backport: partial bitcoin#25717, #25960, #25963, #25978, #26012, #26172, #26184, #26355, #26387, #26954, #30761 (anti-DOS headers sync)

src/Makefile.am

@@ -245,6 +245,7 @@ BITCOIN_CORE_H = \
   flat-database.h \
   flatfile.h \
   fs.h \
+  headerssync.h \
   httprpc.h \
   httpserver.h \
   i2p.h \
@@ -391,6 +392,7 @@ BITCOIN_CORE_H = \
   undo.h \
   unordered_lru_cache.h \
   util/bip32.h \
+  util/bitdeque.h \
   util/bytevectorhash.h \
   util/check.h \
   util/edge.h \
@@ -531,6 +533,7 @@ libbitcoin_node_a_SOURCES = \
   governance/vote.cpp \
   governance/votedb.cpp \
   gsl/assert.cpp \
+  headerssync.cpp \
   httprpc.cpp \
   httpserver.cpp \
   i2p.cpp \

src/Makefile.test.include

@@ -124,6 +124,7 @@ BITCOIN_TESTS =\
   test/coinjoin_dstxmanager_tests.cpp \
   test/coinjoin_queue_tests.cpp \
   test/hash_tests.cpp \
+  test/headers_sync_chainwork_tests.cpp \
   test/httpserver_tests.cpp \
   test/i2p_tests.cpp \
   test/interfaces_tests.cpp \
@@ -283,6 +284,7 @@ test_fuzz_fuzz_SOURCES = \
  test/fuzz/base_encode_decode.cpp \
  test/fuzz/bech32.cpp \
  test/fuzz/bip324.cpp \
+ test/fuzz/bitdeque.cpp \
  test/fuzz/block.cpp \
  test/fuzz/block_header.cpp \
  test/fuzz/blockfilter.cpp \

src/chain.cpp

@@ -29,32 +29,33 @@ void CChain::SetTip(CBlockIndex& block)
     }
 }
 
-CBlockLocator CChain::GetLocator(const CBlockIndex *pindex) const {
-    int nStep = 1;
-    std::vector<uint256> vHave;
-    vHave.reserve(32);
-
-    if (!pindex)
-        pindex = Tip();
-    while (pindex) {
-        vHave.push_back(pindex->GetBlockHash());
-        // Stop when we have added the genesis block.
-        if (pindex->nHeight == 0)
-            break;
+std::vector<uint256> LocatorEntries(const CBlockIndex* index)
+{
+    int step = 1;
+    std::vector<uint256> have;
+    if (index == nullptr) return have;
+
+    have.reserve(32);
+    while (index) {
+        have.emplace_back(index->GetBlockHash());
+        if (index->nHeight == 0) break;
         // Exponentially larger steps back, plus the genesis block.
-        int nHeight = std::max(pindex->nHeight - nStep, 0);
-        if (Contains(pindex)) {
-            // Use O(1) CChain index if possible.
-            pindex = (*this)[nHeight];
-        } else {
-            // Otherwise, use O(log n) skiplist.
-            pindex = pindex->GetAncestor(nHeight);
-        }
-        if (vHave.size() > 10)
-            nStep *= 2;
+        int height = std::max(index->nHeight - step, 0);
+        // Use skiplist.
+        index = index->GetAncestor(height);
+        if (have.size() > 10) step *= 2;
     }
+    return have;
+}
 
-    return CBlockLocator(vHave);
+CBlockLocator GetLocator(const CBlockIndex* index)
+{
+    return CBlockLocator{LocatorEntries(index)};
+}
+
+CBlockLocator CChain::GetLocator() const
+{
+    return ::GetLocator(Tip());
 }
 
 const CBlockIndex *CChain::FindFork(const CBlockIndex *pindex) const {

src/chain.h

@@ -472,8 +472,8 @@ class CChain
     /** Set/initialize a chain with a given tip. */
     void SetTip(CBlockIndex& block);
 
-    /** Return a CBlockLocator that refers to a block in this chain (by default the tip). */
-    CBlockLocator GetLocator(const CBlockIndex* pindex = nullptr) const;
+    /** Return a CBlockLocator that refers to the tip in of this chain. */
+    CBlockLocator GetLocator() const;
 
     /** Find the last common block between this chain and a block index entry. */
     const CBlockIndex* FindFork(const CBlockIndex* pindex) const;
@@ -482,4 +482,10 @@ class CChain
     CBlockIndex* FindEarliestAtLeast(int64_t nTime, int height) const;
 };
 
+/** Get a locator for a block index entry. */
+CBlockLocator GetLocator(const CBlockIndex* index);
+
+/** Construct a list of hash entries to put in a locator.  */
+std::vector<uint256> LocatorEntries(const CBlockIndex* index);
+
 #endif // BITCOIN_CHAIN_H

src/consensus/validation.h

@@ -57,6 +57,7 @@ enum class BlockValidationResult {
     BLOCK_INVALID_PREV,      //!< A block this one builds on is invalid
     BLOCK_TIME_FUTURE,       //!< block timestamp was > 2 hours in the future (or our clock is bad)
     BLOCK_CHECKPOINT,        //!< the block failed to meet one of our checkpoints
+    BLOCK_HEADER_LOW_WORK,   //!< the block header may be on a too-little-work chain
     BLOCK_CHAINLOCK,         //!< the block conflicts with the ChainLock
 };