Remove contention on signature cache during block validation

Since block validation happens in parallel, multiple threads may be
accessing the signature cache simultaneously. To prevent contention:
* Turn the signature cache lock into a shared mutex
* Make reading from the cache only acquire a shared lock
* Let block validations not store their results in the cache

src/main.cpp

@@ -1627,6 +1627,9 @@ bool CBlock::ConnectBlock(CBlockIndex* pindex, CCoinsViewCache &view, bool fJust
     int64 nBIP16SwitchTime = 1333238400;
     bool fStrictPayToScriptHash = (pindex->nTime >= nBIP16SwitchTime);
 
+    unsigned int flags = SCRIPT_VERIFY_NOCACHE |
+                         (fStrictPayToScriptHash ? SCRIPT_VERIFY_P2SH : SCRIPT_VERIFY_NONE);
+
     CBlockUndo blockundo;
 
     CCheckQueueControl<CScriptCheck> control(fScriptChecks && nScriptCheckThreads ? &scriptcheckqueue : NULL);
@@ -1663,7 +1666,7 @@ bool CBlock::ConnectBlock(CBlockIndex* pindex, CCoinsViewCache &view, bool fJust
             nFees += tx.GetValueIn(view)-tx.GetValueOut();
 
             std::vector<CScriptCheck> vChecks;
-            if (!tx.CheckInputs(view, fScriptChecks, fStrictPayToScriptHash ? SCRIPT_VERIFY_P2SH : SCRIPT_VERIFY_NONE, nScriptCheckThreads ? &vChecks : NULL))
+            if (!tx.CheckInputs(view, fScriptChecks, flags, nScriptCheckThreads ? &vChecks : NULL))
                 return false;
             control.Add(vChecks);
         }

src/script.cpp

@@ -16,7 +16,7 @@ using namespace boost;
 #include "sync.h"
 #include "util.h"
 
-bool CheckSig(vector<unsigned char> vchSig, vector<unsigned char> vchPubKey, CScript scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType);
+bool CheckSig(vector<unsigned char> vchSig, vector<unsigned char> vchPubKey, CScript scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType, int flags);
 
 
 
@@ -1007,7 +1007,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, co
 
                     bool fSuccess = (!fStrictEncodings || (IsCanonicalSignature(vchSig) && IsCanonicalPubKey(vchPubKey)));
                     if (fSuccess)
-                        fSuccess = CheckSig(vchSig, vchPubKey, scriptCode, txTo, nIn, nHashType);
+                        fSuccess = CheckSig(vchSig, vchPubKey, scriptCode, txTo, nIn, nHashType, flags);
 
                     popstack(stack);
                     popstack(stack);
@@ -1069,7 +1069,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, co
                         // Check signature
                         bool fOk = (!fStrictEncodings || (IsCanonicalSignature(vchSig) && IsCanonicalPubKey(vchPubKey)));
                         if (fOk)
-                            fOk = CheckSig(vchSig, vchPubKey, scriptCode, txTo, nIn, nHashType);
+                            fOk = CheckSig(vchSig, vchPubKey, scriptCode, txTo, nIn, nHashType, flags);
 
                         if (fOk) {
                             isig++;
@@ -1199,13 +1199,13 @@ private:
      // sigdata_type is (signature hash, signature, public key):
     typedef boost::tuple<uint256, std::vector<unsigned char>, std::vector<unsigned char> > sigdata_type;
     std::set< sigdata_type> setValid;
-    CCriticalSection cs_sigcache;
+    boost::shared_mutex cs_sigcache;
 
 public:
     bool
     Get(uint256 hash, const std::vector<unsigned char>& vchSig, const std::vector<unsigned char>& pubKey)
     {
-        LOCK(cs_sigcache);
+        boost::shared_lock<boost::shared_mutex> lock(cs_sigcache);
 
         sigdata_type k(hash, vchSig, pubKey);
         std::set<sigdata_type>::iterator mi = setValid.find(k);
@@ -1223,7 +1223,7 @@ public:
         int64 nMaxCacheSize = GetArg("-maxsigcachesize", 50000);
         if (nMaxCacheSize <= 0) return;
 
-        LOCK(cs_sigcache);
+        boost::unique_lock<boost::shared_mutex> lock(cs_sigcache);
 
         while (static_cast<int64>(setValid.size()) > nMaxCacheSize)
         {
@@ -1246,7 +1246,7 @@ public:
 };
 
 bool CheckSig(vector<unsigned char> vchSig, vector<unsigned char> vchPubKey, CScript scriptCode,
-              const CTransaction& txTo, unsigned int nIn, int nHashType)
+              const CTransaction& txTo, unsigned int nIn, int nHashType, int flags)
 {
     static CSignatureCache signatureCache;
 
@@ -1271,7 +1271,9 @@ bool CheckSig(vector<unsigned char> vchSig, vector<unsigned char> vchPubKey, CSc
     if (!key.Verify(sighash, vchSig))
         return false;
 
-    signatureCache.Set(sighash, vchSig, vchPubKey);
+    if (!(flags & SCRIPT_VERIFY_NOCACHE))
+        signatureCache.Set(sighash, vchSig, vchPubKey);
+
     return true;
 }
 
@@ -1761,7 +1763,7 @@ static CScript CombineMultisig(CScript scriptPubKey, const CTransaction& txTo, u
             if (sigs.count(pubkey))
                 continue; // Already got a sig for this pubkey
 
-            if (CheckSig(sig, pubkey, scriptPubKey, txTo, nIn, 0))
+            if (CheckSig(sig, pubkey, scriptPubKey, txTo, nIn, 0, 0))
             {
                 sigs[pubkey] = sig;
                 break;

src/script.h

@@ -32,6 +32,7 @@ enum
     SCRIPT_VERIFY_NONE      = 0,
     SCRIPT_VERIFY_P2SH      = (1U << 0),
     SCRIPT_VERIFY_STRICTENC = (1U << 1),
+    SCRIPT_VERIFY_NOCACHE   = (1U << 2),
 };
 
 enum txnouttype