update_verifier: Fix the wrong computation with group_range_count.

'group_range_count' doesn't properly consider the pair-wise range
structure. It may split the ranges into wrong pairs if it evaluates to
an odd number.

For example, for an input range string of "6,0,2,10,12,20,22" with 4
threads, group_range_count becomes 1. It would then try to verify (0,2),
(2,10), (10,12) and (12,20). Note that (2,10) and (12,20) are not valid
ranges to be verified, and with (20,22) uncovered.

Bug: 68343761
Test: Trigger update_verifier verification. Check the number of verified
      blocks against the one in care_map.txt.
Change-Id: I7c5769325d9866be06c45e7dbcc0c8ea266de714
(cherry picked from commit 62caeb5f48c9d7b1a8ed97c4a021195b8499b804)
(cherry picked from commit 559a6d1)

Author: Tao Bao

update_verifier/update_verifier.cpp

@@ -137,11 +137,12 @@ static bool read_blocks(const std::string& partition, const std::string& range_s
     LOG(ERROR) << "Error in parsing range string.";
     return false;
   }
+  range_count /= 2;
 
   std::vector<std::future<bool>> threads;
   size_t thread_num = std::thread::hardware_concurrency() ?: 4;
-  thread_num = std::min(thread_num, range_count / 2);
-  size_t group_range_count = range_count / thread_num;
+  thread_num = std::min(thread_num, range_count);
+  size_t group_range_count = (range_count + thread_num - 1) / thread_num;
 
   for (size_t t = 0; t < thread_num; t++) {
     auto thread_func = [t, group_range_count, &dm_block_device, &ranges, &partition]() {
@@ -154,7 +155,8 @@ static bool read_blocks(const std::string& partition, const std::string& range_s
         return false;
       }
 
-      for (size_t i = 1 + group_range_count * t; i < group_range_count * (t + 1) + 1; i += 2) {
+      for (size_t i = group_range_count * 2 * t + 1;
+           i < std::min(group_range_count * 2 * (t + 1) + 1, ranges.size()); i += 2) {
         unsigned int range_start, range_end;
         bool parse_status = android::base::ParseUint(ranges[i], &range_start);
         parse_status = parse_status && android::base::ParseUint(ranges[i + 1], &range_end);