Avoid writing too much new data per transfer

Limit the number of blocks in each 'new' command to 1024. This should
decrease the chance of fsync error during an OTA update, similiar to
what we have done in b/29535618.

Bug: 29607757
Change-Id: I89f0a845d71eb810766e39860ab667c61b61a417

tools/releasetools/blockimgdiff.py

@@ -342,19 +342,20 @@ class BlockImageDiff(object):
     return ctx.hexdigest()
 
   def WriteTransfers(self, prefix):
-    def WriteTransfersZero(out, to_zero):
-      """Limit the number of blocks in command zero to 1024 blocks.
+    def WriteSplitTransfers(out, style, target_blocks):
+      """Limit the size of operand in command 'new' and 'zero' to 1024 blocks.
 
       This prevents the target size of one command from being too large; and
       might help to avoid fsync errors on some devices."""
 
-      zero_blocks_limit = 1024
+      assert (style == "new" or style == "zero")
+      blocks_limit = 1024
       total = 0
-      while to_zero:
-        zero_blocks = to_zero.first(zero_blocks_limit)
-        out.append("zero %s\n" % (zero_blocks.to_string_raw(),))
-        total += zero_blocks.size()
-        to_zero = to_zero.subtract(zero_blocks)
+      while target_blocks:
+        blocks_to_write = target_blocks.first(blocks_limit)
+        out.append("%s %s\n" % (style, blocks_to_write.to_string_raw()))
+        total += blocks_to_write.size()
+        target_blocks = target_blocks.subtract(blocks_to_write)
       return total
 
     out = []
@@ -478,7 +479,7 @@ class BlockImageDiff(object):
 
       if xf.style == "new":
         assert xf.tgt_ranges
-        out.append("%s %s\n" % (xf.style, xf.tgt_ranges.to_string_raw()))
+        assert tgt_size == WriteSplitTransfers(out, xf.style, xf.tgt_ranges)
         total += tgt_size
       elif xf.style == "move":
         assert xf.tgt_ranges
@@ -538,7 +539,7 @@ class BlockImageDiff(object):
       elif xf.style == "zero":
         assert xf.tgt_ranges
         to_zero = xf.tgt_ranges.subtract(xf.src_ranges)
-        assert WriteTransfersZero(out, to_zero) == to_zero.size()
+        assert WriteSplitTransfers(out, xf.style, to_zero) == to_zero.size()
         total += to_zero.size()
       else:
         raise ValueError("unknown transfer style '%s'\n" % xf.style)
@@ -568,7 +569,7 @@ class BlockImageDiff(object):
 
     # Zero out extended blocks as a workaround for bug 20881595.
     if self.tgt.extended:
-      assert (WriteTransfersZero(out, self.tgt.extended) ==
+      assert (WriteSplitTransfers(out, "zero", self.tgt.extended) ==
               self.tgt.extended.size())
       total += self.tgt.extended.size()