fixup! ZBigFile: Add ZBlk format option 'h' (heuristic) (3)

Use type objects instead of strings to refer to formats because with
strings scattered here and there it is very easy to cause misbehaviour
due to e.g. a typo. With referering to objects by their name, a type is
automatically caught either at runtime, or statically via e.g. pyflakes.

bigfile/file_zodb.py

@@ -481,15 +481,19 @@ class ZBlk1(ZBlkBase):
 # backward compatibility (early versions wrote ZBlk0 named as ZBlk)
 ZBlk = ZBlk0
 
+# _ZBlk_auto indicates to heuristically select ZBlk format
+_ZBlk_auto = object()
+
 # format-name -> blk format type
 ZBlk_fmt_registry = {
     'ZBlk0':    ZBlk0,
     'ZBlk1':    ZBlk1,
+    'auto':     _ZBlk_auto,
 }
 
 # format for updated blocks
 ZBlk_fmt_write = os.environ.get('WENDELIN_CORE_ZBLK_FMT', 'auto')
-if ZBlk_fmt_write != "auto" and ZBlk_fmt_write not in ZBlk_fmt_registry:
+if ZBlk_fmt_write not in ZBlk_fmt_registry:
     raise RuntimeError('E: Unknown ZBlk format %r' % ZBlk_fmt_write)
 
 
@@ -551,13 +555,12 @@ class ZBigFile(LivePersistent):
     # store data    dirty page -> ZODB obj
     def storeblk(self, blk, buf):
         zblk = self.blktab.get(blk)
-        zblk_fmt = ZBlk_fmt_write
-        if zblk_fmt == "auto":  # apply heuristic
-            zblk_fmt = self._zblk_fmt_heuristic(zblk, blk, buf)
-        self._setzblk(blk, zblk, buf, zblk_fmt)
+        zblk_type_write = ZBlk_fmt_registry[ZBlk_fmt_write]
+        if zblk_type_write is _ZBlk_auto:  # apply heuristic
+            zblk_type_write = self._zblk_fmt_heuristic(zblk, blk, buf)
+        self._setzblk(blk, zblk, buf, zblk_type_write)
 
-    def _setzblk(self, blk, zblk, buf, zblk_fmt):  # helper
-        zblk_type_write = ZBlk_fmt_registry[zblk_fmt or ZBlk_fmt_write]
+    def _setzblk(self, blk, zblk, buf, zblk_type_write):  # helper
         # if zblk was absent or of different type - we (re-)create it anew
         if zblk is None  or \
            type(zblk) is not zblk_type_write:
@@ -590,14 +593,14 @@ class ZBigFile(LivePersistent):
                 previous_blk = blk - 1
                 previous_zblk = self.blktab.get(previous_blk)
                 if previous_zblk is not None:
-                    self._setzblk(previous_blk, previous_zblk, previous_zblk.loadblkdata(), "ZBlk0")
-            return "ZBlk1"
+                    self._setzblk(previous_blk, previous_zblk, previous_zblk.loadblkdata(), ZBlk0)
+            return ZBlk1
         else:  # it's changing
             # kirr: "to support sporadic small changes over initial big fillup [...]
             # we could introduce e.g. a ZBlkδ object, which would refer to base
             # underlying ZBlk object and add "patch" information on top of that [...]."
             # See https://lab.nexedi.com/nexedi/wendelin.core/merge_requests/20#note_196084
-            return 'ZBlk1'
+            return ZBlk1
 
 
     # invalidate data   .blktab[blk] invalidated -> invalidate page

bigfile/tests/test_filezodb.py

@@ -17,7 +17,7 @@
 #
 # See COPYING file for full licensing terms.
 # See https://www.nexedi.com/licensing for rationale and options.
-from wendelin.bigfile.file_zodb import ZBigFile, ZBlk_fmt_registry
+from wendelin.bigfile.file_zodb import ZBigFile, ZBlk_fmt_registry, _ZBlk_auto
 from wendelin.bigfile import file_zodb, ram_reclaim
 from wendelin.bigfile.tests.test_thread import NotifyChannel
 from wendelin.lib.zodb import LivePersistent, dbclose
@@ -631,6 +631,8 @@ def test_bigfile_filezodb_fmt_change():
         for dst_fmt, dst_type in ZBlk_fmt_registry.items():
             if src_fmt == dst_fmt:
                 continue    # skip checking e.g. ZBlk0 -> ZBlk0
+            if src_type is _ZBlk_auto   or  dst_type is _ZBlk_auto:
+                continue    # skip checking e.g. * -> auto
 
             file_zodb.ZBlk_fmt_write = src_fmt
             struct.pack_into('p', vma, 0, b(src_fmt))