fixup! X bigfile/_file_zodb: Fix logic around ZSync usage

Don't use regular mutex to protect _zsyncReg updates as this can
deadlock because one of _zsyncReg mutators (on_zconn_dealloc) is invoked
by automatic GC that can be triggered any time.

bigfile/_file_zodb.pyx

@@ -58,7 +58,6 @@ from cpython cimport PyCapsule_New
 from ZODB.Connection import Connection as ZConnection
 from ZODB.utils import u64
 from wendelin.lib.zodb import zconn_at
-from golang import sync as pysync
 import weakref
 
 
@@ -159,8 +158,7 @@ cdef wcfs.PyConn pywconnOf(zconn):
 # back into DB pool).
 #
 # ZSync cares itself to stay alive as long as zconn stays alive.
-_zsyncRegMu = pysync.Mutex()
-_zsyncReg   = {} # id(zsync) -> zsync
+_zsyncReg   = {} # id(zsync) -> zsync   (protected by GIL)
 class ZSync:
     # .zconn_ref    weakref[zodb.Connection]
     # .wconn        (py) wcfs.Connection
@@ -178,13 +176,19 @@ class ZSync:
         zconn.onResyncCallback(zsync)
 
         # keep zsync in _zsyncReg for zsync to stay alive independently of the caller.
-        with _zsyncRegMu:
+        #
+        # NOTE we cannot use regular mutex to protect _zsyncReg updates because
+        #      the other _zsyncReg mutator (on_zconn_dealloc) is invoked by
+        #      automatic GC that can be triggered any time.
+        #
+        #      on CPython dict updates are "atomic" - they happen without releasing GIL.
+        if 1: # = `with gil:`  (GIL already held in python code)
             _zsyncReg[id(zsync)] = zsync
 
     # .zconn dealloc -> wconn.close; release zsync
     def on_zconn_dealloc(zsync, _):
         zsync.wconn.close()
-        with _zsyncRegMu:
+        if 1: # = `with gil:`  (see note in __init__)
             del _zsyncReg[id(zsync)]
 
     # DB resyncs .zconn onto new database view.