explicitly check the superblock for differences

When generating incrementals for the system and vendor partitions,
check the first block (which contains the superblock) of the partition
to see if it's what we expect.  If this check fails, give an explicit
log message about the partition having been remounted R/W (the most
likely explanation) and the need to flash to get OTAs working again.

Bug: 17393999
Change-Id: Ifd2132b428dbc4907527291712690204a3664ac0

tools/releasetools/common.py

@@ -30,6 +30,7 @@ import time
 import zipfile
 
 import blockimgdiff
+from rangelib import *
 
 try:
   from hashlib import sha1 as sha1
@@ -1023,10 +1024,11 @@ def ComputeDifferences(diffs):
 
 
 class BlockDifference:
-  def __init__(self, partition, tgt, src=None):
+  def __init__(self, partition, tgt, src=None, check_first_block=False):
     self.tgt = tgt
     self.src = src
     self.partition = partition
+    self.check_first_block = check_first_block
 
     b = blockimgdiff.BlockImageDiff(tgt, src, threads=OPTIONS.worker_threads)
     tmpdir = tempfile.mkdtemp()
@@ -1043,6 +1045,9 @@ class BlockDifference:
       self._WriteUpdate(script, output_zip)
 
     else:
+      if self.check_first_block:
+        self._CheckFirstBlock(script)
+
       script.AppendExtra('if range_sha1("%s", "%s") == "%s" then' %
                          (self.device, self.src.care_map.to_string_raw(),
                           self.src.TotalSha1()))
@@ -1072,6 +1077,18 @@ class BlockDifference:
             (self.device, partition, partition, partition))
     script.AppendExtra(script._WordWrap(call))
 
+  def _CheckFirstBlock(self, script):
+    r = RangeSet((0, 1))
+    h = sha1()
+    for data in self.src.ReadRangeSet(r):
+      h.update(data)
+    h = h.hexdigest()
+
+    script.AppendExtra(('(range_sha1("%s", "%s") == "%s") || '
+                        'abort("%s has been remounted R/W; '
+                        'reflash device to reenable OTA updates");')
+                       % (self.device, r.to_string_raw(), h, self.device))
+
 
 DataImage = blockimgdiff.DataImage