zodbdump: Retrieve transaction metadata as raw from ZODB if storage support is available

This patch test at runtime whether used storage can provide transaction
metadata in raw form and if so used thi form directly without going
through long fragile way of stable pickling extension back to bytes.

Corresponding ZODB support is at

https://github.com/zopefoundation/ZODB/pull/183

zodbtools/zodbdump.py

@@ -46,8 +46,8 @@ Dump format:
 quote:      quote string with " with non-printable and control characters \-escaped
 hashfunc:   one of sha1, sha256, sha512 ...
 
-(*) On best-effort basis as it is not generally possible to obtain transaction
-    metadata in raw form.
+(*) It is possible to obtain transaction metadata in raw form only in recent ZODB.
+    See https://github.com/zopefoundation/ZODB/pull/183 for details.
 
 TODO also protect txn record by hash.
 """
@@ -62,6 +62,27 @@ from zodbpickle.slowpickle import Pickler as pyPickler
 import sys
 import logging
 
+# txn_raw_extension returns raw extension from txn metadata
+def txn_raw_extension(stor, txn):
+    # if txn provides ZODB.interfaces.IStorageTransactionInformationRaw - use it directly
+    raw_extension = getattr(txn, "extension_bytes", None)
+    if raw_extension is not None:
+        return raw_extension
+
+    # otherwise do best effort to generate raw_extension from txn.extension
+    # in a rational way
+    stor_name = "(%s, %s)" % (type(stor).__name__, stor.getName())
+    if stor_name not in _already_warned_notxnraw:
+        logging.warn("%s: storage does not provide IStorageTransactionInformationRaw ...", stor_name)
+        logging.warn("... will do best-effort to dump pickles in stable order but this cannot be done 100% correctly")
+        logging.warn("... please upgrade your ZODB & storage: see https://github.com/zopefoundation/ZODB/pull/183 for details.")
+        _already_warned_notxnraw.add(stor_name)
+
+    return serializeext(txn.extension)
+
+# set of storage names already warned for not providing IStorageTransactionInformationRaw
+_already_warned_notxnraw = set()
+
 # zodbdump dumps content of a ZODB storage to a file.
 # please see module doc-string for dump format and details
 def zodbdump(stor, tidmin, tidmax, hashonly=False, out=sys.stdout):
@@ -79,7 +100,7 @@ def zodbdump(stor, tidmin, tidmax, hashonly=False, out=sys.stdout):
             vskip, ashex(txn.tid), escapeqq(txn.status),
             escapeqq(txn.user),
             escapeqq(txn.description),
-            escapeqq(serializeext(txn.extension)) ))
+            escapeqq(txn_raw_extension(stor, txn)) ))
 
         objv = txnobjv(txn)