Merge #19905: Remove dead CheckForkWarningConditionsOnNewFork

fa7eed5be704ccdbdce5c9aedb953dd9c8b30446 doc: Clarify that vpindexToConnect is in reverse order (MarcoFalke)
fa62304c9760f0de9838e56150008816e7a9bacb Remove dead CheckForkWarningConditionsOnNewFork (MarcoFalke)

Pull request description:

  The function has several code and logic bugs, which prevent it from working at all:

  * `vpindexToConnect.back()` is passed to `CheckForkWarningConditionsOnNewFork`, which is the earliest connected block (least work block), *not* the new fork tip
  * `ActivateBestChainStep` will never try to connect a block that descends from an invalid block, so the invalid fork will only ever be of height 1, never hitting the 7 block minimum condition

  Instead of dragging the dead and wrong code around through every change in validation, remove it. In the future it could make sense to add a fork detection somewhere outside of the `ActivateBestChainStep` logic (maybe net_processing).

ACKs for top commit:
  jnewbery:
    utACK fa7eed5be704ccdbdce5c9aedb953dd9c8b30446
  fjahr:
    Code review ACK fa7eed5be704ccdbdce5c9aedb953dd9c8b30446
  glozow:
    utACK https://github.com/bitcoin/bitcoin/commit/fa7eed5be704ccdbdce5c9aedb953dd9c8b30446 I see that it's dead code

Tree-SHA512: 815bdbac7c1eb5b7594b0866a2dbd3c7619797afaadb03a5269fb96739ffb83b05b8e4f7c1e68d48d7886132dd0b12c14c3fb4ee0e72de1074726050ed203e1a

3 files changed, 13 insertions, 94 deletions

diff --git a/src/validation.cpp b/src/validation.cpp
index feb7502a0f..8c87c53ac7 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -1312,8 +1312,6 @@ bool CChainState::IsInitialBlockDownload() const
     return false;
 }
 
-static CBlockIndex *pindexBestForkTip = nullptr, *pindexBestForkBase = nullptr;
-
 static void AlertNotify(const std::string& strMessage)
 {
     uiInterface.NotifyAlertChanged();
@@ -1339,73 +1337,16 @@ static void CheckForkWarningConditions() EXCLUSIVE_LOCKS_REQUIRED(cs_main)
     AssertLockHeld(cs_main);
     // Before we get past initial download, we cannot reliably alert about forks
     // (we assume we don't get stuck on a fork before finishing our initial sync)
-    if (::ChainstateActive().IsInitialBlockDownload())
+    if (::ChainstateActive().IsInitialBlockDownload()) {
         return;
-
-    // If our best fork is no longer within 72 blocks (+/- 12 hours if no one mines it)
-    // of our head, drop it
-    if (pindexBestForkTip && ::ChainActive().Height() - pindexBestForkTip->nHeight >= 72)
-        pindexBestForkTip = nullptr;
-
-    if (pindexBestForkTip || (pindexBestInvalid && pindexBestInvalid->nChainWork > ::ChainActive().Tip()->nChainWork + (GetBlockProof(*::ChainActive().Tip()) * 6)))
-    {
-        if (!GetfLargeWorkForkFound() && pindexBestForkBase)
-        {
-            std::string warning = std::string("'Warning: Large-work fork detected, forking after block ") +
-                pindexBestForkBase->phashBlock->ToString() + std::string("'");
-            AlertNotify(warning);
-        }
-        if (pindexBestForkTip && pindexBestForkBase)
-        {
-            LogPrintf("%s: Warning: Large valid fork found\n  forking the chain at height %d (%s)\n  lasting to height %d (%s).\nChain state database corruption likely.\n", __func__,
-                   pindexBestForkBase->nHeight, pindexBestForkBase->phashBlock->ToString(),
-                   pindexBestForkTip->nHeight, pindexBestForkTip->phashBlock->ToString());
-            SetfLargeWorkForkFound(true);
-        }
-        else
-        {
-            LogPrintf("%s: Warning: Found invalid chain at least ~6 blocks longer than our best chain.\nChain state database corruption likely.\n", __func__);
-            SetfLargeWorkInvalidChainFound(true);
-        }
     }
-    else
-    {
-        SetfLargeWorkForkFound(false);
-        SetfLargeWorkInvalidChainFound(false);
-    }
-}
 
