bigfile/zodb: Apply auto format as default only in WCFS mode

This semantically reverts 99f262dd (bigfile/zodb: Make auto format the
default) for wendelin.core-1 mode because in non-WCFS mode there are
known problems with data corruption on BTree topology changes(*) and
auto mode actually does change those topologies with first setting
ZBigFile[blk] -> ZBlk1 and then updating the same block to point to
ZBlk0 object.

Avoid pressuring those problems and use auto as default only in WCFS
mode that should handle invalidations with all those BTree topology
changes well.

The patch is based on suggestion by Levin Zimmermann: nexedi/wendelin.core!20 (comment 212405)

We have to move _default_use_wcfs because now it is invoked at module
import time and needs to be already defined at the time of the call.

(*) see nexedi/wendelin.core@8c32c9f6 for details.

bigfile/file_zodb.py

@@ -91,10 +91,10 @@ on change pattern and works relatively good regarding both access speed and
 database size for append-like workloads::
 
   $WENDELIN_CORE_ZBLK_FMT
-      ZBlk0             fast reads
-      ZBlk1             small changes
-      auto  (default)   heuristically use either ZBlk0 or ZBlk1
-                        depending on change pattern
+      ZBlk0     fast reads                                  (default in !wcfs mode)
+      ZBlk1     small changes
+      auto      heuristically use either ZBlk0 or ZBlk1     (default in wcfs mode)
+                depending on change pattern
 
 Description of block formats follow:
 
@@ -491,8 +491,19 @@ ZBlk_fmt_registry = {
     'auto':     _ZBlk_auto,
 }
 
+# _default_use_wcfs returns whether default virtmem setting is to use wcfs or not.
+def _default_use_wcfs():
+    virtmem = os.environ.get("WENDELIN_CORE_VIRTMEM", "rw:uvmm")    # unset -> !wcfs
+    virtmem = virtmem.lower()
+    return {"r:wcfs+w:uvmm": True, "rw:uvmm": False}[virtmem]
+
 # format for updated blocks
-ZBlk_fmt_write = os.environ.get('WENDELIN_CORE_ZBLK_FMT', 'auto')
+ZBlk_fmt_write = os.environ.get('WENDELIN_CORE_ZBLK_FMT',
+                            # auto is used only with wcfs because !wcfs mode does
+                            # not handle BTree topology well leading to data corruption
+                            # https://lab.nexedi.com/nexedi/wendelin.core/commit/8c32c9f6
+                            'auto' if _default_use_wcfs() else
+                            'ZBlk0')
 if ZBlk_fmt_write not in ZBlk_fmt_registry:
     raise RuntimeError('E: Unknown ZBlk format %r' % ZBlk_fmt_write)
 
@@ -678,21 +689,13 @@ class ZBigFile(LivePersistent):
     # - not set -> behave according to global default
     def fileh_open(self, _use_wcfs=None):
         if _use_wcfs is None:
-            _use_wcfs = self._default_use_wcfs()
+            _use_wcfs = _default_use_wcfs()
 
         fileh = _ZBigFileH(self, _use_wcfs)
         self._v_filehset.add(fileh)
         return fileh
 
 
-    # _default_use_wcfs returns whether default virtmem setting is to use wcfs or not.
-    @staticmethod
-    def _default_use_wcfs():
-        virtmem = os.environ.get("WENDELIN_CORE_VIRTMEM", "rw:uvmm")    # unset -> !wcfs
-        virtmem = virtmem.lower()
-        return {"r:wcfs+w:uvmm": True, "rw:uvmm": False}[virtmem]
-
-
 
 # BigFileH wrapper that also acts as DataManager proxying changes ZODB <- virtmem
 # at two-phase-commit (TPC), and ZODB -> virtmem on objects invalidation.