Do not trigger a DoS ban if SCRIPT_VERIFY_NULLDUMMY fails

src/main.cpp

@@ -1588,14 +1588,26 @@ bool CheckInputs(const CTransaction& tx, CValidationState &state, CCoinsViewCach
                     pvChecks->push_back(CScriptCheck());
                     check.swap(pvChecks->back());
                 } else if (!check()) {
-                    if (flags & SCRIPT_VERIFY_STRICTENC) {
-                        // For now, check whether the failure was caused by non-canonical
-                        // encodings or not; if so, don't trigger DoS protection.
-                        CScriptCheck check(coins, tx, i, flags & (~SCRIPT_VERIFY_STRICTENC), 0);
+                    if (flags & STANDARD_NOT_MANDATORY_VERIFY_FLAGS) {
+                        // Check whether the failure was caused by a
+                        // non-mandatory script verification check, such as
+                        // non-standard DER encodings or non-null dummy
+                        // arguments; if so, don't trigger DoS protection to
+                        // avoid splitting the network between upgraded and
+                        // non-upgraded nodes.
+                        CScriptCheck check(coins, tx, i,
+                                flags & ~STANDARD_NOT_MANDATORY_VERIFY_FLAGS, 0);
                         if (check())
-                            return state.Invalid(false, REJECT_NONSTANDARD, "non-canonical");
-                    }
-                    return state.DoS(100,false, REJECT_NONSTANDARD, "non-canonical");
+                            return state.Invalid(false, REJECT_NONSTANDARD, "non-mandatory-script-verify-flag");
+                    }
+                    // Failures of other flags indicate a transaction that is
+                    // invalid in new blocks, e.g. a invalid P2SH. We DoS ban
+                    // such nodes as they are not following the protocol. That
+                    // said during an upgrade careful thought should be taken
+                    // as to the correct behavior - we may want to continue
+                    // peering with non-upgraded nodes even after a soft-fork
+                    // super-majority vote has passed.
+                    return state.DoS(100,false, REJECT_INVALID, "mandatory-script-verify-flag-failed");
                 }
             }
         }

src/script.h

@@ -49,6 +49,9 @@ enum
 // them to be valid. (but old blocks may not comply with) Currently just P2SH,
 // but in the future other flags may be added, such as a soft-fork to enforce
 // strict DER encoding.
+//
+// Failing one of these tests may trigger a DoS ban - see CheckInputs() for
+// details.
 static const unsigned int MANDATORY_SCRIPT_VERIFY_FLAGS = SCRIPT_VERIFY_P2SH;
 
 // Standard script verification flags that standard transactions will comply
@@ -58,6 +61,9 @@ static const unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY
                                                          SCRIPT_VERIFY_STRICTENC |
                                                          SCRIPT_VERIFY_NULLDUMMY;
 
+// For convenience, standard but not mandatory verify flags.
+static const unsigned int STANDARD_NOT_MANDATORY_VERIFY_FLAGS = STANDARD_SCRIPT_VERIFY_FLAGS & ~MANDATORY_SCRIPT_VERIFY_FLAGS;
+
 enum txnouttype
 {
     TX_NONSTANDARD,