-static void CheckForkWarningConditionsOnNewFork(CBlockIndex* pindexNewForkTip) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
-{
-    AssertLockHeld(cs_main);
-    // If we are on a fork that is sufficiently large, set a warning flag
-    CBlockIndex* pfork = pindexNewForkTip;
-    CBlockIndex* plonger = ::ChainActive().Tip();
-    while (pfork && pfork != plonger)
-    {
-        while (plonger && plonger->nHeight > pfork->nHeight)
-            plonger = plonger->pprev;
-        if (pfork == plonger)
-            break;
-        pfork = pfork->pprev;
-    }
-
-    // We define a condition where we should warn the user about as a fork of at least 7 blocks
-    // with a tip within 72 blocks (+/- 12 hours if no one mines it) of ours
-    // We use 7 blocks rather arbitrarily as it represents just under 10% of sustained network
-    // hash rate operating on the fork.
-    // or a chain that is entirely longer than ours and invalid (note that this should be detected by both)
-    // We define it this way because it allows us to only store the highest fork tip (+ base) which meets
-    // the 7-block condition and from this always have the most-likely-to-cause-warning fork
-    if (pfork && (!pindexBestForkTip || pindexNewForkTip->nHeight > pindexBestForkTip->nHeight) &&
-            pindexNewForkTip->nChainWork - pfork->nChainWork > (GetBlockProof(*pfork) * 7) &&
-            ::ChainActive().Height() - pindexNewForkTip->nHeight < 72)
-    {
-        pindexBestForkTip = pindexNewForkTip;
-        pindexBestForkBase = pfork;
+    if (pindexBestInvalid && pindexBestInvalid->nChainWork > ::ChainActive().Tip()->nChainWork + (GetBlockProof(*::ChainActive().Tip()) * 6)) {
+        LogPrintf("%s: Warning: Found invalid chain at least ~6 blocks longer than our best chain.\nChain state database corruption likely.\n", __func__);
+        SetfLargeWorkInvalidChainFound(true);
+    } else {
+        SetfLargeWorkInvalidChainFound(false);
     }
-
-    CheckForkWarningConditions();
 }
 
 // Called both upon regular invalid block discovery *and* InvalidateBlock
@@ -2746,8 +2687,8 @@ bool CChainState::ActivateBestChainStep(BlockValidationState& state, const CChai
     AssertLockHeld(cs_main);
     AssertLockHeld(m_mempool.cs);
 
-    const CBlockIndex *pindexOldTip = m_chain.Tip();
-    const CBlockIndex *pindexFork = m_chain.FindFork(pindexMostWork);
+    const CBlockIndex* pindexOldTip = m_chain.Tip();
+    const CBlockIndex* pindexFork = m_chain.FindFork(pindexMostWork);
 
     // Disconnect active blocks which are no longer in the best chain.
     bool fBlocksDisconnected = false;
@@ -2767,7 +2708,7 @@ bool CChainState::ActivateBestChainStep(BlockValidationState& state, const CChai
         fBlocksDisconnected = true;
     }
 
-    // Build list of new blocks to connect.
+    // Build list of new blocks to connect (in descending height order).
     std::vector<CBlockIndex*> vpindexToConnect;
     bool fContinue = true;
     int nHeight = pindexFork ? pindexFork->nHeight : -1;
@@ -2777,7 +2718,7 @@ bool CChainState::ActivateBestChainStep(BlockValidationState& state, const CChai
         int nTargetHeight = std::min(nHeight + 32, pindexMostWork->nHeight);
         vpindexToConnect.clear();
         vpindexToConnect.reserve(nTargetHeight - nHeight);
-        CBlockIndex *pindexIter = pindexMostWork->GetAncestor(nTargetHeight);
+        CBlockIndex* pindexIter = pindexMostWork->GetAncestor(nTargetHeight);
         while (pindexIter && pindexIter->nHeight != nHeight) {
             vpindexToConnect.push_back(pindexIter);
             pindexIter = pindexIter->pprev;
@@ -2785,7 +2726,7 @@ bool CChainState::ActivateBestChainStep(BlockValidationState& state, const CChai
         nHeight = nTargetHeight;
 
