Zero out blocks that may be touched by dm-verity.

dm-verity may touch some blocks that are not in the care_map due to
block device read-ahead. It will fail if such blocks contain
non-zeroes. As a workaround, we mark them as extended blocks and
zero out explicitly to avoid dm-verity failures.

Bug: 20881595
Change-Id: I54e24e70ad822c0d6d7af43301f74d24505f4461

tools/releasetools/rangelib.py

@@ -238,6 +238,28 @@ class RangeSet(object):
         out.append(offset + p - start)
     return RangeSet(data=out)
 
+  def extend(self, n):
+    """Extend the RangeSet by 'n' blocks.
+
+    The lower bound is guaranteed to be non-negative.
+
+    >>> RangeSet("0-9").extend(1)
+    <RangeSet("0-10")>
+    >>> RangeSet("10-19").extend(15)
+    <RangeSet("0-34")>
+    >>> RangeSet("10-19 30-39").extend(4)
+    <RangeSet("6-23 26-43")>
+    >>> RangeSet("10-19 30-39").extend(10)
+    <RangeSet("0-49")>
+    """
+    out = self
+    for i in range(0, len(self.data), 2):
+      s, e = self.data[i:i+2]
+      s1 = max(0, s - n)
+      e1 = e + n
+      out = out.union(RangeSet(str(s1) + "-" + str(e1-1)))
+    return out
+
 
 if __name__ == "__main__":
   import doctest