bigfile/zodb: Simple test to verify how ZBlk format changes at runtime

We check all pairs of possible formats and for every pair write some
data at file 0 block, check that it is in source format; then change
default format, write another data, check that file.blktab[0] changed
its type.

I.e. with this test we verify that the we can read data in old format
and change it incrementally to new. Or the other way, if data is in new
format, and we decide to migrate to old one, it also works.

/cc @Tyagov, @klaus

bigfile/tests/test_filezodb.py

@@ -15,8 +15,8 @@
 # warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 #
 # See COPYING file for full licensing terms.
-from wendelin.bigfile.file_zodb import LivePersistent, ZBigFile
-from wendelin.bigfile import ram_reclaim
+from wendelin.bigfile.file_zodb import LivePersistent, ZBigFile, ZBlk_fmt_registry
+from wendelin.bigfile import file_zodb, ram_reclaim
 from wendelin.lib.zodb import dbclose
 from wendelin.lib.testing import getTestDB
 from persistent import UPTODATE, GHOST, CHANGED
@@ -26,6 +26,8 @@ from ZODB.POSException import ConflictError
 from numpy import ndarray, array_equal, uint8, zeros
 from threading import Thread
 from six.moves import _thread
+from six import b
+import struct
 import weakref
 import gc
 
@@ -721,3 +723,41 @@ def test_bigfile_filezodb_fileh_gc():
 
     del vma2, fh2, f2
     dbclose(root2)
+
+
+# verify how zblk format change works
+def test_bigfile_filezodb_fmt_change():
+    root = dbopen()
+    root['zfile5'] = f = ZBigFile(blksize)
+    transaction.commit()
+
+    fh  = f.fileh_open()    # TODO + ram
+    vma = fh.mmap(0, blen)
+
+    # save/restore original ZBlk_fmt_write
+    fmt_write_save = file_zodb.ZBlk_fmt_write
+
+    try:
+        # check all combinations of format pairs via working with blk #0 and
+        # checking internal f structure
+        for src_fmt, src_type in ZBlk_fmt_registry.items():
+            for dst_fmt, dst_type in ZBlk_fmt_registry.items():
+                if src_fmt == dst_fmt:
+                    continue    # skip checking e.g. ZBlk0 -> ZBlk0
+
+                file_zodb.ZBlk_fmt_write = src_fmt
+                struct.pack_into('p', vma, 0, b(src_fmt))
+                transaction.commit()
+
+                assert type(f.blktab[0]) is src_type
+
+                file_zodb.ZBlk_fmt_write = dst_fmt
+                struct.pack_into('p', vma, 0, b(dst_fmt))
+                transaction.commit()
+
+                assert type(f.blktab[0]) is dst_type
+
+    finally:
+        file_zodb.ZBlk_fmt_write = fmt_write_save
+
+    dbclose(root)