         // Connect new blocks.
-        for (CBlockIndex *pindexConnect : reverse_iterate(vpindexToConnect)) {
+        for (CBlockIndex* pindexConnect : reverse_iterate(vpindexToConnect)) {
             if (!ConnectTip(state, chainparams, pindexConnect, pindexConnect == pindexMostWork ? pblock : std::shared_ptr<const CBlock>(), connectTrace, disconnectpool)) {
                 if (state.IsInvalid()) {
                     // The block violates a consensus rule.
@@ -2821,11 +2762,7 @@ bool CChainState::ActivateBestChainStep(BlockValidationState& state, const CChai
     }
     m_mempool.check(&CoinsTip());
 
-    // Callbacks/notifications for a new best chain.
-    if (fInvalidFound)
-        CheckForkWarningConditionsOnNewFork(vpindexToConnect.back());
-    else
-        CheckForkWarningConditions();
+    CheckForkWarningConditions();
 
     return true;
 }
diff --git a/src/warnings.cpp b/src/warnings.cpp
index 501bf7e637..1dec663a73 100644
--- a/src/warnings.cpp
+++ b/src/warnings.cpp
@@ -14,7 +14,6 @@
 
 static Mutex g_warnings_mutex;
 static bilingual_str g_misc_warnings GUARDED_BY(g_warnings_mutex);
-static bool fLargeWorkForkFound GUARDED_BY(g_warnings_mutex) = false;
 static bool fLargeWorkInvalidChainFound GUARDED_BY(g_warnings_mutex) = false;
 
 void SetMiscWarning(const bilingual_str& warning)
@@ -23,18 +22,6 @@ void SetMiscWarning(const bilingual_str& warning)
     g_misc_warnings = warning;
 }
 
-void SetfLargeWorkForkFound(bool flag)
-{
-    LOCK(g_warnings_mutex);
-    fLargeWorkForkFound = flag;
-}
-
-bool GetfLargeWorkForkFound()
-{
-    LOCK(g_warnings_mutex);
-    return fLargeWorkForkFound;
-}
-
 void SetfLargeWorkInvalidChainFound(bool flag)
 {
     LOCK(g_warnings_mutex);
@@ -60,10 +47,7 @@ bilingual_str GetWarnings(bool verbose)
         warnings_verbose.emplace_back(warnings_concise);
     }
 
-    if (fLargeWorkForkFound) {
-        warnings_concise = _("Warning: The network does not appear to fully agree! Some miners appear to be experiencing issues.");
-        warnings_verbose.emplace_back(warnings_concise);
-    } else if (fLargeWorkInvalidChainFound) {
+    if (fLargeWorkInvalidChainFound) {
         warnings_concise = _("Warning: We do not appear to fully agree with our peers! You may need to upgrade, or other nodes may need to upgrade.");
         warnings_verbose.emplace_back(warnings_concise);
     }
diff --git a/src/warnings.h b/src/warnings.h
index 28546eb753..e87b64a86d 100644
--- a/src/warnings.h
+++ b/src/warnings.h
@@ -11,8 +11,6 @@
 struct bilingual_str;
 
 void SetMiscWarning(const bilingual_str& warning);
-void SetfLargeWorkForkFound(bool flag);
-bool GetfLargeWorkForkFound();
 void SetfLargeWorkInvalidChainFound(bool flag);
 /** Format a string that describes several potential problems detected by the core.
  * @param[in] verbose bool