bigfile/file.h: C interface for defining BigFile backends

Users can inherit from BigFile and provide custom ->loadblk() and
->storeblk() to load/store file blocks from a database or some other
storage. The system then could use such files to memory map them into
user address space (see next patch).

include/wendelin/bigfile/file.h

+#ifndef _WENDELIN_BIGFILE_FILE_H_
+#define _WENDELIN_BIGFILE_FILE_H_
+
+/* Wendelin.bigfile | Base file class
+ * Copyright (C) 2014-2015  Nexedi SA and Contributors.
+ *                          Kirill Smelkov <kirr@nexedi.com>
+ *
+ * This program is free software: you can Use, Study, Modify and Redistribute
+ * it under the terms of the GNU General Public License version 3, or (at your
+ * option) any later version, as published by the Free Software Foundation.
+ *
+ * You can also Link and Combine this program with other software covered by
+ * the terms of any of the Open Source Initiative approved licenses and Convey
+ * the resulting work. Corresponding source of such a combination shall include
+ * the source code for all other software used.
+ *
+ * This program is distributed WITHOUT ANY WARRANTY; without even the implied
+ * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ *
+ * See COPYING file for full licensing terms.
+ *
+ * ~~~~~~~~
+ *
+ * TODO description
+ *
+ * Clients usually work with bigfiles via mapping files to memory -
+ * see wendelin/bigfile/virtmem.h and BigFileH for details. XXX
+ */
+
+#include <stddef.h>
+#include <wendelin/bigfile/types.h>
+
+
+/* BigFile base class
+ *
+ * BigFile is a file of fixed size blocks. It knows how to load/store blocks
+ * to/from memory. Nothing else.
+ *
+ * Concrete file implementations subclass BigFile and define their file_ops.
+ */
+struct BigFile {
+    const struct bigfile_ops *file_ops;
+
+    size_t blksize;     /* size of block */
+};
+typedef struct BigFile BigFile;
+
+struct bigfile_ops {
+    /* load/store file block to/from memory.
+     * NOTE len(buf) must be = file->blksize
+     *
+     * @return  0 - ok      !0 - fail
+     */
+    // XXX loadblk  - called from SIGSEGV context;
+    //     storeblk - called from usual context
+    //                (in the future maybe in SIGSEGV context too?)
+    //
+    // TODO maybe also/instead use ram-fd based interface and then
+    // loadblk/storeblk would use splice(2) into/from that?
+    int  (*loadblk)  (BigFile *file, blk_t blk, void *buf);
+    int  (*storeblk) (BigFile *file, blk_t blk, const void *buf);
+
+    /* release file */
+    void (*release)  (BigFile *file);
+};
+
+
+#endif