Simplify DisconnectBlock arguments/return value

DisconnectBlock currently has a complicated interface:

  Situation       Return value
                  pfClean != nullptr   pfClean == nullptr

  All good:       true                 true
  Failure:        false                false
  Unclean rewind: true                 false
                  with *pfClean=false

Change this to return a tristate enum instead. As an added bonus,
remove the ValidationState& argument which was unused.

src/validation.cpp

@@ -1526,28 +1526,36 @@ bool ApplyTxInUndo(const CTxInUndo& undo, CCoinsViewCache& view, const COutPoint
     return fClean;
 }
 
+enum DisconnectResult
+{
+    DISCONNECT_OK,      // All good.
+    DISCONNECT_UNCLEAN, // Rolled back, but UTXO set was inconsistent with block.
+    DISCONNECT_FAILED   // Something else went wrong.
+};
+
 /** Undo the effects of this block (with given index) on the UTXO set represented by coins.
- *  In case pfClean is provided, operation will try to be tolerant about errors, and *pfClean
- *  will be true if no problems were found. Otherwise, the return value will be false in case
- *  of problems. Note that in any case, coins may be modified. */
-static bool DisconnectBlock(const CBlock& block, CValidationState& state, const CBlockIndex* pindex, CCoinsViewCache& view, bool* pfClean = NULL)
+ *  When UNCLEAN or FAILED is returned, view is left in an indeterminate state. */
+static DisconnectResult DisconnectBlock(const CBlock& block, const CBlockIndex* pindex, CCoinsViewCache& view)
 {
     assert(pindex->GetBlockHash() == view.GetBestBlock());
 
-    if (pfClean)
-        *pfClean = false;
-
     bool fClean = true;
 
     CBlockUndo blockUndo;
     CDiskBlockPos pos = pindex->GetUndoPos();
-    if (pos.IsNull())
-        return error("DisconnectBlock(): no undo data available");
-    if (!UndoReadFromDisk(blockUndo, pos, pindex->pprev->GetBlockHash()))
-        return error("DisconnectBlock(): failure reading undo data");
+    if (pos.IsNull()) {
+        error("DisconnectBlock(): no undo data available");
+        return DISCONNECT_FAILED;
+    }
+    if (!UndoReadFromDisk(blockUndo, pos, pindex->pprev->GetBlockHash())) {
+        error("DisconnectBlock(): failure reading undo data");
+        return DISCONNECT_FAILED;
+    }
 
-    if (blockUndo.vtxundo.size() + 1 != block.vtx.size())
-        return error("DisconnectBlock(): block and undo data inconsistent");
+    if (blockUndo.vtxundo.size() + 1 != block.vtx.size()) {
+        error("DisconnectBlock(): block and undo data inconsistent");
+        return DISCONNECT_FAILED;
+    }
 
     // undo transactions in reverse order
     for (int i = block.vtx.size() - 1; i >= 0; i--) {
@@ -1576,8 +1584,10 @@ static bool DisconnectBlock(const CBlock& block, CValidationState& state, const
         // restore inputs
         if (i > 0) { // not coinbases
             const CTxUndo &txundo = blockUndo.vtxundo[i-1];
-            if (txundo.vprevout.size() != tx.vin.size())
-                return error("DisconnectBlock(): transaction and undo data inconsistent");
+            if (txundo.vprevout.size() != tx.vin.size()) {
+                error("DisconnectBlock(): transaction and undo data inconsistent");
+                return DISCONNECT_FAILED;
+            }
             for (unsigned int j = tx.vin.size(); j-- > 0;) {
                 const COutPoint &out = tx.vin[j].prevout;
                 const CTxInUndo &undo = txundo.vprevout[j];
@@ -1590,12 +1600,7 @@ static bool DisconnectBlock(const CBlock& block, CValidationState& state, const
     // move best block pointer to prevout block
     view.SetBestBlock(pindex->pprev->GetBlockHash());
 
-    if (pfClean) {
-        *pfClean = fClean;
-        return true;
-    }
-
-    return fClean;
+    return fClean ? DISCONNECT_OK : DISCONNECT_UNCLEAN;
 }
 
 void static FlushBlockFile(bool fFinalize = false)
@@ -2131,7 +2136,7 @@ bool static DisconnectTip(CValidationState& state, const CChainParams& chainpara
     int64_t nStart = GetTimeMicros();
     {
         CCoinsViewCache view(pcoinsTip);
-        if (!DisconnectBlock(block, state, pindexDelete, view))
+        if (DisconnectBlock(block, pindexDelete, view) != DISCONNECT_OK)
             return error("DisconnectTip(): DisconnectBlock %s failed", pindexDelete->GetBlockHash().ToString());
         bool flushed = view.Flush();
         assert(flushed);
@@ -3656,15 +3661,17 @@ bool CVerifyDB::VerifyDB(const CChainParams& chainparams, CCoinsView *coinsview,
         }
         // check level 3: check for inconsistencies during memory-only disconnect of tip blocks
         if (nCheckLevel >= 3 && pindex == pindexState && (coins.DynamicMemoryUsage() + pcoinsTip->DynamicMemoryUsage()) <= nCoinCacheUsage) {
-            bool fClean = true;
-            if (!DisconnectBlock(block, state, pindex, coins, &fClean))
+            DisconnectResult res = DisconnectBlock(block, pindex, coins);
+            if (res == DISCONNECT_FAILED) {
                 return error("VerifyDB(): *** irrecoverable inconsistency in block data at %d, hash=%s", pindex->nHeight, pindex->GetBlockHash().ToString());
+            }
             pindexState = pindex->pprev;
-            if (!fClean) {
+            if (res == DISCONNECT_UNCLEAN) {
                 nGoodTransactions = 0;
                 pindexFailure = pindex;
-            } else
+            } else {
                 nGoodTransactions += block.vtx.size();
+            }
         }
         if (ShutdownRequested())
             return true;