Merge bitcoin/bitcoin#27823: init: return error when block index is non-contiguous, fix feature_init.py file perturbation

d27b9a2248476439ddab7700327f074005a810d5 test: fix feature_init.py file perturbation (Martin Zumsande)
ad66ca1e475d2546dbbda206465307613108a15d init: abort loading of blockindex in case of missing height. (Martin Zumsande)

Pull request description:

  When the block index database is non-contiguous due to file corruption (i.e. it contains indexes of height `x-1` and `x+1`, but not `x`), bitcoind can currently crash with an assert in `BuildSkip()` / `GetAncestor()` during `BlockManager::LoadBlockIndex()`:
  ```
  bitcoind: chain.cpp:112: const CBlockIndex* CBlockIndex::GetAncestor(int) const: Assertion `pindexWalk->pprev' failed.
  ```
  This PR changes it such that we instead return an `InitError` to the user.

  I stumbled upon this because I noticed that the file perturbation in `feature_init.py`  wasn't working as intended, which is fixed in the second commit:
  * Opening the file twice in one `with` statement would lead to `tf_read` being empty, so the test wouldn't perturb anything but replace the file with a new one. Fixed by first opening for read, then for write.
  * We need to restore the previous state after perturbations, so that only the current perturbation is active and not a mix of the current and previous ones.
  * I also added `checkblocks=200` to the startup parameters so that corruption in earlier blocks of `blk00000.dat` is detected during init verification and not ignored.

  After fixing `feature_init.py` like that I'd run into the `assert` mentioned above (so running the testfix from the second commit without the first one is a way to reproduce it).

ACKs for top commit:
  achow101:
    ACK d27b9a2248476439ddab7700327f074005a810d5
  furszy:
    Code ACK d27b9a224
  fjahr:
    Code review ACK d27b9a2248476439ddab7700327f074005a810d5

Tree-SHA512: 2e54da6030c5813c86bd58f816401e090bb43c5b834764a5e3c0e55dbfe09e423f88042cab823db3742088204b274d4ad2abf58a3832a4b18328b11a30bf7094

1 files changed, 5 insertions, 0 deletions

diff --git a/src/node/blockstorage.cpp b/src/node/blockstorage.cpp
index 5e61ed3100..706b62ea9b 100644
--- a/src/node/blockstorage.cpp
+++ b/src/node/blockstorage.cpp
@@ -410,8 +410,13 @@ bool BlockManager::LoadBlockIndex(const std::optional<uint256>& snapshot_blockha
     std::sort(vSortedByHeight.begin(), vSortedByHeight.end(),
               CBlockIndexHeightOnlyComparator());
 
+    CBlockIndex* previous_index{nullptr};
     for (CBlockIndex* pindex : vSortedByHeight) {
         if (m_interrupt) return false;
+        if (previous_index && pindex->nHeight > previous_index->nHeight + 1) {
+            return error("%s: block index is non-contiguous, index of height %d missing", __func__, previous_index->nHeight + 1);
+        }
+        previous_index = pindex;
         pindex->nChainWork = (pindex->pprev ? pindex->pprev->nChainWork : 0) + GetBlockProof(*pindex);
         pindex->nTimeMax = (pindex->pprev ? std::max(pindex->pprev->nTimeMax, pindex->nTime) : pindex->nTime);