Fix Blob bug which prevented opening of blobs with no committed data using either mode 'r+' or 'a'.

src/CHANGES.txt

@@ -22,6 +22,9 @@ Bugs Fixed
 - Objects defining _p_deactivate methods that didn't call base methods
   weren't loaded properly. https://bugs.launchpad.net/zodb/+bug/185066
 
+- Opening a blob with modes 'r+' or 'a' would fail when the blob had no
+  committed changes.
+
 3.9.0b5 (2009-08-06)
 ====================
 

src/ZODB/blob.py

@@ -169,14 +169,15 @@ class Blob(persistent.Persistent):
                 if self._p_blob_uncommitted is None:
                     self._create_uncommitted_file()
                 result = BlobFile(self._p_blob_uncommitted, mode, self)
-            else:
+            else: # 'r+' and 'a'
                 if self._p_blob_uncommitted is None:
                     # Create a new working copy
                     self._create_uncommitted_file()
                     result = BlobFile(self._p_blob_uncommitted, mode, self)
-                    utils.cp(file(self._p_blob_committed), result)
-                    if mode == 'r+':
-                        result.seek(0)
+                    if self._p_blob_committed:
+                        utils.cp(open(self._p_blob_committed), result)
+                        if mode == 'r+':
+                            result.seek(0)
                 else:
                     # Re-use existing working copy
                     result = BlobFile(self._p_blob_uncommitted, mode, self)

src/ZODB/tests/blob_basic.txt

@@ -156,6 +156,19 @@ Blobs are always opened in binary mode::
     'rb'
     >>> f9.close()
 
+Blobs that have not been committed can be opened using any mode,
+except for "c"::
+
+    >>> from ZODB.blob import BlobError, valid_modes
+    >>> for mode in valid_modes:
+    ...     try:
+    ...         f10 = Blob().open(mode)
+    ...     except BlobError:
+    ...         print 'open failed with mode "%s"' % mode
+    ...     else:
+    ...         f10.close()
+    open failed with mode "c"
+
 Some cleanup in this test is needed::
 
     >>> import transaction