Fix for Collector issue #671: PData chains are of equal length, except the

*first* entry (and not the last one as assumed before).

- Adjust test to create a big file that will have a Pdata entry of smaller
  size

- Fix pdata_map lookups with first-entry-size in mind.

lib/python/OFS/Image.py

@@ -12,7 +12,7 @@
 ##############################################################################
 """Image object"""
 
-__version__='$Revision: 1.142 $'[11:-2]
+__version__='$Revision: 1.143 $'[11:-2]
 
 import Globals, struct
 from OFS.content_types import guess_content_type
@@ -323,7 +323,13 @@ class File(Persistent, Implicit, PropertyManager,
                             # calculations allow us to fast-forward through the
                             # Pdata chain without a lot of dereferencing if we
                             # did the work already.
-                            closest_pos = start - (start % (1<<16))
+                            first_size = len(pdata_map[0].data)
+                            if start < first_size:
+                                closest_pos = 0
+                            else:
+                                closest_pos = (
+                                    ((start - first_size) >> 16 << 16) +
+                                    first_size)
                             pos = min(closest_pos, max(pdata_map.keys()))
                             data = pdata_map[pos]
 

lib/python/OFS/tests/testRanges.py

@@ -33,7 +33,8 @@ def makeConnection():
 def createBigFile():
     # Create a file that is several 1<<16 blocks of data big, to force the
     # use of chained Pdata objects.
-    size = (1<<16) * 5
+    # Make sure we create a file that isn't of x * 1<<16 length! Coll #671
+    size = (1<<16) * 5 + 12345
     file = cStringIO.StringIO()
 
     def addLetter(x, add=file.write, l=string.letters,