Discourage NOPs reserved for soft-fork upgrades

NOP1 to NOP10 are reserved for future soft-fork upgrades. In the event
of an upgrade such NOPs have *VERIFY behavior, meaning that if their
arguments are not correct the script fails. Discouraging these NOPs by
rejecting transactions containing them from the mempool ensures that
we'll never accept transactions, nor mine blocks, with scripts that are
now invalid according to the majority of hashing power even if we're not
yet upgraded. Previously this wasn't an issue as the IsStandard() rules
didn't allow upgradable NOPs anyway, but 7f3b4e95 relaxed the
IsStandard() rules for P2SH redemptions allowing any redeemScript to be
spent.

We *do* allow upgradable NOPs in scripts so long as they are not
executed. This is harmless as there is no opportunity for the script to
be invalid post-upgrade.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/script/script_error.cpp b/src/script/script_error.cpp
index 4a3df268ec..793fc0da41 100644
--- a/src/script/script_error.cpp
+++ b/src/script/script_error.cpp
@@ -59,6 +59,8 @@ const char* ScriptErrorString(const ScriptError serror)
             return "Non-canonical signature: S value is unnecessarily high";
         case SCRIPT_ERR_SIG_NULLDUMMY:
             return "Dummy CHECKMULTISIG argument must be zero";
+        case SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS:
+            return "NOPx reserved for soft-fork upgrades";
         case SCRIPT_ERR_UNKNOWN_ERROR:
         case SCRIPT_ERR_ERROR_COUNT:
         default: break;