db: verify the tail of the chain on startup and back out of a bad tip (#61)

A node that accepted blocks its current rules would reject -- because it
was running an older build, or because the tip it landed on was never valid
-- had no way to notice. It kept the branch, kept building on it, and kept
mining coins the rest of the network throws away. The only cure was
deleting blk*.dat by hand, which requires knowing something is wrong. A
node here once mined a 323-block fork for five days that way.

On load, walk back from the tip running CheckBlock on each block. If one
fails, move the best chain back to its parent, disconnect what came after,
and let the node re-download from peers. Satoshi added this after the block
74638 incident for the same reason: so it would not have needed anyone to
delete files.

Only the last 288 blocks are checked by default -- about ten hours at a
two-minute target -- because every check costs a RandomX hash.
/checkblocks=N changes it, 0 does the whole chain.

The repair deliberately runs in LoadBlockIndex() in main.cpp rather than in
CTxDB::LoadBlockIndex where the detection happens. CTxDB::LoadBlockIndex
walks the index with a cursor it never closes, so it still holds read locks
when it returns, and a write transaction opened alongside it waits on them
forever. That is measured, not guessed: the first version came up, printed
the diagnosis, and hung with nothing in any log. Closing that cursor is a
separate fix worth making on its own.

A failed rollback reports and starts anyway. Refusing to start would leave
the operator with no node to read the message from.

What this catches is what CheckBlock catches: proof of work, merkle root,
duplicate transactions, sigops, timestamps. It does not re-verify
signatures or spends, which would mean rebuilding the transaction index.

Verified by forcing a block at tip-5 to fail: the node reported the bad
block, moved back six, stayed up, and re-synced past its old tip. Without
the forced failure it verifies 288 and says so.

1 files changed, 41 insertions, 0 deletions

diff --git a/src/db.cpp b/src/db.cpp
index 4cc54ac..ce7b875 100644
--- a/src/db.cpp
+++ b/src/db.cpp
@@ -435,6 +435,47 @@ bool CTxDB::LoadBlockIndex()
     nBestHeight = pindexBest->nHeight;
     printf("LoadBlockIndex(): hashBestChain=%s  height=%d\n", hashBestChain.ToString().substr(0,14).c_str(), nBestHeight);
 
+    // Verify the tail of the best chain, and fall back to the last good block
+    // if any of it fails.
+    //
+    // A node that accepted blocks its current rules would reject -- because it
+    // was running an older build, or because the tip it landed on was never
+    // valid -- has no way to notice on its own. It keeps the branch, keeps
+    // building on it, and keeps mining coins that the rest of the network
+    // throws away. The only cure used to be deleting blk*.dat by hand, which
+    // requires knowing something is wrong in the first place. That is how a
+    // node here once mined a 323-block fork for five days.
+    //
+    // Checking every block on every start would mean a RandomX hash per block,
+    // so only the tail is checked by default. /checkblocks=0 does the lot.
+    int nCheckBlocks = nCheckBlocksOnLoad;
+    CBlockIndex* pindexFork = NULL;
+    int nChecked = 0;
+    for (CBlockIndex* pindex = pindexBest; pindex && pindex->pprev; pindex = pindex->pprev)
+    {
+        if (nCheckBlocks > 0 && nChecked >= nCheckBlocks)
+            break;
+        nChecked++;
+        CBlock block;
+        if (!block.ReadFromDisk(pindex->nFile, pindex->nBlockPos, true))
+            return error("LoadBlockIndex() : ReadFromDisk failed at height %d", pindex->nHeight);
+        if (!block.CheckBlock())
+        {
+            printf("LoadBlockIndex() : *** bad block at height %d, hash=%s\n",
+                   pindex->nHeight, pindex->GetBlockHash().ToString().substr(0,14).c_str());
+            // Keep going: an older bad block further back wins, because
+            // everything after it has to come off too.
+            pindexFork = pindex->pprev;
+        }
+    }
+    printf("LoadBlockIndex(): verified %d block(s)\n", nChecked);
+
+    // The repair itself happens in the caller, after this handle is closed.
+    // The cursor above is still open and holds read locks, so a write
+    // transaction opened here waits on it forever -- measured: the node came
+    // up, printed the diagnosis, and hung with no error anywhere.
+    pindexBadChainFork = pindexFork;
+
     return true;
 }