Add SCRIPT_VERIFY_CLEANSTACK (BIP62 rule 6)

Based on an earlier patch by Peter Todd, though the rules here are different
(P2SH scripts should not have a CLEANSTACK check before the P2SH evaluation).

src/script/interpreter.cpp

@@ -1096,7 +1096,6 @@ bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, unsigne
         return false;
     if (stack.empty())
         return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
-
     if (CastToBool(stack.back()) == false)
         return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
 
@@ -1126,8 +1125,18 @@ bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, unsigne
             return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
         if (!CastToBool(stack.back()))
             return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
-        else
-            return set_success(serror);
+    }
+
+    // The CLEANSTACK check is only performed after potential P2SH evaluation,
+    // as the non-P2SH evaluation of a P2SH script will obviously not result in
+    // a clean stack (the P2SH inputs remain).
+    if ((flags & SCRIPT_VERIFY_CLEANSTACK) != 0) {
+        // Disallow CLEANSTACK without P2SH, as otherwise a switch CLEANSTACK->P2SH+CLEANSTACK
+        // would be possible, which is not a softfork (and P2SH should be one).
+        assert((flags & SCRIPT_VERIFY_P2SH) != 0);
+        if (stack.size() != 1) {
+            return set_error(serror, SCRIPT_ERR_CLEANSTACK);
+        }
     }
 
     return set_success(serror);

src/script/interpreter.h

@@ -67,8 +67,14 @@ enum
     // discouraged NOPs fails the script. This verification flag will never be
     // a mandatory flag applied to scripts in a block. NOPs that are not
     // executed, e.g.  within an unexecuted IF ENDIF block, are *not* rejected.
-    SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS  = (1U << 7)
+    SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS  = (1U << 7),
 
+    // Require that only a single stack element remains after evaluation. This changes the success criterion from
+    // "At least one stack element must remain, and when interpreted as a boolean, it must be true" to
+    // "Exactly one stack element must remain, and when interpreted as a boolean, it must be true".
+    // (softfork safe, BIP62 rule 6)
+    // Note: CLEANSTACK should never be used without P2SH.
+    SCRIPT_VERIFY_CLEANSTACK = (1U << 8),
 };
 
 uint256 SignatureHash(const CScript &scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType);

src/script/script_error.h

@@ -43,6 +43,7 @@ typedef enum ScriptError_t
     SCRIPT_ERR_SIG_HIGH_S,
     SCRIPT_ERR_SIG_NULLDUMMY,
     SCRIPT_ERR_PUBKEYTYPE,
+    SCRIPT_ERR_CLEANSTACK,
 
     /* softfork safeness */
     SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS,