Fix deleteObject in history-preserving mode to conform to ZODB interface

[navytux]

ZODB specifies deleteObject to create new revision that indicates object
removal:

def deleteObject(oid, serial, transaction):
    """Mark an object as deleted

    This method marks an object as deleted VIA A NEW OBJECT
    REVISION.  Subsequent attempts to load current data for the
    object will fail with a POSKeyError, but loads for
    non-current data will succeed if there are previous
    non-delete records.  The object will be removed from the
    storage when all not-delete records are removed.

https://github.com/zopefoundation/ZODB/blob/bc13ca74/src/ZODB/interfaces.py#L1292-L1307
(emphasis mine)

However currently for history-preserving mode, as explained in
zopefoundation/ZODB#318 (comment),
RelStorage purges latest object revision instead of creating new one with
whiteout indication. This goes against deleteObject specification and, as
demonstrated by attached test program, against particular FileStorage
behaviour.

-> Fix it.

P.S. I'm complete RelStorage newbie and looked only briefly. It could be that
my patch is e.g. incomplete, or not optimal. However it demonstrates a real
problem, and it fixes both adjusted testcase and failure of attached tdelete.py

P.P.S. Tested only with SQLite backend.

---- 8< ---- (tdelete.py)
#!/usr/bin/env python
"""tdelete.py demonstrates that deleteObject should create new whiteout
record, and that older data records should be still accessible.

e.g. with FileStorage:

   $ ./tdelete.py file://1.db
   @03e40964a0766f33 (= 280359404597309235) obj<0000000000000001>  ->  int(0)
   @03e40964a0790944 (= 280359404597479748) obj<0000000000000001>  ->  int(1)

   --------

   @03e40964a0766f33  obj<0000000000000001>  ->  int(0)    # must be int(0)
   @03e40964a0790944  obj<0000000000000001>  ->  int(1)    # must be int(1)

However it currently fails with RelStorage, because deleteObject does not
create new whiteout revision and instead purges already committed data:

    $ rm x/*; ./tdelete.py sqlite://?data_dir=`pwd`/x
    @03e40972d5408022 (= 280359465612509218) obj<0000000000000001>  ->  int(0)
    @03e40972d541ddee (= 280359465612598766) obj<0000000000000001>  ->  int(1)

    --------

    @03e40972d5408022  obj<0000000000000001>  ->  int(0)    # must be int(0)
    Traceback (most recent call last):
      File "./tdelete.py", line 84, in <module>
        main()
      File "./tdelete.py", line 80, in main
        dumpObjAt(at1, "must be int(1)")
      File "./tdelete.py", line 75, in dumpObjAt
        obj = conn.get(oid)
      File "/home/kirr/src/wendelin/z/ZODB/src/ZODB/Connection.py", line 238, in get
        obj = self._reader.getGhost(p)
      File "/home/kirr/src/wendelin/z/ZODB/src/ZODB/serialize.py", line 598, in getGhost
        unpickler = self._get_unpickler(pickle)
      File "/home/kirr/src/wendelin/z/ZODB/src/ZODB/serialize.py", line 478, in _get_unpickler
        file = BytesIO(pickle)
    TypeError: StringIO() argument 1 must be string or buffer, not None
"""

from __future__ import print_function

import zodburi
from persistent import Persistent
from ZODB.DB import DB
from ZODB.Connection import TransactionMetaData
from ZODB.utils import u64
import transaction
import sys

class PInt(Persistent):
    def __init__(self, i):
        self.i = i
    def __str__(self):
        return "int(%d)" % self.i

def h(tid):
    return tid.encode('hex')

def dump(obj):
    print("@%s (= %d) obj<%s>  ->  %s" % (h(obj._p_serial), u64(obj._p_serial), h(obj._p_oid), obj))

def main():
    zurl = sys.argv[1]
    zstoropen, dbkw = zodburi.resolve_uri(zurl)

    stor = zstoropen()
    db = DB(stor, **dbkw)

    conn = db.open()
    root = conn.root()

    root['X'] = obj = PInt(0)
    transaction.commit()
    dump(obj)
    at0 = obj._p_serial
    oid = obj._p_oid

    obj.i += 1
    transaction.commit()
    dump(obj)
    at1 = obj._p_serial

    txn_meta = TransactionMetaData()
    stor.tpc_begin(txn_meta)
    stor.deleteObject(oid, at1, txn_meta)
    stor.tpc_vote(txn_meta)
    stor.tpc_finish(txn_meta)

    print('\n--------\n')

    def dumpObjAt(at, comment):
        conn = db.open(at=at)
        obj = conn.get(oid)
        print("@%s  obj<%s>  ->  %s\t# %s" % (h(at), h(oid), obj, comment))
        conn.close()

    dumpObjAt(at0, "must be int(0)")
    dumpObjAt(at1, "must be int(1)")

if __name__ == '__main__':
    main()

P.P.P.S. SQLite URI resolver is currently broken after 08259fa (Finer
control over sqlite storage locking, oid allocation and stats). I've used the
following local patch as a workaround:

--- a/src/relstorage/zodburi_resolver.py
+++ b/src/relstorage/zodburi_resolver.py
@@ -121,14 +121,14 @@ def factory(options):
         return factory, unused

 class SqliteAdapterHelper(Resolver):
-    _string_args = ('path',)
+    _string_args = ('data_dir',)

     def __call__(self, parsed_uri, kw):
         kw, unused = self.interpret_kwargs(kw)

         def factory(options):
             from relstorage.adapters.sqlite.adapter import Sqlite3Adapter
-            return Sqlite3Adapter(options=options, **kw)
+            return Sqlite3Adapter(options=options, pragmas={}, **kw)
         return factory, unused

 # The relstorage support is inspired by django-zodb.

[navytux]

@jamadden, all AppVeyor tests passed. This pull-request is no longer work-in-progress.