream(struct fs_file *file);
+int fs_wrapper_write_stream_finish(struct fs_file *file, bool success);
+int fs_wrapper_lock(struct fs_file *file, unsigned int secs,
+		    struct fs_lock **lock_r);
+void fs_wrapper_unlock(struct fs_lock *_lock);
+int fs_wrapper_exists(struct fs_file *file);
+int fs_wrapper_stat(struct fs_file *file, struct stat *st_r);
+int fs_wrapper_copy(struct fs_file *src, struct fs_file *dest);
+int fs_wrapper_rename(struct fs_file *src, struct fs_file *dest);
+int fs_wrapper_delete(struct fs_file *file);
+struct fs_iter *
+fs_wrapper_iter_init(struct fs *fs, const char *path,
+		     enum fs_iter_flags flags);
+const char *fs_wrapper_iter_next(struct fs_iter *iter);
+int fs_wrapper_iter_deinit(struct fs_iter *iter);
+
+#endif