From: Rusty Russell <rusty@rustcorp.com.au>
To: Andrew Poelstra <apoelstra@wpsoftware.net>
In-Reply-To: <ZTcbKM+XTCaJ2kIP@camus>
References: <CAEM=y+XDB7GGa5BTAWrQHqTqQHBE2VRyd7VWjEb+zCOMzRP+Lg@mail.gmail.com>
 <871qdmulvt.fsf@rustcorp.com.au> <ZTZ4H2y6+5pxRcs/@camus>
 <871qdku9pj.fsf@rustcorp.com.au> <ZTcbKM+XTCaJ2kIP@camus>
Date: Tue, 24 Oct 2023 14:15:49 +1030
Message-ID: <87r0lksmxe.fsf@rustcorp.com.au>
MIME-Version: 1.0
Content-Type: text/plain
Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Proposed BIP for OP_CAT
Precedence: list

Andrew Poelstra <apoelstra@wpsoftware.net> writes:
> I had a similar thought. But my feeling is that replacing the stack
> interpreter data structure is still too invasive to justify the benefit.
>
> Also, one of my favorite things about this BIP is the tiny diff.

To be fair, this diff is even smaller than the OP_CAT diff :)

Though I had to strongly resist refactoring, that interpreter code
needs a good shake!  Using a class for the stack is worth doing anyway
(macros, really??).

diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
index dcaf28c2472..2ee2034115f 100644
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -403,6 +403,19 @@ static bool EvalChecksig(const valtype& sig, const valtype& pubkey, CScript::con
     assert(false);
 }
 
+// First 520 bytes is free, after than you consume an extra slot!
+static size_t effective_size(const std::vector<std::vector<unsigned char> >& stack)
+{
+    size_t esize = stack.size();
+
+    for (const auto& v : stack)
+    {
+        if (v.size() > MAX_SCRIPT_ELEMENT_SIZE) 
+            esize += (v.size() - 1) / MAX_SCRIPT_ELEMENT_SIZE;
+    }
+    return esize;
+}
+    
 bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker, SigVersion sigversion, ScriptExecutionData& execdata, ScriptError* serror)
 {
     static const CScriptNum bnZero(0);
@@ -1239,7 +1252,7 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
             }
 
             // Size limits
-            if (stack.size() + altstack.size() > MAX_STACK_SIZE)
+            if (effective_size(stack) + effective_size(altstack) > MAX_STACK_SIZE)
                 return set_error(serror, SCRIPT_ERR_STACK_SIZE);
         }
     }